Download Electrical Hazards - MEO COC Oral and Objective Exam for Marine

BOOK OF ELECTRICAL
ELECTRICAL ENGINEERING
CONTENTS
Electrical Hazards ............................................................................................................... 2
Physiological effects of electricity ...................................................................................... 3
SOLAS Requirements……………………………………………………………………..6
Safety Requirements of Electrical equipments ................................................................... 6
Ship’s Electrical system .................................................................................................... 23
Power Distribution System ............................................................................................... 24
Insulated and earthed neutral system ................................................................................ 26
Distribution Circuit Breakers ............................................................................................ 31
Transformers ..................................................................................................................... 31
Instrument Transformers ................................................................................................... 35
Shore Supply Connection ................................................................................................. 36
Circuit Protection .............................................................................................................. 38
Cables................................................................................................................................ 43
Principle of 3 phase A.C. generation ................................................................................ 45
Alternator construction ..................................................................................................... 47
Automatic Voltage Regulation ......................................................................................... 50
Alternators in parallel ....................................................................................................... 51
Generator protection ......................................................................................................... 53
Main Switchboard ............................................................................................................. 53
Main Circuit Breakers ....................................................................................................... 54
Single phase a.c. motor ..................................................................................................... 57
3 Phase AC induction motors ........................................................................................... 58
3 phase squirrel cage induction motor .............................................................................. 60
Motor Starters ................................................................................................................... 61
Contactors ......................................................................................................................... 65
Induction Motor Speed Control ........................................................................................ 66
Motor Protection ............................................................................................................... 68
Single-phasing................................................................................................................... 70
Motor enclosures ............................................................................................................... 71
Insulation class .................................................................................................................. 72
Electrical Practice for Tankers .......................................................................................... 73
Stroboscopic effect of light ............................................................................................... 76
Navigation and Signal Lights............................................................................................ 77
Electrical Diagrams ........................................................................................................... 78
Battery ............................................................................................................................... 81
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Electrical Hazards
1. Electrical shock
2. Electrical fire
3. Non-operation of essential equipments
4. Incorrect operation of equipments
1. Electrical shock
A person may be electrocuted. We will study the physiological effects of electrocution on
human body and immediate assistance and first aid to be provided to the victim later on.
2. Electrical Fires
There is a second major hazard of fire or an explosion due to electrical causes. When, you
read in a newspaper regarding any big fires, the cause normally given is “due to electrical short
circuit”.
ELECTRICAL CAUSES OF FIRE
I.
Fires due to use of electricity as a form of heat
II.
Fires caused due to overloading of wires and equipment
III.
Fault in wiring
IV.
Things stored below transmission line due to earth fault currents
Fires due to use of electricity as a form of heat
This group includes such fires as those due to pans of fat boiling over or material being
dropped over electric heaters. Leaving electric irons ‘on’ and then going to do some petty job and
totally forgetting about the iron has caused many fires.
Fires caused due to overloading of wires and equipment
Overloading leads to overheating which ultimately leads to fire. Fuses and miniature
circuit breakers (MCB’s) are meant to prevent overloading.
Fault in wiring
The shorts occurring between phase wires or between phase and neutral or earth gives
rise to sparks and heat. If faults occur in places where easily combustible materials are lying
around, fire will take place. Fuse or MCB’s cannot stop this as they come into action only after
the short circuit has taken place. The heat and sparks produced cause the damage and they are
already there although the fuse has blown or the MCB has tripped.
Things stored below transmission line due to earth fault currents
It is better to leave the space below transmission line free of easily combustible materials.
With proper earth wires such fires are totally avoided but if earth wire is broken it takes some
time to replace it. In that interval some mishap can take place.
Fire Extinguishing
Great care is required in case of extinguishing fire where electricity is also present. Water
is not the right fire-extinguishing agent where electricity is concerned. Since water is a good
conductor of electricity, there are chances that the person pouring water may get severe shock.
The right extinguishing agent for such fires is carbon dioxide or dry powder.
Burns
Fire and burns generally go hand in hand. Human beings are intolerant of even quite
moderate heat. Temperatures above 65 degree C are tolerated for only limited periods. Above 90
degree C, the tolerance time drops sharply. 120 degree C can be tolerated for 15 minutes, 145
degree C becomes intolerable in 5 minutes and at 175 degree C, irreversible injury occurs to the
skin in less than one minute. Generally in a fire, temperature may reach 150 degree C 3 metres
ahead of the blaze and over 500 degree C above the fire.
Burns classification
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The severity of burn is determined by many factors. They are intensity of heat, the length
of exposure, size of body area burnt, the thickness of skin, the age and health of victim and the
speed of subsequent cooling.
Degree of burn
• First degree - When skin is reddened but there is no blistering
• Second degree - Includes reddening and blistering but does not destroy the full thickness
of the skin. In such cases new skin can grow from uninjured cells below
• Third degree - Where full thickness of skin is destroyed, injuring all cells below. No
chances of new skin growing. Skin grafting will become necessary
Care of burn
Generally if a burn hurts, it is a good sign. A full thickness burn (3rd degree) destroys the
nerve endings and does not hurt. A wound that hurts will heal if proper care is taken. Danger is
always involved if a burn becomes infected. Pathogenic bacteria i.e. the germs that can cause
infection are everywhere. Hence no burn should be neglected. Any burn more than 3 cm square
needs a doctor’s care.
Cold Water
One can actually decrease the severity of a burn if the skin is cooled quickly by pouring
cold water. If you wait for 5 minutes and then put cold water on broken blisters, more harm than
good is done by introducing pathogenic bacteria in the wound. Quickness is essential, if cold
water is to be used.
3. Non-operation of essential equipment
This may make the ship unsafe resulting in loss of life, loss of ship, loss of cargo and
possibly a hazard to nearby people and floating/fixed objects.
4. Incorrect operation of equipment
Can be injurious/ harmful to operator and others on board the ship
Physiological effects of electricity
1. Heat and burn
2. Preventing muscles from acting
3. Effect heart muscles
Heat and burn
Effect of electricity on living tissue: current makes it heat up. If the amount of heat
generated is sufficient, the tissue may be burnt. The effect is physiologically the same as damage
caused by an open flame or other high-temperature source of heat, except that electricity has the
ability to burn tissue well beneath the skin of a victim, even burning internal organs.
Preventing muscles from acting
“Nerve cells" process and conduct the multitude of signals responsible for regulation of
many body functions. The brain, spinal cord, and sensory/motor organs in the body function
together to allow it to sense, move, respond, think, and remember. Nerve cells communicate to
each other by acting as "transducers:" creating electrical signals (very small voltages and
currents).
Prevents muscles from acting
If electric current of sufficient magnitude is conducted through a human body, its effect
will be to override the tiny electrical impulses normally generated by the nerve cells, overloading
the nervous system and preventing both reflex and volitional signals from being able to actuate
muscles. Muscles triggered by an external current will involuntarily contract, Medically, this
condition of involuntary muscle contraction is called tetanus.
Temporary immobilization
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Even when the current is stopped, the victim may not regain voluntary control over their
muscles for a while, as the neurotransmitter chemistry has been thrown into disarray. A wellplaced shock has the effect of temporarily (a few minutes) immobilizing the victim.
Effect of current on heart
The diaphragm muscle controlling the lungs and the heart (which is a muscle in itself)
can also be "frozen" in a state of tetanus by electric current. Currents are able to scramble nerve
cell signals so that the heart cannot beat properly, sending the heart into a condition known as
fibrillation.
Fibrillating heart
A fibrillating heart flutters rather than beats, and is ineffective at pumping blood to vital
organs in the body. In any case, death from asphyxiation and/or cardiac arrest will surely result
from a strong enough electric current through the body.
Shock current path
Without two contact points on the body for current to enter and exit, respectively, there is
no hazard of shock. Body resistance varies depending on how contact is made with the skin: is it
from hand-to-hand, hand-to-foot, foot-to-foot, hand-to-elbow, etc.
Decrease in body resistance
Sweat, being rich in salts and minerals, is an excellent conductor of electricity for being a
liquid. So is blood, with its similarly high content of conductive chemicals. Thus, contact with a
wire made by a sweaty hand or open wound will offer much less resistance to current than contact
made by clean, dry skin.
Effect of current 60 Hz A.C.
• Slight sensation felt at hand – 0.4 mA
• Threshold of perception – 1.1 mA
• Painful, but voluntary muscle control maintained – 9 mA
• Painful, unable to let go wires – 16 mA
• Severe pain, difficulty in breathing – 23 mA
• Possible heart fibrillation after 3 sec. – 100 mA
Resistance of human body
• Clean dry skin – 1MΏ
• Water on fingers or with sweat – 17 kΏ
• With ring on a finger – 1 kΏ
Current to induce tetanus
• Size of shock current is related to applied voltage and body resistance.
• For 16 mA to induce tetanus
E=IR
• Dry skin: E=(16mA)(1MΏ)=16 kV
• Wet hand: E=(16mA)(17kΏ)=272 V
• With gold ring: E=(16mA)(1kΏ)=16 V
Heart fibrillation
For 100 mA to induce heart fibrillation
E=IR
• Dry skin: E=(100mA)(1MΏ)=100 kV
• Wet hand: E=(100mA)(17kΏ)=1700 V
• With gold ring: E=(100mA)(1kΏ)=100 V
Portable tools and hand lamps
In marine industry, voltage of 60 V and below is considered safe for portable hand. Step
down transformers are used with portable tools and hand lamps. It is still an excellent idea to keep
one's hands clean and dry, and remove all metal jewelry when working around electricity.
Metal jewelry
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Even around lower voltages, metal jewelry can present a hazard by conducting enough
current to burn the skin if brought into contact between two points in a circuit. Metal rings,
especially, have been the cause of more than a few burnt fingers by bridging between points in a
low-voltage, high-current circuit.
Hand to hand shock current
The path current takes through the human body makes a difference as to how harmful it is.
Current will affect whatever muscles are in its path, and since the heart and lung (diaphragm)
muscles are probably the most critical to one's survival, shock paths traversing the chest are the
most dangerous. This makes the hand-to-hand shock current path a very likely mode of injury and
fatality.
Work with right hand
To guard against such an occurrence, it is advisable to only use only one hand to work on live
circuits. Of course, it is always safer to work on a circuit when it is unpowered. For one-handed
work, the right hand is generally preferred over the left for two reasons:
1. most people are right-handed (thus granting additional coordination when working),
2. and the heart is usually situated to the left of center in the chest cavity.
Add resistance
The best protection against shock from a live circuit is resistance, and resistance can be
added to the body through the use of insulated tools, gloves, boots, and other gear. Current in a
circuit is a function of available voltage divided by the total resistance in the path of the flow.
Emergency response
If you see someone lying unconscious or "froze on the circuit," the very first thing to do
is shut off the power by opening the appropriate circuit breaker. If you do not know the
appropriate circuit breaker, then trip the generator from main switchboard. Remember life is very
precious.
Don’t be a dead hero
If someone touches another person being shocked, there may be enough voltage dropped
across the body of the victim to shock the would-be rescuer, thereby "freezing" two people
instead of one. Don't be a hero. Electrons don't respect heroism. Make sure the situation is safe
for you to step into, or else you will be the next victim, and nobody will benefit from your efforts.
Cardiopulmonary resuscitation (CPR)
Once the victim has been safely disconnected from the source of electric power, the
immediate medical concerns for the victim should be respiration and circulation (breathing and
pulse). You should follow the appropriate steps of checking for breathing and pulse, then
applying CPR as necessary to keep the victim's body from deoxygenating. The cardinal rule of
CPR is to keep going until you have been relieved by qualified personnel.
Physiological shock
There is the possibility of the victim going into a state of physiological shock (a condition
of insufficient blood circulation) so he should be kept as warm and comfortable as possible. An
electrical shock insufficient to cause immediate interruption of the heartbeat may be strong
enough to cause heart irregularities or a heart attack up to several hours later, so the victim should
pay close attention to their own condition after the incident.
Safe Practices
If possible, shut off the power to the circuit before performing any work on it. Some basic
safety considerations are given below:
1. Isolate all equipment and apparatus from the supply before carrying out any work on
circuits or equipment in them.
2. Apply tests (using lamps or other indicating devices) to ensure that a circuit is ‘dead’
before working on it.
3. Wear protective clothing where you are required to do so
4. Use the correct tools and equipment
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5. Double-check all apparatus and the electrical supply before switching on
6. When current flow in a highly inductive circuit is suddenly interrupted, the collapsing
magnetic field will induce a large current flow: also, if a circuit containing capacitors
has been isolated, considerable electric charge may be stored in the system.
7. Check all apparatus to ensure that there is no danger from faulty or poor connections
or damaged insulation.
8. Isolate equipment or apparatus from the supply if there are indications of overheating
or sparking.
Action after incident
1. Isolate the supply from the equipment and the person
2. No one approaches the person without isolating the supply to the equipment.
3. A casualty will have suffered burns and shock and must be treated according to the
degree of damage sustained – even to applying artificial respiration or chest compression
in serious incidents.
Safety Requirements of Electrical equipments
The ship is designed and built and the operating staff is to maintain the vessel and its
electrical installation to their requirements of following throughout the ship’s lifetime.
1. Various national organisations
2. International Maritime Organization (IMO)
3. Classification Societies
4. Various international organisations
1. Various National Organisations
For Indian registered ships, it is necessary to comply, with:
• The Merchant Shipping Act, 1958
• Bureau of Indian Standards (BIS)
• Institute of Electrical Engineers (IEE)
2. International Maritime Organization (IMO)
The International Maritime Organization (IMO) is a specialised agency of the United
Nations devoted to maritime affairs. Of all the international conventions made by IMO, the most
important is the International Convention for the Safety of Life at Sea,1974 (SOLAS 1974),
which covers a wide range of measures designed to improve the safety of shipping.
SOLAS 1974
This Convention has twelve chapters. Electrical regulations are part of Chapter II-1, which
outlines the requirements for Ship construction - sub-division and stability, machinery and
electrical installations. This Chapter has five Parts as follows:
• Part A - General
• Part B - Sub-division and stability
• Part C - Machinery installations
• Part D - Electrical installations
• Part E - Additional requirements for periodically unattended machinery spaces
Electrical Installations
The electrical installations (Part D) is sub-divided into regulations as:
• Regulation 40 - General
• Regulation 41 - Main source of electrical power and lighting systems
• Regulation 42 - Emergency source of electrical power in passenger ships
• Regulation 43 - Emergency source of electrical power in cargo ships
• Regulation 44 - Starting arrangements for emergency generator sets
• Regulation 45 - Precautions against shock, fire and other hazards of electrical origin
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Regulation 40
General
1. Electrical installations shall be such that:
1.1 All electrical auxiliary services necessary for maintaining the ship in normal
operational and habitable conditions will be ensured without recourse to the
emergency source of electrical power
1.2 Electrical services essential for safety will be ensured under various emergency
conditions, and
1.3 The safety of passengers, crew and ship from electrical hazards will be ensured.
2. The Administration shall take appropriate steps to ensure uniformity
in the implementation and application of the provisions of this part in
respect of electrical installations.
Regulation 41
Main source of electrical power and lighting systems
1.1 A main source of electrical power of sufficient capacity to supply all those services
mentioned in regulation 40.1.1 shall be provided. This main source of electrical power
shall consist of at least two generating sets.
1.2 The capacity of these generating sets shall be such that in the event of any one
generating set being stopped it will still be possible to supply those services necessary to
provide normal operational conditions of propulsion and safety. Minimum comfortable
conditions of habitability shall also be ensured which include at least adequate services
for cooking, heating, domestic refrigeration, mechanical ventilation, sanitary and fresh
water.
1.3 The arrangements of the ship’s main source of electrical power shall be such that the
services referred to in regulation 40.1.1 can be maintained regardless of the speed and
direction of rotation of the propulsion machinery or shafting.
1.4 In addition, the generating sets shall be such as to ensure that with any one generator or
its primary source of power out of operation, the remaining generating sets shall be
capable of providing the electrical services necessary to start the main propulsion plant
from a dead ship condition. The emergency source of electrical power may be used for
the purpose of starting from a dead ship condition if its capability either alone or
combined with that of any other source of electrical power is sufficient to provide at the
same time those services required to be supplied by regulations 42.2.1 to 42.2.3 or
43.2.1 to 43.2.4.
1.5 Where transformers constitute an essential part of the electrical supply system required
by this paragraph, the system shall be so arranged as to ensure the same continuity of
the supply as is stated in this paragraph.
2.1
A main electric lighting system which shall provide illumination throughout those parts of
the
Ship normally accessible to and used by passengers or crew shall be supplied from the
main
source of electrical power.
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2.2
The arrangement of the main electric lighting system shall be such that a fire or other
casualty
. in spaces containing the main source of electrical power,
associated transforming equipment,
if any, the main switchboard and the main lighting switchboard, will not render the
emergency
electric lighting system required by regulations 42.2.1 and 42.2.2 or 43.2.1, 43.2.2
and 43.2.3 inoperative
2.3
The arrangement of the emergency electric lighting system shall be such that a fire or
other
casualty in spaces containing the emergency source of electrical power,
associated transforming equipment, if any, the emergency switchboard and the emergency
lighting switchboard will not
render the main electric lighting system required by this regulation inoperative.
3.
The main switchboard shall be so placed relative to one main generating station that, as far
as is practicable, the integrity of the normal electrical supply may be affected only by a fire or
other casualty in one space. An environmental enclosure for the main switchboard, such as
may be provided by a machinery control room situated within the main boundaries of the
space, is not to be considered as separating the switchboards from the generators.
4.
Where the total installed electrical power of the main generating sets is in excess of 3 MW,
the main busbars shall be subdivided into at least two parts which shall normally be
connected by removable links or other approved means; so far as is practicable, the
connection of generating sets and any other duplicated equipment shall be equally divided
between the parts. Equivalent arrangements may be permitted to the satisfaction of the
Administration.
5.
Ships constructed on or after 1 July 1998:
.1 in addition to paragraphs 1 to 3, shall comply with the following:
.1.1 where the main source of electrical power is necessary for propulsion and steering of
the ship, the system shall be so arranged that the electrical supply to equipment necessary
for propulsion and steering and to ensure safety of the ship will be maintained or
immediately restored in the case of loss of any one of the generators in service;
.1.2 load shedding or other equivalent arrangements shall be provided to protect the
generators required by this regulation against sustained overload;
.1.3 where the main source of electrical power is necessary for propulsion of the ship, the
main busbar shall be subdivided into at least two parts which shall normally be connected by
circuit breakers or other approved means; so far as is practicable, the connection of
generating sets and other duplicated equipment shall be equally divided between the parts;
and
.2 need not comply with paragraph 4.
Regulation 42
Emergency source of electrical power in passenger ships
1.1 A self-contained emergency source of electrical power shall be provided.
1.2 The emergency source of electrical power, associated transforming equipment, if any,
transitional source of emergency power, emergency switchboard and emergency lighting
switchboard shall be located above the uppermost continuous deck and shall be readily
accessible from the open deck. They shall not be located forward of the collision bulkhead.
1.3 The location of the emergency source of electrical power and associated transforming
equipment, if any, the transitional source of emergency power, the emergency switchboard
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and the emergency electric lighting switchboards in relation to the main source of electrical
power, associated transforming equipment, if any, and the main switchboard shall be such
as to ensure to the satisfaction of the Administration that a fire or other casualty in spaces
containing the main source of electrical power, associated transforming equipment, if any,
and the main switchboard or in any machinery space of category A will not interfere with the
supply, control and distribution of emergency electrical power. As far as practicable, the
space containing the emergency source of electrical power, associated transforming
equipment, if any, the transitional source of emergency electrical power and the emergency
switchboard shall not be contiguous to the boundaries of machinery spaces of category A or
those spaces containing the main source of electrical power, associated transforming
equipment, if any, or the main switchboard. be such as to ensure to the satisfaction of the
Administration that a fire or other casualty in spaces containing the main source of electrical
power,
associated transforming equipment, if any, and the main switchboard or in
any machinery space of category A will not interfere with the supply,
control and distribution of emergency electrical power. As far as practicable,
the space containing the emergency source of electrical power, associated
transforming equipment, if any, the transitional source of emergency
electrical power and the emergency switchboard shall not be contiguous to
the boundaries of machinery spaces of category A or those spaces containing
the main source of electrical power, associated transforming equipment, if
any, or the main switchboard.
1.4 Provided that suitable measures are taken for safeguarding independent
emergency operation under all circumstances, the emergency
generator may be used exceptionally, and for short periods, to supply
non-emergency circuits.
The electrical power available shall be sufficient to supply all those
services that are essential for safety in an emergency, due regard being paid
to such services as may have to be operated simultaneously. The emergency
source of electrical power shall be capable, having regard to starting currents
and the transitory nature of certain loads, of supplying simultaneously at
least the following services for the periods specified hereinafter, if they
depend upon an electrical source for their operation:
For a period of 36 h, emergency lighting:
.1
at every muster and embarkation station and over the sides as
required by regulations III/11.4 and III/16.7;
.2
in alleyways, stairways and exits giving access to the muster and
embarkation stations, as required by regulation III/11.5;
3
in all service and accommodation alleyways, stairways and exits,
personnel lift cars;
.4
in the machinery spaces and main generating stations including
their control positions;
.5
in all control stations, machinery control rooms, and at each
main and emergency switchboard;
.6
at all stowage positions for firemen’s outfits;
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.7
at the steering gear; and
.8
at the fire pump, the sprinkler pump and the emergency bilge
pump referred to in paragraph 2.4 and at the starting position of
their motors.
For a period of 36 h:
.1
the navigation lights and other lights required by the International
Regulations for Preventing Collisions at Sea in force; and
.2
on ships constructed on or after 1 February 1995, the VHF radio
installation required by regulation IV/7.1.1 and IV/7.1.2; and, if
applicable:
.2.1 the MF radio installation required by regulations IV/9.1.1,
IV/9.1.2, IV/10.1.2 and IV/10.1.3;
.2.2 the ship earth station required by regulation IV/10.1.1; and
.2.3 the MF/HF radio installation required by regulations IV/
10.2.1, IV/10.2.2 and IV/11.1.
For a period of 36 h:
.1 all internal communication equipment required in an emergency;
.2 the shipborne navigational equipment as required by regulation
V/12*; where such provision is unreasonable or impracticable
the Administration may waive this requirement for ships of less
than 5,000 gross tonnage;
.3
the fire detection and fire alarm system, and the fire door
holding and release system; and
.4
for intermittent operation of the daylight signalling lamp, the
ship’s whistle, the manually operated call points, and all internal
signals that are required in an emergency;
unless such services have an independent supply for the period of 36 h from
an accumulator battery suitably located for use in an emergency.
For a period of 36 h:
.1 one of the fire pumps required by regulation II-2/4.3.1 and
4.3.3;{
.2
the automatic sprinkler pump, if any; and
.3
the emergency bilge pump and all the equipment essential for the
operation of electrically powered remote controlled bilge valves.
2.5 For the period of time required by regulation 29.14 the steering gear if
required to be so supplied by that regulation.
2.6 For a period of half an hour:
.1
any watertight doors required by regulation 15 to be poweroperated
together with their indicators and warning signals;
Part D: Electrical installations Regulation 42
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This relates to the chapter V in force before 1 July 2002. The equivalent in the amended
chapter V is regulation 19.
{ These relate to the chapter II-2 in force before 1 July 2002. The equivalents in the amended
chapter II-2 are 10.2.2.2 and 10.2.2.3.
.2
the emergency arrangements to bring the lift cars to deck level
for the escape of persons. The passenger lift cars may be brought
to deck level sequentially in an emergency.
2.7 In a ship engaged regularly on voyages of short duration, the
Administration if satisfied that an adequate standard of safety would be
attained may accept a lesser period than the 36 h period specified in
paragraphs 2.1 to 2.5 but not less than 12 h.
3 The emergency source of electrical power may be either a generator
or an accumulator battery, which shall comply with the following:
3.1 Where the emergency source of electrical power is a generator, it shall
be:
.1 driven by a suitable prime mover with an independent supply of
fuel having a flashpoint (closed cup test) of not less than 438C;
.2 started automatically upon failure of the electrical supply from
the main source of electrical power and shall be automatically
connected to the emergency switchboard; those services referred
to in paragraph 4 shall then be transferred automatically to the
emergency generating set. The automatic starting system and
the characteristic of the prime mover shall be such as to permit
the emergency generator to carry its full rated load as quickly as
is safe and practicable, subject to a maximum of 45 s; unless a
second independent means of starting the emergency generating
set is provided, the single source of stored energy shall be
protected to preclude its complete depletion by the automatic
starting system; and
.3 provided with a transitional source of emergency electrical
power according to paragraph 4.
3.2 Where the emergency source of electrical power is an accumulator
battery, it shall be capable of:
.1 carrying the emergency electrical load without recharging while
maintaining the voltage of the battery throughout the discharge
period within 12% above or below its nominal voltage;
.2 automatically connecting to the emergency switchboard in the
event of failure of the main source of electrical power; and
.3 immediately supplying at least those services specified in
paragraph 4.
3.3 The following provisions in paragraph 3.1.2 shall not apply to ships
constructed on or after 1 October 1994:
Unless a second independent means of starting the emergency
generating set is provided, the single source of stored energy shall
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Chapter II-1: Construction – structure, stability, installations
be protected to preclude its complete depletion by the automatic
starting system.
3.4 For ships constructed on or after 1 July 1998, where electrical power
is necessary to restore propulsion, the capacity shall be sufficient to restore
propulsion to the ship in conjunction with other machinery, as appropriate,
from a dead ship condition within 30 min after blackout.
4 The transitional source of emergency electrical power required by
paragraph 3.1.3 shall consist of an accumulator battery suitably located for
use in an emergency which shall operate without recharging while
maintaining the voltage of the battery throughout the discharge period
within 12% above or below its nominal voltage and be of sufficient capacity
and so arranged as to supply automatically in the event of failure of either
the main or emergency source of electrical power at least the following
services, if they depend upon an electrical source for their operation:
4. 1 For half an hour:
1 the lighting required by paragraphs 2.1 and 2.2;
2 all services required by paragraphs 2.3.1, 2.3.3 and 2.3.4 unless
such services have an independent supply for the period
specified from an accumulator battery suitably located for use
in an emergency.
4.2 Power to operate the watertight doors, as required by regulation
15.7.3.3, but not necessarily all of them simultaneously, unless an
independent temporary source of stored energy is provided. Power to the
control, indication and alarm circuits as required by regulation 15.7.2 for
half an hour.
5.1 The emergency switchboard shall be installed as near as is practicable
to the emergency source of electrical power.
5.2 Where the emergency source of electrical power is a generator, the
emergency switchboard shall be located in the same space unless the
operation of the emergency switchboard would thereby be impaired.
5.3 No accumulator battery fitted in accordance with this regulation shall
be installed in the same space as the emergency switchboard. An indicator
shall be mounted in a suitable place on the main switchboard or in the
machinery control room to indicate when the batteries constituting either
the emergency source of electrical power or the transitional source of
emergency electrical power referred to in paragraph 3.1.3 or 4 are being
discharged.
5.4 The emergency switchboard shall be supplied during normal
operation from the main switchboard by an interconnector feeder which
is to be adequately protected at the main switchboard against overload and
short circuit and which is to be disconnected automatically at the
Part D: Electrical installations Regulation 4 129
emergency switchboard upon failure of the main source of electrical power.
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Where the system is arranged for feedback operation, the interconnector
feeder is also to be protected at the emergency switchboard at least against
short circuit.
5.5 In order to ensure ready availability of the emergency source of
electrical power, arrangements shall be made where necessary to disconnect
automatically non-emergency circuits from the emergency switchboard to
ensure that power shall be available to the emergency circuits.
6 The emergency generator and its prime mover and any emergency
accumulator battery shall be so designed and arranged as to ensure that they
will function at full rated power when the ship is upright and when inclined
at any angle of list up to 22.58 or when inclined up to 108 either in the fore
or aft direction, or is in any combination of angles within those limits.
7 Provision shall be made for the periodic testing of the complete
emergency system and shall include the testing of automatic starting
arrangements.
Regulation 42-1
Supplementary emergency lighting for ro–ro passenger ships
the approach to the means of escape can be readily seen.
The source of power for the supplementary lighting shall consist
of accumulator batteries located within the lighting units that are
continuously charged, where practicable, from the emergency
switchboard. Alternatively, any other means of lighting which is
at least as effective may be accepted by the Administration. The
supplementary lighting shall be such that any failure of the lamp
will be immediately apparent. Any accumulator battery provided
shall be replaced at intervals having regard to the specified
service life in the ambient conditions that they are subject to in
service; and 130
Chapter II-1: Construction – structure, stability, installations
.2 a portable rechargeable battery operated lamp shall be provided in
every crew space alleyway, recreational space and every working
space which is normally occupied unless supplementary emergency
lighting, as required by subparagraph .1, is provided.
Regulation 43
Emergency source of electrical power in cargo ships
1.1 A self-contained emergency source of electrical power shall be
provided.
1.2 The emergency source of electrical power, associated transforming
equipment, if any, transitional source of emergency power, emergency
switchboard and emergency lighting switchboard shall be located above the
uppermost continuous deck and shall be readily accessible from the open
deck. They shall not be located forward of the collision bulkhead, except
where permitted by the Administration in exceptional circumstances.
1.3 The location of the emergency source of electrical power, associated
transforming equipment, if any, the transitional source of emergency power,
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the emergency switchboard and the emergency lighting switchboard in
relation to the main source of electrical power, associated transforming
equipment, if any, and the main switchboard shall be such as to ensure to
the satisfaction of the Administration that a fire or other casualty in the space
containing the main source of electrical power, associated transforming
equipment, if any, and the main switchboard, or in any machinery space of
category A will not interfere with the supply, control and distribution of
emergency electrical power. As far as practicable the space containing the
emergency source of electrical power, associated transforming equipment, if
any, the transitional source of emergency electrical power and the
emergency switchboard shall not be contiguous to the boundaries of
machinery spaces of category A or those spaces containing the main source
of electrical power, associated transforming equipment, if any, and the main
switchboard.
1.4 Provided that suitable measures are taken for safeguarding independent
emergency operation under all circumstances, the emergency
generator may be used, exceptionally, and for short periods, to supply
non-emergency circuits
2
The electrical power available shall be sufficient to supply all those
services that are essential for safety in an emergency, due regard being paid
to such services as may have to be operated simultaneously. The emergency
source of electrical power shall be capable, having regard to starting currents
and the transitory nature of certain loads, of supplying simultaneously at
least the following services for the periods specified hereinafter, if they
depend upon an electrical source for their operation:
Part D: Electrical installations Regulation 42-1, 43
2.1 For a period of 3 h, emergency lighting at every muster and
embarkation station and over the sides as required by regulations III/11.4
and III/16.7.
2.2 For a period of 18 h, emergency lighting:
1 in all service and accommodation alleyways, stairways and exits,
personnel lift cars and personnel lift trunks;
2 in the machinery spaces and main generating stations including
their control positions;
.3 in all control stations, machinery control rooms, and at each
main and emergency switchboard;
.4 at all stowage positions for firemen’s outfits;
.5 at the steering gear;
.6 at the fire pump referred to in paragraph 2.5, at the sprinkler
pump, if any, and at the emergency bilge pump, if any, and at
the starting positions of their motors; and
7 in all cargo pump-rooms of tankers constructed on or after 1
July 2002.
2.3 For a period of 18 h:
1 the navigation lights and other lights required by the International
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Regulations for Preventing Collisions at Sea in force;
.2 on ships constructed on or after 1 February 1995 the VHF radio
installation required by regulation IV/7.1.1 and IV/7.1.2; and, if
applicable:
.2.1 the MF radio installation required by regulations IV/9.1.1,
IV/9.1.2, IV/10.1.2 and IV/10.1.3;
.2.2 the ship earth station required by regulation IV/10.1.1; and
.2.3 the MF/HF radio installation required by regulations IV/10.2.1,
IV/10.2.2 and IV/11.1.
2.4 For a period of 18 h:
.1 all internal communication equipment as required in an
emergency;
.2 the shipborne navigational equipment as required by
regulation V/12;* where such provision is unreasonable or
impracticable the Administration may waive this requirement
for ships of less than 5,000 gross tonnage;
.3 the fire detection and fire alarm system; and
.4 intermittent operation of the daylight signalling lamp, the ship’s
whistle, the manually operated call points and all internal signals
that are required in an emergency;
This relates to the chapter V in force before 1 July 2002. The equivalent in the amended
chapter V is regulation 19.
Chapter II-1: Construction – structure, stability, installations
unless such services have an independent supply for the period of 18 h from
an accumulator battery suitably located for use in an emergency.
2.5 For a period of 18 h one of the fire pumps required by regulation
II-2/4.3.1 and 4.3.3* if dependent upon the emergency generator for its
source of power.
2.6.1 For the period of time required by regulation 29.14 the steering gear
where it is required to be so supplied by that regulation.
2.6.2 In a ship engaged regularly in voyages of short duration, the
Administration if satisfied that an adequate standard of safety would be
attained may accept a lesser period than the 18 h period specified in
paragraphs 2.2 to 2.5 but not less than 12 h.
3
The emergency source of electrical power may be either a generator
or an accumulator battery, which shall comply with the following:
3.1 Where the emergency source of electrical power is a generator, it shall be:
.1 driven by a suitable prime mover with an independent supply of
fuel, having a flashpoint (closed cup test) of not less than 438C;
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.2 started automatically upon failure of the main source of electrical
power supply unless a transitional source of emergency electrical
power in accordance with paragraph
3.1.3 is provided; where the
emergency generator is automatically started, it shall be automatically
connected to the emergency switchboard; those services
referred to in paragraph
4 shall then be connected automatically to
the emergency generator; and unless a second independent means
of starting the emergency generator is provided the single source
of stored energy shall be protected to preclude its complete
depletion by the automatic starting system; and
.3 provided with a transitional source of emergency electrical
power as specified in paragraph 4 unless an emergency generator
is provided capable both of supplying the services mentioned in
that paragraph and of being automatically started and supplying
the required load as quickly as is safe and practicable subject to a
maximum of 45 s.
3.2 Where the emergency source of electrical power is an accumulator
battery it shall be capable of:
.1 carrying the emergency electrical load without recharging while
maintaining the voltage of the battery throughout the discharge
period within 12% above or below its nominal voltage;
Part D: Electrical installations Regulation 43
These relate to the chapter II-2 in force before 1 July 2002. The equivalents in the amended
chapter II-2 are 10.2.2.2 and 10.2.2.3.
133
.2 automatically connecting to the emergency switchboard in the
event of failure of the main source of electrical power; and
.3 immediately supplying at least those services specified in
paragraph 4.
3.3 The following provision in paragraph 3.1.2 shall not apply to ships
constructed on or after 1 October 1994:
Unless a second independent means of starting the emergency
generating set is provided, the single source of stored energy shall
be protected to preclude its complete depletion by the automatic
starting system.
3.4 For ships constructed on or after 1 July 1998, where electrical power
is necessary to restore propulsion, the capacity shall be sufficient to restore
propulsion to the ship in conjunction with other machinery, as appropriate,
from a dead ship condition within 30 min after blackout.
4
The transitional source of emergency electrical power where required
by paragraph 3.1.3 shall consist of an accumulator battery suitably located
for use in an emergency which shall operate without recharging while
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maintaining the voltage of the battery throughout the discharge period
within 12% above or below its nominal voltage and be of sufficient capacity
and shall be so arranged as to supply automatically in the event of failure of
either the main or the emergency source of electrical power for half an hour
at least the following services if they depend upon an electrical source for
their operation:
1
the lighting required by paragraphs 2.1, 2.2 and 2.3.1. For this
transitional phase, the required emergency electric lighting, in
respect of the machinery space and accommodation and service
spaces may be provided by permanently fixed, individual,
automatically charged, relay operated accumulator lamps; and
2 all services required by paragraphs 2.4.1, 2.4.3 and 2.4.4 unless
such services have an independent supply for the period
specified from an accumulator battery suitably located for use
in an emergency.
5.1 The emergency switchboard shall be installed as near as is practicable
to the emergency source of electrical power.
5.2 Where the emergency source of electrical power is a generator, the
emergency switchboard shall be located in the same space unless the
operation of the emergency switchboard would thereby be impaired.
5.3 No accumulator battery fitted in accordance with this regulation shall
be installed in the same space as the emergency switchboard. An indicator
shall be mounted in a suitable place on the main switchboard or in the
machinery control room to indicate when the batteries constituting either
the emergency source of electrical power or the transitional source of
electrical power referred to in paragraph 3.2 or 4 are being discharged.
134
Chapter II-1: Construction – structure, stability, installations
5.4 The emergency switchboard shall be supplied during normal
operation from the main switchboard by an interconnector feeder which
is to be adequately protected at the main switchboard against overload and
short circuit and which is to be disconnected automatically at the
emergency switchboard upon failure of the main source of electrical power.
Where the system is arranged for feedback operation, the interconnector
feeder is also to be protected at the emergency switchboard at least against
short circuit.
5.5 In order to ensure ready availability of the emergency source of
electrical power, arrangements shall be made where necessary to disconnect
automatically non-emergency circuits from the emergency switchboard to
ensure that electrical power shall be available automatically to the
emergency circuits.
6 The emergency generator and its prime mover and any emergency
accumulator battery shall be so designed and arranged as to ensure that they
will function at full rated power when the ship is upright and when inclined
at any angle of list up to 22.58 or when inclined up to 108 either in the fore
or aft direction, or is in any combination of angles within those limits.
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7 Provision shall be made for the periodic testing of the complete
emergency system and shall include the testing of automatic starting
arrangements.
Regulation 44
Starting arrangements for emergency generating sets
1
Emergency generating sets shall be capable of being readily started in
their cold condition at a temperature of 08C. If this is impracticable, or if
lower temperatures are likely to be encountered, provision acceptable to the
Administration shall be made for the maintenance of heating arrangements,
to ensure ready starting of the generating sets.
2 Each emergency generating set arranged to be automatically started
shall be equipped with starting devices approved by the Administration with
a stored energy capability of at least three consecutive starts. A second
source of energy shall be provided for an additional three starts within
30 min unless manual starting can be demonstrated to be effective.
2.1 Ships constructed on or after 1 October 1994, in lieu of the provision
of the second sentence of paragraph 2, shall comply with the following
requirements:
The source of stored energy shall be protected to preclude critical
depletion by the automatic starting system, unless a second
independent means of starting is provided. In addition, a second
Part D: Electrical installations Regulation 44
135 source of energy shall be provided for an additional three starts within
30 in unless manual starting can be demonstrated to be effective.
2.
.1
The stored energy shall be maintained at all times, as follows:
electrical and hydraulic starting systems shall be maintained from
the emergency switchboard;
.2
compressed air starting systems may be maintained by the main
or auxiliary compressed air receivers through a suitable nonreturn
valve or by an emergency air compressor which, if
electrically driven, is supplied from the emergency switchboard;
.3
all of these starting, charging and energy storing devices shall be
located in the emergency generator space; these devices are not
to be used for any purpose other than the operation of the
emergency generating set. This does not preclude the supply to
the air receiver of the emergency generating set from the main
or auxiliary compressed air system through the non-return valve
fitted in the emergency generator space.
4.1 Where automatic starting is not required, manual starting is
permissible, such as manual cranking, inertia starters, manually charged
hydraulic accumulators, or powder charge cartridges, where they can be
demonstrated as being effective.
4.2 When manual starting is not practicable, the requirements of
paragraphs 2 and 3 shall be complied with except that starting may be
manually initiated.
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Regulation 45
Precautions against shock, fire and other hazards of electrical origin
1.1 Exposed metal parts of electrical machines or equipment which are
not intended to be live but which are liable under fault conditions to
become live shall be earthed unless the machines or equipment are:
.1
supplied at a voltage not exceeding 50 V direct current or 50 V
root mean square between conductors; auto-transformers shall
not be used for the purpose of achieving this voltage; or
.2
supplied at a voltage not exceeding 250 V by safety isolating
transformers supplying only one consuming device; or
.3
constructed in accordance with the principle of double
insulation.
1.2 The Administration may require additional precautions for portable
electrical equipment for use in confined or exceptionally damp spaces where
particular risks due to conductivity may exist.
1.3 All electrical apparatus shall be so constructed and so installed as not to
cause injury when handled or touched in the normal manner.
2
Main and emergency switchboards shall be so arranged as to give easy
access as may be needed to apparatus and equipment, without danger to
personnel. The sides and the rear and, where necessary, the front of
switchboards shall be suitably guarded. Exposed live parts having voltages to
earth exceeding a voltage to be specified by the Administration shall not be
installed on the front of such switchboards. Where necessary, nonconducting
mats or gratings shall be provided at the front and rear of the
switchboard.
3.1 The hull return system of distribution shall not be used for any
purpose in a tanker, or for power, heating, or lighting in any other ship of
1,600 gross tonnage and upwards.
3.2 The requirement of paragraph 3.1 does not preclude under conditions
approved by the Administration the use of:
1
2
impressed current cathodic protective systems;
limited and locally earthed systems; or
.3 insulation level monitoring devices provided the circulation
current does not exceed 30 mA under the most unfavourable
conditions.
3.2–1 For ships constructed on or after 1 October 1994, the requirement of
paragraph 3.1 does not preclude the use of limited and locally earthed
systems, provided that any possible resulting current does not flow directly
through any dangerous spaces.
3.3
Where the hull return system is used, all final subcircuits, i.e. all
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circuits fitted after the last protective device, shall be two-wire and special
precautions shall be taken to the satisfaction of the Administration.
4.1
Earthed distribution systems shall not be used in a tanker. The
Administration may exceptionally permit in a tanker the earthing of the
neutral for alternating current power networks of 3,000 V (line to line) and
over, provided that any possible resulting current does not flow directly
through any of the dangerous spaces.
4.2 When a distribution system, whether primary or secondary, for
power, heating or lighting, with no connection to earth is used, a device
capable of continuously monitoring the insulation level to earth and of
giving an audible or visual indication of abnormally low insulation values
shall be provided.
Part D: Electrical installations Regulation 45
4.3 Ships constructed on or after 1 October 1994, in lieu of the provisions
of paragraph 4.1, shall comply with the following requirements:
1 Except as permitted by paragraph 4.3.2, earthed distribution
systems shall not be used in a tanker.
2
The requirement of paragraph 4.3.1 does not preclude the use of
earthed intrinsically safe circuits and in addition, under
conditions approved by the Administration, the use of the
following earthed systems:
2.1 power-supplied control circuits and instrumentation circuits
where technical or safety reasons preclude the use of a system
with no connection to earth, provided the current in the hull is
limited to not more than 5 A in both normal and fault
conditions; or
2.2 limited and locally earthed systems, provided that any possible
resulting current does not flow directly through any of the
dangerous spaces; or
2.3 alternating current power networks of 1,000 V root mean
square (line to line) and over, provided that any possible
resulting current does not flow directly through any of the
dangerous spaces.
5.1 Except as permitted by the Administration in exceptional circumstances,
all metal sheaths and armour of cables shall be electrically
continuous and shall be earthed.
5.2 All electric cables and wiring external to equipment shall be at least of
a flame-retardant type and shall be so installed as not to impair their original
flame-retarding properties. Where necessary for particular applications the
Administration may permit the use of special types of cables such as radio
frequency cables, which do not comply with the foregoing.
5.3
Cables and wiring serving essential or emergency power, lighting,
internal communications or signals shall so far as practicable be routed clear
of galleys, laundries, machinery spaces of category A and their casings and
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other high fire risk areas. In ro–ro passenger ships, cabling for emergency
alarms and public address systems installed on or after 1 July 1998 shall be
approved by the Administration having regard to the recommendations
developed by the Organization.* Cables connecting fire pumps to the
emergency switchboard shall be of a fire-resistant type where they pass
through high fire risk areas. Where practicable all such cables should be run
in such a manner as to preclude their being rendered unserviceable by
heating of the bulkheads that may be caused by a fire in an adjacent space.
heating of the bulkheads that may be caused by a fire in an adjacent space.
* Refer
to MSC/Circ.808, Recommendation on performance standards for public address
systems on passenger ships, including cabling.
5.4 Where cables which are installed in hazardous areas introduce the risk
of fire or explosion in the event of an electrical fault in such areas, special
precautions against such risks shall be taken to the satisfaction of the
Administration.
5.5 Cables and wiring shall be installed and supported in such a manner as
to avoid chafing or other damage.
5.6 Terminations and joints in all conductors shall be so made as to retain
the original electrical, mechanical, flame-retarding and, where necessary,
fire-resisting properties of the cable.
6.1 Each separate circuit shall be protected against short circuit and against
overload, except as permitted in regulations 29 and 30 or where the
Administration may exceptionally otherwise permit.
6.2 The rating or appropriate setting of the overload protective device for
each circuit shall be permanently indicated at the location of the protective
device.
7
Lighting fittings shall be so arranged as to prevent temperature rises
which could damage the cables and wiring, and to prevent surrounding
material from becoming excessively hot.
8
All lighting and power circuits terminating in a bunker or cargo space
shall be provided with a multiple-pole switch outside the space for
disconnecting such circuits.
9.1 Accumulator batteries shall be suitably housed, and compartments
used primarily for their accommodation shall be properly constructed and
efficiently ventilated.
9.2 Electrical or other equipment which may constitute a source of
ignition of flammable vapours shall not be permitted in these compartments
except as permitted in paragraph 10.
9.3 Accumulator batteries shall not be located in sleeping quarters except
where hermetically sealed to the satisfaction of the Administration.
10 No electrical equipment shall be installed in any space where
flammable mixtures are liable to collect including those on board tankers
or in compartments assigned principally to accumulator batteries, in paint
lockers, acetylene stores or similar spaces, unless the Administration is
satisfied that such equipment is:
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1
essential for operational purposes;
2 of a type which will not ignite the mixture concerned;
3 appropriate to the space concerned; and
4 appropriately certified for safe usage in the dusts, vapours or
gases likely to be encountered.
11In a passenger ship, distribution systems shall be so arranged that fire in
any main vertical zone as is defined in regulation II-2/3.9* will not interfere
with services essential for safety in any other such zone. This requirement
will be met if main and emergency feeders passing through any such zone are
separated both vertically and horizontally as widely as is practicable.
3. Classification Societies
Electrical safety requirements and regulations adopted by various classifications societies
generally cover installation such as distribution systems, generators, MCB, motors and starters,
cables, batteries and other ancillary services.
Classification Regulation
1. Location
2. Construction
3. Earthing
Location
Area, where electrical equipments are installed, should be always well ventilated, easily
accessible, adequately lighted, and clear of any flammable material, away from steam, oil and
water pipes or otherwise properly protected from splashing of oil or water, and properly shielded
so as not to have any interference on magnetic compasses. If located in dangerous zones then the
equipment to be of safe type (flame proof or similar).
Construction
Generally main aspects of construction such as material, sharp corners on edges,
insulation grades, resistance to moisture, oil, vapour etc. should be taken care off at the stage of
manufacturing. However, one important aspect, that requires special attention, is tightening of
screws and nuts used in connection with current carrying parts and other working parts. If
neglected, it can lead to serious hazards. Creepage (distance between line metal) distances to be
adequate.
Earthing
It is essential that non-current carrying exposed metal parts of the equipment (including
portable) be effectively earthed. Protected against damage and electrolytic action exception - < 55
V. Earthing connections are of copper and
1. Equal to cross-section of cable conductors up to 16 mm2.
2. At least half the cross-section above 10 mm2 with minimum of 16 mm2
4. Various international organisations
It has also to follow ‘Safe Working Practices’ as laid down by international bodies or
institutes
• BS – British Standardization Institute
• IEC – International Electrotechnical Commission
• API – American Petroleum Institute or
• IP – Institute of Petroleum, UK
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Ship’s Electrical system
Auxiliary services on board ship range from engine room pumps, compressors and fans,
deck winches and windlasses, to general lighting, catering and air conditioning. Electrical power
is used to drive the majority of these auxiliary services. The electrical power system on board
ship is designed to provide a secure supply to all loads with adequate built-in protection for the
equipment and operating personnel. The general scheme of a ship’s electrical power system is
common to nearly all ships.
Alternators
The main a.c. generators (sometimes called alternators) produce the electrical power. It is
supplied to the main switchboard and then distributed to the various auxiliary services comprising
the electrical load.
Emergency generator
An emergency generator and emergency switchboard maintain supplies in the event of a
main power failure
Prime mover of alternators
The generators may be driven by a diesel engine, by a steam or gas turbine, or by the
main propulsion engine as a shaft generator. The type of prime mover is determined by the design
of the ships and by economic factors. The combined power rating of the generators is determined
by the overall demand of the ship’s electrical load.
Voltage and Frequency
Electrical power onboard a ship is commonly generated at 440V, 60Hz (sometimes 380V,
50Hz). These values have been adopted because they are standard shore supplies in the USA and
in Europe. A frequency of 60 Hz is generally a US standard and 50 Hz is a UK standard.
Size of Alternators
Large passenger ships usually have four large generators rated at 10 MW or more to
supply the electric propulsion motors and the extensive hotel services on board. A cargo ship may
have two/three main generators typically rated from 350 to 1000 kW which are sufficient to
supply the engine room auxiliaries while at sea and the winches or cranes for handling cargo
while in port.
Emergency generator
The limited load required during an emergency requires that an emergency generator may
be rated from about 10 kW for a small coastal ship to about 300 kW or more for a cargo liner.
The shipbuilder must estimate the number and power rating of the required generators by
assessing the power demand of the load for all situations whether at sea or in port.
Lighting circuit
Lighting and other low power ancillary services usually operate at 110 V or 220 V,
single-phase a.c. Transformers are used to reduce the 440 V system voltage to these lower voltage
levels.
Portable equipments
Where portable equipment is to be used in dangerous, hot and ‘damp locations, it is
advisable to operate at 55 V or even 24 V, supplied by a step-down transformer. Occasionally,
transformers are also used to step-up voltages, e.g. supplying a large 3.3 kV bow thruster motor
from a 440 V switchboard supply.
Batteries
Batteries for various essential services operate at 12 V or 24 V d.c. but sometimes higher
voltages are used if such loads require a large power supply.
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Power Distribution System
The function of a ship’s electrical distribution system is to safely convey the generated
electrical power to every item of consumer equipment connected to it. Probably the most obvious
element in the system is the main distribution centre, i.e. the ship’s main switchboard.
Main Switchboard
The main board supplies bulk power to motor group starter boards (often part of the main
board), section boards and distribution boards. Protection, e.g. circuit-breakers and fuses,
strategically placed throughout the system automatically disconnects a faulty circuit within the
network.
Transformers
Transformers inter-connect the high voltage and low voltage distribution sections of the
system.
Monitoring and protection
The operational state of a distribution system is indicated by the monitors for power,
voltage, current and by protection relays for over currents and earth-faults at each main control
centre.
A.C. Distribution system
The vast majority of ships have an alternating current (a.c.) distribution system in
preference to a direct current (d.c.) system. The required electrical services are broadly
considered as main and emergency supplies.
Advantages of A.C.
An a.c. network is cheaper to install and operate than a d.c. system. In particular, a.c.
offers a higher power/weight ratio for the generation, distribution and utilisation of electricity.
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Simple transformers efficiently step-up or step-down a.c. voltages where required. Three-phase
a.c. is effectively converted into rotary mechanical power in simple and efficient induction
motors. A ship’s electrical distribution scheme generally follows shore practice. This allows
normal industrial equipment to be used on board ship after being “marinised”, where necessary,
to withstand the rigours of a sea-life (e.g. it must withstand the vibration, humidity, high
temperature, ozone, sea-water, etc. encountered in various parts of the ship).
3 phase insulated neutral
The majority of ships have a 3-phase a.c., 3-wire, 440 V insulated-neutral system. This
means that the neutral point of star-connected generators is not earthed to the ship’s hull. For
continental European vessels, a 380 V, 3-phase system is common.
High voltage system
Ships with very large electrical loads have generators operating at high voltages (HV) of
3.3 kV, 6.6 kV and even 11 kV. Such high voltages are economically necessary in high power
systems to reduce the size of current, and hence reduce the size of conductors and equipment
required.
Frequency
The frequency of an a.c. power system can be 50 Hz or 60 Hz. In Europe and most of the
world the national frequency is 50 Hz but is 60 Hz in North America and in a few other countries.
The most common power frequency adopted for use on board ships and offshore platforms is 60
Hz. This higher frequency means that motors and generators run at higher speeds with a
consequent reduction in size for a given power rating.
Lighting and low power
Lighting and low power single-phase supplies usually operate at the lower voltage of 220
V a.c. although 110 V a.c. is also used. These voltages are derived from step-down transformers
connected to the 440 V system.
Branching system
The system is called a radial or branching system. This distribution system has a simple
and logical structure. Each item of load is supplied at its rated voltage via the correct size of cable
and is protected by the correctly rated protection device.
Essential and non-essential
The main electrical load is divided into essential and non-essential services. Essential
services are those required for the safety of personnel and for the safe navigation and propulsion
of the ship. They include certain supplies to navigational aids, communications, machinery spaces,
control stations and steering gear. The essential services may be supplied directly from the main
switchboard or via section boards or distribution boards.
Emergency supplies
Emergency supplies are necessary for loads which are required to handle a potentially
dangerous situation.
Preferential load shedding
To maintain generator operation during an overload, a preferential load shedding
arrangement is employed. This is achieved by a special overload relay, called a preference trip
relay. If a generator overload develops, the preference trip relay sets an alarm and acts to trip
selected non-essential loads. This reduces the generator load so that it may continue to supply
essential circuits.
Over current relay
Each generator has its own normal over current relay to trip its own circuit-breaker which
is typically high set at 150% with a 20 seconds delay. In addition, each generator has its own
preference overload trip, this being low set generally at 110% current, instantaneous operation. If
a generator overload condition develops, its preference overload trip will operate to energise the
timing relay.
Timing relay
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The timing relay then operates to disconnect non-essential services in a definite order at set
time intervals, e.g.
• 1st trip - air conditioning and ventilation - 5 seconds
• 2nd trip - refrigerated cargo plant -10 seconds
• 3rd trip - deck equipment -15 seconds
When sufficient non-essential load has been disconnected, the preference overload trip resets
and no further load is disconnected. The generator preference trip system can also be initiated by
low generator frequency or by low speed at the generator prime-mover.
Emergency supply
An emergency electrical power service must be provided on board in the event of a main
power failure. Such a supply is required for emergency lighting, alarms, communications,
watertight doors and other services necessary to maintain safety and to permit safe evacuation of
the ship. Regulations require that the emergency power source be a generator, or batteries, or both.
The emergency power source must be self-contained and not dependent upon any other engine
room power supply. A battery when fully charged is obviously self-contained. An emergency
generator must have an internal combustion engine as prime mover and have its own fuel supply
tank, starting equipment and switchboard in the near vicinity. The emergency power source must
come into action following a total mains failure. Emergency batteries can be arranged to be
switched into service immediately following a main power failure. Emergency generators can be
hand cranked, but are usually automatically started by compressed air or a battery to ensure
immediate run-up following a main power failure. Although regulations may permit a battery to
be the sole source of emergency power, in practice a suitable battery may be physically very large
and hence a diesel driven generator is usually installed with its own small starting battery or airstart supply. Other small batteries may also be installed to locally supply control and
communication equipment.
On passenger ships, regulations require that the primary emergency power supply be
provided by a diesel driven generator for up to 36 hours (18 hours for non-passenger vessels).
In addition, an emergency transitional battery must also be installed to maintain vital services
(mainly lighting) for a short period - typically a minimum of 3 hours. This emergency battery is
to ensures that a total blackout cannot occur in the transitional period between loss of main power
and the connection of the emergency generator.
Testing of emergency power
The emergency power system must be ready and available at all times. Such reliability
requires special care and maintenance. At regular intervals it must be tested to confirm that it
does operate correctly. The testing is normally carried out during the weekly emergency fire and
boat drill practice sessions. The main generators are not shut down but the emergency power
sources are energised and connected to supply the emergency services for the period of the
practice session.
Insulated and earthed neutral system
An insulated system is one that is totally electrically insulated from earth (ship’s hull).
An earth system has the supply neutral point connected to the earth. Shipboard main systems at
440V a.c. are normally insulated from earth (ship’s hull). Similar system shore are normally
earthed to the ground.
Continuity – single earth fault
The priority requirement on board ship is to maintain continuity of the electrical supply to
essential equipment in the event of a single earth fault occurring. The priority requirement ashore
is the immediate isolation of earth faulted equipment, which is automatically achieved by an
earthed system.
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Circuit consists of two parts
1. Conductor, the part which carries current through the
circuit
2. Insulation, the part which keeps the current inside
the conductor.
Basic faults
1. An open circuit fault is due to a break in the
conductor, as at A, so that current cannot flow.
2. An earth fault is due to a break in the insulation, as
at B, allowing the conductor to touch the hull or an
earthed metal enclosure.
3. A short-circuit fault is due to a double break in the
insulation, as at C, allowing both conductors to be
connected so that a very large current by-passes or
“short-circuits” the load.
QUESTION
A 10 A motor operates from a 220 V insulated system. The supply cables have a total
impedance of 0.01 Q. What circuit current would flow due to (a) an open-circuit fault,
(b) an earth fault and (c) a short-circuit fault occurred.
Answers
(a) The open circuit fault has infinite impedance.
I=V/Z, I=220/∞=0
(b) The earth fault has NO effect on circuit current. I=10A (This is an insulated system).
(c) The short circuit fault impedance is only the 0.01Ώ of the cables. Since I=V/Z = 220/0.01 Ώ =
22000 A
Need for earthing
The majority of earth faults occur within electrical equipment due to an insulation failure
or a loose wire, which allows a live conductor to come into contact with its earthed metal
enclosure. To protect against the dangers of electric shock and fire that may result from earth
faults, the metal enclosures and other non-current carrying metal parts of electrical equipment
must be earthed.
Equipment at zero volts
The earthing conductor connects the metal enclosure to earth (the ship’s hull) to prevent
it from attaining a dangerous voltage with respect to earth. Such earth bonding of equipment
ensures that it always remains at zero volts.
Significance of Earth Faults
If a single earth fault occurs on the live line of an EARTHED DISTRIBUTION
SYSTEM, it would be equivalent to a short-circuit fault across the generator through the ship’s
hull. The resulting large earth fault current would immediately cause the line protective device
(fuse or circuit breaker) and to trip out the ‘faulty circuit.
Loss of power
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The faulted electrical equipment would be immediately isolated from the supply and so
rendered safe. However, the loss of power supply could create a hazardous situation, especially
if the equipment was classed essential, e.g. steering gear. The large fault current could also cause
arcing damage at the fault location.
Single earth – insulated system
In contrast, a single earth fault “A” occurring on one line of an insulated distribution
system will not cause any protective trip to operate and the system would continue to function
normally. The equipment continues to operate with a single earth fault as it does not provide a
complete circuit so no earth fault current will flow.
Second earth – insulated system
If a second earth fault at “B” occurred on another line in the insulated system, the two
earth faults together would be equivalent to a short-circuit fault (via the ship’s hull) and the
resulting large current would operate protection devices and cause disconnection of perhaps
essential services creating a risk to the safety of the ship.
Difference – earthed & insulated
An insulated distribution system therefore requires two earth faults on two different lines
to cause an earth fault current to flow. In contrast, an earthed distribution system requires only
one earth fault on the line conductor to create an earth fault current which will trip out the faulty
circuit.
Marine – insulated system
An insulated system is, therefore, more effective than an earthed system in maintaining continuity
of supply to essential services. Hence its adoption for most marine electrical systems. Doublepole switches with fuses in both lines are necessary in an insulated single-phase circuit.
Earth fault monitor
Regulations require that an
earth fault monitor is fitted to the
main switchboard to indicate the
presence of an earth fault on each
isolated section of a distribution
system, e.g. on the 440 V and 220 V
sections. An earth fault monitor can
be either a set of indicator lamps or an
instrument (calibrated in kΏ or MΏ) to show the system IR value to earth.
Principle of earth lamps
When the system is healthy (no earth faults) then the lamps glow with equal half
brilliance. If an earth fault occurs on one line, the lamp connected to that line goes dim or
extinguished. The other lamps experience an increased voltage so will glow brighter than before.
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Earth indication lamps have been the most common method used for many years, being an
inexpensive installation which is easy to understand. Their major disadvantage is that they are not
very sensitive and will fail to indicate the presence of a high impedance earth fault.
Instrument type earth fault indicator
This has led to the development of instrument type earth fault indicators which are being
increasingly used. One common type of earth fault instrument-type monitor connects a small d.c.
voltage to the distribution system. Any resulting d.c. current is a measure of the insulation
resistance of the system. The injection-type instrument limits the maximum earth fault monitoring
current to only 1 mA (compared with about60 mA for earth lamps), and the meter indicates
insulation resistance directly in kΏ or MΏ. The monitor triggers an alarm when its set value is
reached.
Earth fault monitoring by dc
injection
Search and clear earth fault
If the earth fault monitor on the
switch-board shows the presence of
an earth fault on the distribution
system. It is up to the maintenance
staff to trace (search for) the exact
location of the fault and then to clear
it as quickly as possible.
Earth fault relay can be used for indication or
tripping. In this arrangement a CT is
connected across three phases. Under normal
operation there will be no current flow as
phaser sum of current is zero. When one of
the phase earths earth fault current flows and
phaser sum of current is not zero.
Method of search
An apparently simple method
would be to open the circuit-breakers
feeding loads A, B, C, etc. one at a
time and by watching the earth fault
monitor while observing which
circuit-breaker, when tripped, clears
the earth fault. The earth fault must
then be on that particular circuit. In
practice,
circuits
cannot
be
disconnected at random in this way.
Some vital service may be interrupted
causing the main engines to stop,
perhaps in dangerous narrow waters.
Tracing the earth fault must be coordinated with the operational requirements of the ship’s electrical services.
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Earth fault – lighting circuit
Suppose the earth fault monitor on the 220 V lighting distribution board (d.b.) indicates
the presence of an earth fault. Switches A, B, C, are sequentially opened and closed in turn until
the earth fault monitor indicates the earth faulted circuit.
Suppose this is switch B. Circuit B supplies a distribution fuse-board (d.f.b.) located near
its lighting circuits. Here there is no earth fault monitor so an IR (megger) tester must he used. At
this d.f.b. fuse-pair No. 1 is removed to isolate the supply to the load. The IR tester (megger) is
now connected with one lead to earth (hull) and the other lead to “b” (the outgoing terminal as
shown), and a test applied. If healthy (IR >1 MΏ), connect the test lead to “a” and repeat the test.
If both “a” and “b” are healthy, circuit 1 is healthy and fuse-pair 1 can be replaced. Fuse-pair 2 is
now removed and tested a t “a” and “b”. If an earth fault is indicated (IR = low) then the faulted
circuit has been located. All fuse-pairs are checked in turn to confirm whether healthy or faulted.
At the faulted circuit, the fuses should be removed, all switches should be opened, and all lamps
taken out. This breaks the circuit into several isolated conductor sections. At the supply
distribution board, test at “a” and then at “b”. If both have an IR > 1 MΏ then the conductors
connected to “a” and “b” are clear and healthy. Close the switch and re-test at “a”. If the IR is low
then the earth fault lies on the conductors beyond the switch.
At lamp 1 remove the
fitting and
disconnect
the
conductors as shown to further
break down the circuit. Use the IR
tester on each
of these
disconnected leads. If one
conductor is indicated as having
an earth fault (suppose it is the
conductor between L1 and L2)
then the earth fault lies at lamp 1 or lamp 2 or on the conductor. Both lamp fittings must now be
opened and visually inspected to trace the exact location of earth fault. The method of tracing the
earth fault is essentially that of continually breaking down the circuit into smaller and smaller
sections until it is finally located. When located, the damaged insulation must be repaired. The
method of repairing the earth fault depends upon the cause of the earth fault and this is
determined by visual examination. A lamp fitting that is damaged must be replaced. Dampness in
insulation must be dried out by gentle heat and then some precaution must be taken to prevent the
future ingress of moisture. Insulation that has been mechanically damaged or weakened by
overheating must be made good again. If surface dirt is the cause, a thorough cleaning will
probably cure the fault.
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Distribution Circuit Breakers
The function of any circuit-breaker is to safely make onto and break open the prospective
short-circuit fault current expected at that point in the circuit. The main contacts must open
rapidly while the resulting arc is transferred to special arcing contacts above the main contacts.
Arc chutes with arc-splitters quickly stretch and cool the arc until it snaps. The CB is open when
the arc is quenched. Feeder and distribution circuits are usually protected by the moulded-case
(MCCB) type or the miniature (MCB) type of circuit-breakers.
MCCBs
These are small, compact air circuit-breakers fitted in a moulded plastic case. They have
a lower normal current rating (50-1500 A) than main breakers and a lower breaking capacity.
They usually have an adjustable thermal overcurrent setting and an adjustable or fixed magnetic
overcurrent trip for short-circuit protection built into the case. An undervoltage trip coil may also
be included within the case. Operation to close is usually by a hand operated lever but motorcharged spring closing can also be fitted.
MCCBs are reliable, trouble free and require negligible maintenance. If the breaker operates in
the ON position for long periods it should be tripped and closed a few times to free the
mechanism and clean the contacts. Terminals should be checked for tightness otherwise
overheating damage will develop.
The front cover of larger MCCBs (around 1000 A rating) can usually be removed for
visual inspection and cleaning. Following tripping under a circuit fault, the breaker should be
inspected for damage, checked for correct operation, and its insulation resistance measured. A test
result of at least 5 MΏ is usually required. Any other faulty operation usually requires
replacement or overhaul by the manufacturer.
MCCBs can be used for every application on board ship from generator breakers to small
distribution breakers. The limited breaking capacity may demand back-up fuses be fitted for very
high prospective short-circuit fault levels.
MCBs
These are very small air circuit breakers fitted in small moulded cases. They have current
rating of 5-100 A, generally thermal over current and magnetic short circuit protection. They have
a very limited breaking capacity (about 3000 A) and are commonly used in final distribution
boards instead of fuses. The d.b. is supplied via a fuse or MCCB with required breaking capacity.
MCBs must be replaced if fault develops.
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Details of parts of MCB
1. Actuator lever - used to manually trip and reset the circuit breaker. Also indicates
the status of the circuit breaker (On or Off/tripped).
2. Actuator mechanism - forces the contacts together or apart.
3. Contacts - Allow current to flow when touching and break the flow of current
when moved apart.
4. Terminals
5. Bimetallic strip
6. Calibration screw
7. Solenoid
8. Arc divider / extinguisher
Transformers
Magnetic flux, like current, lags applied voltage by 900
A transformer is a device made of two or more inductors, one of which is powered by AC,
inducing an AC voltage across the second inductor. If the second inductor is connected to a load,
power will be electromagnetically coupled from the first inductor's power source to that load. The
powered inductor in a transformer is called the primary winding. The unpowered inductor in a
transformer is called the secondary winding. Magnetic flux in the core lags 900 behind the source
voltage waveform. The current drawn by the primary coil from the source to produce this flux is
called the magnetizing current, and it also lags the supply voltage by 900.
The principle of operation of a single-phase transformer is simple. An applied a.c. voltage V1 to
the primary winding sets up an alternating magnetic flux in the laminated steel core. The flux
induces an emf in the secondary whose size is fixed by the ratio of primary and secondary turns
in the pair of phase windings (N1 and N2) to give: V1 / V2 = N1/ N2
Schematic symbol for
transformer consists of
two inductor symbols,
separated by lines
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indicating a ferromagnetic core.
Ferromagnetic core with primary coil (AC driven) and secondary coil.
Dividing the iron core into thin
insulated laminations minimizes
eddy current loss.
Transformer cross-section cut shows core and windings.
Core Characteristics: Transformers are working on principal of mutual inductance. Core
of the transformer is required to provide passage to current and flux.
Due to alternating current the core is magnetised and demagnetised and therefore, core
material should provide minimum resistance to flux flow otherwise hysteresis loss will
increase. Ferromagnetic materials are used as core material.
Eddy current flows in the core to oppose the flux generated and therefore, material gets
heated up and leading to losses. To minimise this loss core is constructed using laminated
sheets.
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Transformer arrangements
Step-down transformer: (many turns :few turns).
The secondary voltage V2 is available to drive current through a load. It is the load
connected to the secondary that sets the size and power factor angle of the load current I2. This is
matched on the primary side from: V1 / V2 = I1/ I2 .Transformers are rated in apparent power (VA
or kVA) units.
Electrical generation on board ship is typically at 3-phase a.c., 440 V, 60 Hz, while fixed
lighting and other low power loads are supplied with 220 or 110 V a.c. single-phase from very
efficient (typically > 90%) static transformer units.
A three-phase transformer is made of three sets of primary and secondary windings, each
set wound around one leg of an iron core assembly. Essentially it looks like three single-phase
transformers sharing a joined core.
Those sets of primary and secondary windings will be
connected in either Δ or Y configurations to form a complete
unit.
The transformers are generally air cooled, being
mounted in sheet steel enclosures which are often located adjacent to the main switchboard.
Alternatively, they may be fitted within the switchboard so transformer enclosures are not
required. Three-phase 440/220 V lighting trans- formers are usually composed of three separate
single-phase units inter- connected to form a 3-phase arrangement. This enables easy replacement
of a single-phase unit if it develops a fault.
The alternative is to use a single 3-phase unit with all windings mounted on a common
magnetic core. This type has to be completely isolated in the event of a fault on one phase only.
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Transformers for use on 3-phase insulated systems are generally interconnected in a delta-delta
circuit configuration using copper links between the phase windings.
If a fault develops on one phase of such an arrangement, the faulty unit can be disconnected (via
the links) creating an open-delta or ‘V” connection and a 3-phase supply will still be available,
although at a reduced power capacity. This is obviously a useful safeguard. In some cases, a spare
4th transformer is available to replace the faulty unit.
Maintenance
Transformers are static items of equipment which are usually very reliable and troublefree. However, like all electrical equipment, transformers must be subjected to the usual
maintenance checks. At regular specified intervals, transformers must be disconnected, covers
removed and all accumulated dust and deposits removed by a vacuum cleaner and suitable
brushes. Windings must be inspected for any signs of damage or over heating. Winding
continuity resistance values are measured, recorded and compared with each other for balance.
Any differences in continuity readings will indicate winding faults such as short-circuited turns.
The insulation resistance of all windings must be measured both with respect to earth and to the
other phase windings. The cause of any low insulation resistance reading must be investigated
and rectified. Cable connections must be checked’ for tightness. Covers must be securely
replaced and the transformers re-commissioned. All test results and observations should then be
recorded for future reference.
Delta-delta transformer connection.
Instrument Transformers
Transformers are used to supply
instruments and protection relays
with proportionally small currents
and voltages derived from the large
currents and voltages in a high power
network.
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While the PT is a step-down device, the Current Transformer (or CT) is a step-up device
(with respect to voltage), which is what is needed to step down the power line current.
Bar Primary transformer: Current conductor to be
measured is threaded through the opening. Scaled
down current is available on wire leads.
Quite often, CTs are built as donut-shaped devices
through which the power line conductor is run, the power
line itself acting as a single-turn primary winding. The bar
primary type CT is used with very high primary current
ratings the wound primary type being used for small stepdown ratios, e.g. 1000/5 A bar primary; 50/5 A wound
primary. The ratio specified on a VT details its input and output voltages, e.g. 3.3 kV/110 V is
used on a 3.3 kV mains circuit and steps the voltage down to 110 V. The associated instrument
will have its scale calibrated O-3.3 kV and will be marked “3.3 kV/110 V VT ratio”.
The ratio specified on a CT similarly details its input and output currents, e.g. 150/5 A
CT is used on a 150 A mains circuit and steps the current down to 5 A. The associated instrument
will have its scale calibrated “O-150 A” and will be marked “150/5 A CT ratio”. The use of
instrument transformers does not eliminate danger to operators. The 110 V output from a VT will
apply a severe, possibly lethal shock to un-suspecting fingers.
The secondary circuit of a CT must never be opened while mains primary load current is
flowing. Excessive heating will be developed in an open-circuited CT with an extremely high
voltage arising at the open secondary terminals.
If an ammeter is to be removed from circuit, the CT secondary output terminal must be
first short-circuited, with the primary circuit
switched off. The secondary short circuit will not
damage the CT when the primary current is
switched on. For further safety, one end of the
secondary winding of a CT or VT is connected to
earth.
Status indicator lamps on switchboards are
commonly of the transformer type, having a small
transformer built into the lamp fitting. The
transformer provides a 6 V or 12 V output. The
lamp is of low wattage with small bayonet cap
fitting. Although not an accurate instrument
transformer, the lamp transformer is similar in
function to a VT.
Shore Supply Connection
A shore-supply is required so that the ship’s generators and their prime-movers can be
shut down for major overhaul during a dry-docking period. There must be a suitable connection
box conveniently located to accept the shore supply cable.
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Shore connection box and indicators.
The connection box is often located at the entrance to the accommodation or in the
emergency generator room. The connection box must have suitable terminals to accept the shore
supply cable, including an earthing terminal to earth the ship’s hull to the shore earth.
The connection box must have a circuit-breaker or an isolator switch and fuses to protect the
cable linking the connection box to the main switchboard, with a data plate giving details of the
ship’s electrical system (voltage and frequency) and showing the method for connecting the shore
supply cable.
A voltmeter is fitted to indicate polarity of a d.c. shore supply. For an a.c. shore supply a
phase-sequence indicator (P. S. I.) is fitted to indicate correct supply phase sequence. This
indicator may be arranged as two lamps connected as an unbalanced load across the three phases
via resistors and capacitors. The sequence is “right” (or correct) when the right side lamp is bright
and the other is dark.
An alternative P.S.I. indicator is a rotary pointer driven by a small 3-phase induction
motor. At the main switchboard an indicator is provided, usually a lamp, to indicate that the shore
supply is available for connection to the bus-bars via a connecting switch or circuit-breaker.
It is not normally possible to parallel the shore supply with the ship’s generators. The
ship’s generators must, therefore, be disconnected before the shore supply can be connected to the
main switchboard. Normally, the shore supply switch on the main switchboard is interlocked with
the generator circuit-breakers so that it cannot be closed if the generators are still connected.
Correct sequence
By “correct” we mean that it is the same sequence as the ship’s supply (red-yellow- blue).
A reversed phase sequence (red-blue-yellow) will produce a reversed shaft rotation in all 3-phase
motors because the direction of their rotating magnetic fields will be reversed with disastrous
results. This fault is remedied by interchanging any two conductors of the shore supply cable at
the connection box. The shore supply may have a different frequency and/or voltage to that of the
ship’s system.
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A higher frequency will cause motors to run faster, be overloaded and overheat. A higher
voltage will generally cause equipment to take excess current and overheat. It will also cause
motors to accelerate more rapidly and this may overstress the driven loads.
A lower voltage is generally not so serious but may cause motors to run slower and
overheat, and may cause motors to stall. If the shore supply frequency differs from the ship’s
normal frequency then, ideally, the shore supply voltage should differ in the same proportion.
Question: If your ship is designed for 60 Hz at 440 V - what value should the shore
supply voltage be if operating at 50 Hz?
Answer: Supply voltage should be reduced to about 380 V.
Circuit Protection
Many forms of electrical protection are available which are designed to protect the
distribution system when a fault occurs. Protection relays are used to monitor overcurrent,
over/under voltage, over/under frequency, earth leakage, un- balanced loading, over-temperature,
reverse power (for generators) etc.
As most protection relays monitor Current and/or voltage, we will limit our examination
to overcurrent and under-, voltage protection together with an appreciation of protective
discrimination. Reverse power protection will be discussed later along with alternator protections.
No matter how well designed and operated, there is always the possibility of faults developing on
electrical equipment. Faults can develop due to natural wear and tear, incorrect operation,
accidental damage and by neglect.
The breakdown of essential equipment may endanger the ship, but probably the most
serious hazard is FIRE. Overcurrent (I2R resistive heating effect) in cables and equipment will
cause overheating and possibly fire. The size of conductor used in cables and equipment is such
that with rated full load current flowing, the heat developed does not raise the temperature beyond
about 80°C (i.e. 35°C rise above an ambient of 45°C). A copper conductor can withstand very
high temperatures (melts at 10830 C), but its insulation (generally organic materials such as
cotton or plastic compounds) cannot withstand temperatures much in excess of 100°C. At higher
temperatures the insulation suffers irreversible chemical changes, loses its insulation properties
and becomes burnt out. Short-circuit and overload currents must, therefore, be detected and
rapidly cleared before damage occurs.
Reasons why protection equipment is essential in an electrical distribution system:
1. To disconnect and isolate faulty equipment in order to maintain the power supply to the
remaining healthy circuits in the system
2. To prevent damage to equipment from the thermal and magnetic forces that occur during
short circuit and overload faults
3. To protect personnel from electric shock
The protection scheme consists of circuit-breakers, fuses, contactors, overcurrent and
undervoltage relays. A circuit-breaker, fuse or contactor interrupts the fault current. An
overcurrent relay detects the fault current and initiates the trip action. The circuit-breaker or fuse
must be capable of safely and rapidly interrupting a short-circuit current. They must be
mechanically strong enough to withstand the thermal and magnetic forces produced by the fault
current.
The size (strength) of the circuit-breaker or fuse is specified by its breaking capacity which is
the maximum fault current it can safely interrupt. For example, an MCCB may be continuously
rated at 440 V with a rated current of 600 A. Its breaking capacity may be 12.5 MVA which
means it can safely interrupt a fault current of 16,400 A (from 12.5 x 106 /√3.440 =16,400 A).
The prospective fault current level at a point in a circuit is the current that arises due to a shortcircuit at that point.
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Short circuit fault location.
The size of this short-circuit fault current is determined by the total impedance of
generators, cables and transformers in the circuit between the generator and the fault. This total
impedance is generally very small so the maximum fault current (called the prospective fault
current) can be very large.
A 440 V, 5 kW, 0.8 pf 3-phase load is supplied. The normal full load POWER is
P = √3 VL IL cos Φ watts
So the load full load current is IL = P/ √3 VLL cos Φ = 5000/ √3 .440. 0.8 = 8.2 A
Fault circuit
Suppose now a short-circuit fault occurs at the load terminals The total impedance is ZF = 0.025 +
0.01 + 0.015 = 0.05 Ώ
and the prospective short-circuit fault current is IF = V/ZF = 440V/0.05 Ώ = 8,800 A
So the prospective fault current level at the load is 8800 A.
For a short-circuit at the d.b. the 440 V fault level is:
440 V/ (0.025 + O.O1)Ώ = 12,571 A
For a short-circuit at the main switchboard 440 V the fault level is:
440 V/ 0.025 Ώ = 17,600 A
The fault level increases, the nearer the fault occurs to the generator. The circuit-breaker
or fuse must have a breaking-current capacity in excess of the prospective fault current level
expected at the point at which it is fitted. If less, the circuit breaker (or fuse) is liable to explode
and cause fire. The ability of a protection system to disconnect only the faulted circuits and to
maintain the electrical supplies to healthy circuits is called protective discrimination.
Discrimination is achieved by co-ordinating the current ratings and time settings of the fuses and
overcurrent relays used between the generator and the load.
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Protective discrimination scheme.
The protective devices nearest the load having the lowest current rating and shortest
operating time. Those nearest the generator having the highest current rating and longest
operating time. If a short-circuit fault occurs in the lamp holder in above figure, the fault current
will be large enough to operate all protection devices from the generators to the fault. However,
the 5 A fuse protecting the lamp circuit has the lowest current rating and shortest operating time
in the system so will be the quickest to operate. This action will clear the fault and leave all other
healthy circuits still connected. In the case of fuses, it is generally accepted that discrimination
will be achieved if consecutive fuses have a ratio of about 2:l. The shipbuilder specifies the
current ratings of fuses, together with the current and time settings of relays, in the protection
scheme. It is important that the original settings are maintained to achieve correct discrimination.
Overcurrent Protection
The general term “overcurrent” applies to a relatively small increase over the full load
current (FLC) rating (e.g. due to mechanical overloading of a motor) rather than the massive
current increase caused by a short-circuit fault. Generally, an overcurrent, supplied from a CT, is
detected by a relay with an appropriate time-delay to match the protected circuit.
Short circuit faults in LV distribution circuits are mainly detected and cleared almost
instantaneously by fuses, MCCBs or MCBs. Main supply feeders are usually protected against
short-circuits by circuit breakers with instantaneous magnetic trip action.
Overcurrent relay types
All relay types have an inverse current-time characteristic called OCIT (over-current
inverse time), i.e. the bigger the current the faster it will operate. The basic inverse I/t curve
would tend towards zero time for the highest currents. To make the relay action more precise at
very high fault currents the action is arranged to operate at a definite minimum time which is
fixed by the design.
This type is called an OCIDMT (over
current inverse and definite minimum
time) relay action. The OCIDMT can
also be combined with an
instantaneous (high set) trip to give the
fastest action against extremely high
currents due to a short circuit fault.
Magnetic relay
A magnetic relay, directly
converts
the
current
into
an
electromagnetic force to operate a trip
switch. One type is the attracted
armature action similar in construction
to a simple signaling relay but with an
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adjustment for the current setting. The time of operation is fixed at a definite minimum time which
is usually less than 0.2 seconds. This is regarded as instantaneous i.e. with no deliberate timedelay.
To obtain a magnetic inverse-time action, e.g. for motor overload protection, an induction
disc movement is usually employed. This construction is similar to a kWh energy meter used in a
house but the disc movement is constrained by a spring so is not allowed to actually rotate.
The disc travel is very small but sufficient to operate a set of trip switch contacts. Both
current and time settings are adjustable. A combined relay including an attracted armature
element and induction disc element will give an instantaneous action (high set current) and an
inverse/time characteristic.
Thermal relay
Thermal relay utilises the bending action of a bimetallic bar (one per phase) to open a
normally-closed (NC) contact which then trips a contactor or circuit-breaker. A small circuit
current will be allowed to flow directly through the bimetallic strip but larger currents will be
directed through a heater coil surrounding the strip. The three bimetal strips in a three phase relay,
all bend in the same direction with balanced over currents to cause a trip. A mechanical bellcrank trip arrangement can also operate with unbalanced (differential) currents. This is
particularly effective with a single-phasing motor fault. In this case, two of the bimetal strips
bend further in the normal direction with increased line current, while the other cools down
allowing this strip to move relatively backwards (differential action).
The time taken to heat the bimetal strip to cause sufficient bending fixes the required time
to trip. Resetting the relay can only be achieved after the strip has cooled down back to the
ambient temperature. The inverse I/t overcurrent characteristic of a thermal relay is very useful
for the indirect temperature protection of motors. Its thermal time delay is, however, far too long
for a short-circuit fault so back-up instantaneous protection must also be used in the form of fuses
or a circuit breaker.
Electronic relay
An electronic overcurrent relay usually converts the measured current into a proportional
voltage. This is then compared with a set voltage level within the monitoring unit which may be
digital or analogue. In an analogue unit the time delay is obtained by the time taken to charge up a
capacitor. This type of relay has separate adjustments for over current and time settings together
with an instantaneous trip. The electronic amplifiers within the relay require a low voltage d.c.
power supply, e.g. 24 V d.c. derived from a 110 V a.c. auxiliary supply. Here, the input from a
line current transformer (CT) is rectified to produce a d.c. voltage which is proportional to the
line current.
Both the magnetic and electronic relays can be designed to give an almost instantaneous
trip (typically less than 0.05 seconds or 50 ms) to clear a short-circuit fault. Thermal relays are
commonly fitted in moulded case circuit breakers (MCCBs) and in miniature circuit-breakers
(MCBs) to give a “long time” thermal overcurrent trip in addition to a magnetic action for an
instantaneous trip with a short-circuit fault.
Overcurrent protection relays in large power circuits are generally driven by current
transformers (CTs). The CT secondary usually has a 5 A or 1 A rating for full load current in its
primary winding. All overcurrent relays can be tested by injecting calibrated test currents into
them to check their current trip levels and time delay settings.
Primary injection is where a calibrated test current is fed through the normal load circuit.
This requires a large current injection test set. The test set is essentially a transformer and
controller rather like a welding set, i.e. it gives a low voltage - high current output.
Small secondary injection currents (5-50 A) are fed directly into the over current relay
usually via a special test plug/socket wired into the relay. Secondary injection does not prove the
CT performance (as it is disconnected during the test) but is the usual method for testing an over
current relay. The setting up of an overcurrent relay is obviously critical to its protective duty so
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is carried out in strict accordance with the manufacturer’s instructions. Such setting up is done
during new ship trials and at subsequent periodic surveys.
Fuse Protection
A fuse is the most common type of protection against a short-circuit fault in LV
distribution circuits, motor circuits and for portable appliances. It is relatively simple, inexpensive
and reliable. As re-wireable fuses tend to be less reliable than the cartridge type and are open to
abuse (fitting the wrong size of fuse wire), they are not recommended for marine practice.
HRC (high rupturing capacity - e.g. 80 kA) cartridge-type fuse links are normally used.
HRC fuse construction.
A disadvantage of a fuse is
its insensitivity to small over
currents. An HRC fuse will blow at
currents as low as 25% overload, but
only after about 4 hours. The
advantage of a fuse is its very high
speed of operation (a few milli-seconds) at high short-circuit fault current - faster than a circuitbreaker.
Fuses are fitted in circuits to give protection against short-circuits. Protection against
relatively small over currents (e.g. due to shaft overloading on a motor) is provided where
necessary by an over current relay (OCR). A starter over current relay protects the motor against
relatively small over-currents.
The fuse links provide back-up protection for the supply cables and generators against a
short-circuit fault. Motor fuses are typically rated at 2-3 times the motor full load current in order
to withstand the large starting current surge (up to 6 times full load) of the motor.
Figure 1Bottle type HRC fuse
The motor manufacturer will specify the correct rating of fuse link for a particular motor rating.
Hence a typical fuse designation for a motor circuit could be “32M63” which indicates a
continuous rating of 32 A but a rating of 63 A for the brief starting period.
Important points to note concerning fuses are
1. In the event of a fuse blowing, the cause of the fault must be located and repaired before
the fuse link is replaced.
2. The replacement fuse link must be of the correct current rating, grade and type. Usually
this means the replacement fuse link is identical to the blown fuse link.
3. Replace all three fuses in a 3-phase supply even if only one is found blown after a fault.
The others may be seriously weakened which makes them unreliable for future use.
The reference symbols used on an HRC fuse link are devised by the particular manufacturer.
They include the current rating, voltage, application (e.g. motor, transformer, diode, general use),
physical size, and type of fixing arrangement.
Undervoltage Protection TO CHANGE UNDERVOLTAGE COIL DIAGRAM
An undervoltage (U/V) release mechanism is fitted to all generator breakers and some
main feeder circuit-breakers. Its main function is to trip the breaker when a severe voltage dip
(around 50%) occurs. This is achieved by lifting the mechanical latch (which keeps the contacts
closed) to allow the trip spring to function which opens the breaker contacts. The U/V release on
a generator circuit-breaker also prevents it being closed when the generator voltage is very low or
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absent. An undervoltage relay may be magnetic or electronic, also provides back-up protection to
short-circuit protection.
As an example,
suppose during generator paralleling procedures, an attempt was made to close the wrong circuitbreaker e.g. the breaker of a stopped and dead generator. If this circuit-breaker was closed, the
dead generator would be the equivalent of a short-circuit fault on the bus-bars and cause a
blackout. The undervoltage relay prevents the closure of the circuit-breaker of the dead generator.
A short-circuit occurs on the main bus-bars and the short-circuit trip of the running
generator breaker fails to operate. The short-circuit reduces the bus-bar voltage to zero which
causes the U/V release to trip the breaker.
Under voltage protection is also required for motor starters. The starter contactor
normally provides this protection as it drops out when the supply voltage is lost or is drastically
reduced. The starter circuit will not normally allow the motor to re-start when the voltage is
restored, except when automatic restarting is facilities are provided.
Cables
Ship wiring cables have to withstand a wide variety of environmental conditions, e.g.
extremes of ambient temperature, humidity and salinity. Improved materials have led to ship
wiring cables of a fairly standard design that are safe, durable and efficient under all conditions.
The normal distribution voltage on ships is 440 V and cables for use at this voltage are designated
600/1000V, i.e. 600 V to earth or 1000 V between conductors. Higher voltage systems require
cables with appropriate ratings, e.g. for a 3.3 kV 3-phase earthed neutral system the required
cable rating is 1900/3300V. For 3-phase insulated systems the cable rating would be 3300/3300 V.
Cables are constructed of several basic parts
Conductors are of annealed stranded copper which may be circular or shaped. Cables
with shaped conductors and cores are usually smaller and lighter than cables with circular cores.
Cable insulation has a thickness appropriate to the system voltage rating. Insulation materials are
generally organic plastic compounds. Butyl rubber, which is tough and resilient, has good heat,
ozone and moisture resistance. These excellent properties enable butyl rubber to replace natural
rubber as an insulant.
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Even so, butyl rubber has now been largely superseded by ethylene propylene rubber
(EPR) insulation. EPR has similar electrical and physical properties to butyl rubber but with
better resistance to moisture and ozone. It should not, however, be exposed to oils and greases.
Cross-linked polyethylene (XLPE) is also used as an insulant but has inferior mechanical and
thermal properties when compared with EPR.
Polyvinyl chloride (PVC) is not
generally used for ships’ cables, even
though it is very common ashore. PVC
tends to soften and flow at high
temperatures (melts at 15O0C), and
hardens and cracks at low temperatures
(- 8°C). Even at normal temperatures
PVC tends to flow and become
distorted under mechanical stress - for
example necking occurs at cable glands
causing the gland to lose its watertight
properties. Multicore ship wiring
cables have the cores identified by
either colour, printed numerals on
untaped cores or numbered tapes on
taped cores.
Sheath
The, sheath of a cable protects the
insulation from damage and injury – it
is not classed as an insulant. Sheath
materials are required to be heat, oil
and chemical resistant and flame
retardant (HOFR). The sheath must
also
be
tough
and
flexible.
Polychloroprene (PCP or Neoprene) is
a common sheath material but has been
largely
superseded
by
chlorosulphonated polyethylene (CSP
or hypalon). CSP-HOFR sheathing
compound is well suited to shipboard
conditions. It offers good resistance to
cuts and abrasions, resists weather and
ozone, acid fumes and alkalis, and is
flexible.
Armouring
Extra mechanical protection is provided by armouring with basket-woven wire braid of
either galvanised steel or tinned phosphor bronze. The non-magnetic properties of phosphor
bronze are preferred for single-core cables. A protective outer sheath of CSP compound covers
the wire braid. The wire braiding also acts as a screen to reduce interference (caused by magnetic
fields) in adjacent communication and instrumentation circuits.
Will cable materials burn?
All organic materials will eventually burn in a severe fire. Cable sheath materials
commonly in use are organic plastic compounds that are classed as flame retardant, i.e. will not
sustain a fire. Most cable materials now achieve this property by developing chlorine gas and acid
fumes to smother the flame.
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PVC is notorious for its release of deadly acid fumes, but PCP and CSP do the same.
EPR and XLPE do not. Some new materials do not produce acid fumes when burning - an
important feature for fire-fighting personnel. However, burning cable materials still tend to
produce dense black smoke.
Current rating
The current rating of a cable is the current the cable can carry continuously without the
conductor exceeding 800C with an ambient temperature of 450C (i.e. 350C rise). This rating must
be reduced if the ambient temperature exceeds 450C or when cables are bunched together or
enclosed in a pipe or trunking, which reduces the effective cooling.
The volt drop in cables from the main switchboard to the appliance must not exceed 6%
(in practice it is about 2%). The cables installed must comply with both the current rating and the
volt-drop limitation. Cable volt drop only becomes a problem in very long cables.
Cable gland
Cables are insulated, mechanically protected and watertight. They may be armoured and
suitable for installation in a hazardous explosive area. A cable gland maintains these properties
where the cable is terminated at an appliance, e.g. at a motor terminal box.
The cable gland is screwed into the appliance terminal box. Nuts on the gland compress
sealing rings to maintain watertight seals on the inner and outer sheaths and to clamp the armour
braiding. The gland must be matched to the size and type of cable.
Mechanical damage to cables must be made good either by repairing the damage or
replacing that section of cable. Unprotected metal armouring and insulation material are
vulnerable to attack by moisture, chemicals and corrosive gases, while exposed live conductors
are obviously dangerous.
Principle of 3 phase A.C. generation
AC generator (alternator) provides the power in such a way that the rotating magnetic
field passes by three sets of wire windings, each set spaced 120 0 apart around the circumference
of the machine.
Together, the six pole" windings of a three-phase
alternator are connected to comprise three winding pairs, each
pair producing AC voltage with a phase angle 1200 shifted from
either of the other two winding pairs. The three voltage sources
connected together in a Y or star configuration, with one lead of
each source tied to a common point .
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As the magnet rotates, the phase
angle shift of 1200 is a function of
the actual rotational angle shift of
the three pairs of windings. If the
magnet is rotating clockwise,
winding 3 will generate its peak
instantaneous voltage exactly 1200
(of alternator shaft rotation) after
winding 2, which will hits its peak
1200 after winding 1. The magnet passes by each pole pair at different positions in the
rotational movement of the shaft.
Where we decide to place the windings will dictate the amount of phase shift between the
windings' AC voltage waveforms. If we make winding 1 our reference voltage source for
phase angle (00 ), then winding 2 will have a phase angle of 1200 ( lagging, or 2400
leading) and winding 3 an angle of 2400 (or 1200 leading).
This sequence of phase shifts has a definite order. For clockwise rotation of the
shaft, the order is 1-2-3
(winding 1 peaks first,
them winding 2, then
winding 3).
This order keeps
repeating itself as long as
we continue to rotate the
alternator's shaft. However,
if we reverse the rotation of
the alternator's shaft (turn it
anticlockwise), the magnet will pass by the pole pairs in the opposite sequence.
Instead of 1-2-3, we'll have 3-2-1. Now, winding 2's waveform will be leading 1200
ahead of 1 instead of lagging, and 3 will be another 1200 ahead of 2.
Phase sequence
Phase sequence is
produced (the order in which
pole pairs get passed by the
alternator's rotating magnet)
and how it can be changed
by reversing the alternator's
shaft rotation. There is a
much easier way to reverse
phase sequence than reversing alternator rotation: just exchange any two of the three wires going
to a three-phase load.
Voltage
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The useful emf level (E) is called the root mean square (rms) value and all equipment is
rated in rms terms. The voltage available at the generator terminals is V = E - (I.Z) [phasor
calculation]where I is the load current flowing in the stator phase windings. An internal phase
volt-drop of (IZ) occurs due to the impedance Z of a phase winding which is made up from its
resistance and reactance.
Frequency
The frequency f (measured in Hertz) of the emf is the number of wave form cycles per
second. This obviously depends on the rotational speed and the number of poles.
So f = n.p =(N/6O).p where n = speed in rev/s, N = rev/min and p = pairs of poles.
These two basic relationships for emf and frequency dictate how to control the voltage
and frequency output of a generator. In practice the speed is maintained practically constant by
the generator’s prime-mover which fixes the output frequency. The constant speed then allows
the size of generated emf to be directly controlled by the size of pole flux (excitation).
A practical a.c. generator has three sets of coils, called phase windings, located in slots in
the stator surrounding the rotating magnetic poles. The emf induced in each phase is 1200 out of
phase with the other two phases. Three-phase windings are labelled as U-V-W with colour coding
of red, yellow and blue used on terminals and bus-bars. One end of each of the three phase
windings are joined together to form the neutral point of a star connection. The other ends of the
phase windings are connected to outgoing conductors called lines.
The three output line voltages (represented by VL) and the 3 output line currents (represented
by IL) combine to create the three-phase electrical power output of:
• P = √3.VL.ILcosΦ watts
• In a star connection, any line voltage VL, is made up from two phase voltages, where VL
= √3. VPH. The √3 factor is due to the 1200 displacement between phase voltages.
Power factor
The rated values of a machine always refer to line conditions (as stated on rating plate).
Angle Φ is the phase angle between VPH and IPH which is determined by the types of electrical
load on the generator (e.g. lighting, motors, galley equipment etc.).
CosΦ is the power factor of the electrical load and is typically about 0.8 lagging which
means that the current wave form lags about 370 behind the voltage.
Alternator construction
The two main parts of any rotating a.c. machine are its stator and rotor. The fabricated
steel stator frame supports the stator core and its three phase windings. The main outgoing cables
connected to these terminals conduct the generator’s electric power to its circuit-breaker at the
main switchboard.
Rotor
The rotor of a main a.c. generator provides the field excitation from its electromagnetic poles.
Two constructional forms of rotor are available
1. salient pole type
2. cylindrical type
A typical rotating-field ac generator consists of an alternator and a smaller dc generator built into
a single unit. The output of the alternator section supplies alternating voltage to the load. The only
purpose for the dc exciter generator is to supply the direct current required to maintain the
alternator field. This dc generator is referred to as the exciter.
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The exciter is a dc, shunt-wound, self-excited generator. The exciter shunt field (2)
creates an area of intense magnetic flux between its poles. When the exciter armature (3) is
rotated in the exciter-field flux, voltage is induced in the exciter armature windings. The output
from the exciter commutator (4) is connected through brushes and slip rings (5) to the alternator
field. Since this is direct current already converted by the exciter commutator, the current always
flows in one direction through the alternator field (6). Thus, a fixed-polarity magnetic field is
maintained at all times in the alternator field windings. When the alternator field is rotated, its
magnetic flux is passed through and across the alternator armature windings (7).
Salient pole type
The salient pole type has projecting poles bolted or keyed onto the shaft hub. Field
excitation windings are fitted around each pole. This type of rotor is used with medium and slow
shaft speeds (1800 rpm and below) and is the most common arrangement for marine generators.
If you could compare the physical size of the two types of rotors with the same electrical
characteristics, you would see that the salient-pole rotor would have a greater diameter. At the
same number of revolutions per minute, it has a greater centrifugal force than high speed rotor.
Cylindrical type
Cylindrical type rotors are generally used with large power, high speed (1500-3600 rpm)
steam/gas turbine drives. The excitation windings are firmly embedded in slots to withstand the
tremendous centrifugal forces encountered at high speeds. Unwound sections of the rotor form
the pole faces between the winding slots. The windings in the cylindrical type rotor are arranged
to form two or four distinct poles.
Rotor poles
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The rotor poles are supplied with direct current (d.c.) from an exciter. If the exciter
equipment is a conventional d.c. generator or is static, the d.c. excitation current is fed into the
field windings via carbon brushes on a pair of shaft-mounted slip-rings.
Brushless excitation
To eliminate the maintenance problems associated with rotating contacts, a brushless
arrangement is usual for marine generators. All brush gear, commutators and slip rings are
eliminated by using an a.c. exciter with its output being rectified by shaft-mounted silicon diodes.
The diodes are connected as a three phase
a.c./d.c. bridge circuit. The six diodes, mounted on the
shaft, convert the a.c. exciter output to d.c. which-is
then fed directly into the main generator rotor field
windings. The a.c. exciter has its own d.c. fields poles
fitted on its stator while the rotor carries its threephase a.c. exciter output windings. This construction
layout is inverted compared with that of the main
generator.
Brushless excitation scheme.
Cooling
Power losses, typically 10% of the generator rating, cause internal heating in the
windings and magnetic cores of both rotor and stator. This heat must be continuously transferred
out of the generator to prevent excessive temperature rise causing breakdown of winding
insulation. Cooling air is forced through ventilation ducts in the stator core, between rotor poles
and through the air gap (a few millimeters) between stator and rotor.
Heating during stoppage
While the generator is stopped during standby or maintenance periods, low power electric
heaters within the machine prevent internal condensation forming on the winding insulation.
These heaters may be switched on manually or automatically from auxiliary contacts on the
generator circuit-breaker. Heater power supplies are normally 220 V a.c. single-phase supplied
from a distribution box local to the generator.
The stator core is assembled from laminated steel with the windings housed in slots around the
inner periphery of the cylindrical core.
The stator
coils
are
interconnected (in
the end-winding
regions) to form
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three separate phase windings with six ends. These phase ends are found in the stator terminal
box.
Automatic Voltage Regulation
Sudden load current surges (e.g. due to motor starting) cause a corresponding change in
its output voltage. This is due to an internal voltage drop in the generator windings and the effect
is usually called voltage dip. Similarly, load shedding will produce an over voltage at the bus-bars.
An un-regulated or non-compounded generator excitation system would not be realistic on board
ship due to the varying voltage caused by the fluctuating load demand. Automatic voltage
regulation (AVR) equipment is necessary to rapidly correct such voltage changes.
An AVR will control the generator’s voltage to ±2.5% (or better) of its set value over the full load
range. This is its steady-state voltage regulation. Transient voltage dip is usually limited to 15%
for a specified sudden load change with recovery back to rated voltage within 1.5 seconds. The
AVR senses the generator output voltage and acts to alter the field current to maintain the voltage
at its set value.
The control circuit for a modern AVR consists of transformers, rectifiers, zener diodes,
transistors and thyristors. These are mounted on one or more circuit cards fitted either within the
switchboard or local to the generator. Although the AVR control circuit design varies with the
manufacturer, the basic scheme contains the elements shown.
AVR block diagram
The voltage sensing unit transforms down, rectifies and smoothes the generator output
voltage. This produces a low voltage d.c. signal that is proportional to the a.c. generator voltage.
This actual d.c. signal is compared with a set d.c. value produced by a reference circuit of zener
diodes and resistors. An error signal output from the comparator is then amplified and made
suitable for driving the field circuit regulating thyristor(s). A thyristor is a fast-acting electronic
switch controlled by a voltage signal at its gate terminal. This device rectifies and regulates the
generator field current.
Insulation test of generator & wiring of AVR
Electronic components such as transistors, integrated circuit chips, thyristors, etc. are likely to
be damaged during a high voltage (500 V) megger test. To test the generator and its cables to
earth and protect the electronic parts, either:
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1. Short-circuit all outgoing cable terminals during the IR test
2. Remove electronic card(s)
3. Disconnect all cables at both ends and test separately
Alternators in parallel
Main generator units (gas-turbine, steam turbine or diesel drives) have to be run in
parallel to share a total load that exceeds the capacity of a single machine. Changeover of main
and standby generator units requires a brief parallel running period to achieve a smooth transition
without blackout. Essentially, parallel running is achieved in the two stages of:
1. Synchronising
2. Load Sharing
Both operations are carried out automatically but manual control is still in common use and is
generally provided as a back-up to the auto control. The generator already on the bus bars is
called the running machine and the generator to be brought into service is the incoming machine.
To smoothly parallel the incoming generator, it must be synchronised with the live bus-bars.
Breaker closing when not synchronised
At the instant of closing the breaker, the voltage phase difference causes a large
circulating current between the machines which produces a large magnetic force to pull the
generator voltages (and field poles) into synchronism. This means rapid acceleration of one rotor
and deceleration of the other. The large forces may physically damage the generators and their
prime-movers and the large circulating current may trip each generator breaker. Result - Blackout,
danger and embarrassment.
To achieve smooth manual synchronising, the incomer must be brought up to speed to
obtain approximately the same frequency as shown on the bus-bar frequency meter e.g. 60 Hz.
The incoming generator voltage is set by its AVR to be equal to the bus-bar voltage.
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Fine tuning of the speed can
now be observed on the
synchroscope
or
synchronising lamps. The
incomer is adjusted so that the
synchroscope
indicator
rotates slowly clockwise (fast
direction) at about 4 seconds
per indicator revolution.
The circuit-breaker
should be closed as the
indicator approaches the 12
o’clock (in-phase) position.
Breaker closing between 5 to
and 5 past the 12 o’clock
synchroscope position is
satisfactory as long as the
pointer rotation is fairly slow.
Back -up
As
a
back-up,
or
alternative,
to
the
synchroscope a set of lamps
may be used. The correct
synchronised position may be shown by either of the following methods:
1. Lamps dark method (2 lamps)
2. Lamps bright method (2 lamps)
3. Sequence method (3 lamps)
4. In each case the lamps are connected between the incoming generator and the bus-bars.
The sequence method is preferred as it displays a rotation of lamp brightness which
indicates whether the incoming machine is running fast (clockwise) or slow (anticlockwise).
As with the synchroscope, the lamp sequence must appear to rotate slowly clockwise.
Correct synchronisation occurs when the top lamp is dark and the two bottom lamps are equally
bright.
Without synchroscope or lights
Connect a voltmeter across one pole of the
open incoming generator circuit breaker. This
procedure is more easily (and safely) performed at the
synchroscope terminals behind the door of the
synchronising panel at the front of the main
switchboard. Check the circuit diagrams before such
testing. Adjust the generator speed until the voltmeter
very slowly fluctuates from zero to maximum. Close
the breaker when the voltmeter indication passes
through zero
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The total bus-bar load can
now be shared between generators or
totally transferred to the new
machine. Manual kW load sharing is
achieved by raising the governor
setting of the incoming machine
while lowering the setting on the
running machine. The balance of
power sharing is dictated by the
governor (speed) droop of each
generator prime mover. For equal load sharing of kW and kVAr, each machine must have similar
droop characteristics which are typically 2-4% between no-load and full-load values.
Generator protection
The protection of a generator is largely based on the sensing of current and voltage from
CTs and VTs. The number and type of protective relay functions increases with the generator
kVA rating and voltage level. Protective relays are electromagnetic (traditional) or electronic
(increasingly more common) which are mounted on the generator front panel of the main
switchboard.
Over Current Inverse Time
The Over Current Inverse Time relay function monitors general balanced overloading and has
current/time settings determined by the overall protective discrimination scheme.
• Typical setting ranges for current (I) and time (t) are:
• I > : 0.7-2.In, (In = normal or rated generator current) and t: 1-10s
Overcurrent (instantaneous)
“Instantaneous” trip to protect against extremely high overcurrent caused by a shortcircuit fault. Typical setting ranges are: I > : 2-10.In, and t: 0.1-1s
Reverse Power
Generators intended to operate in parallel must have reverse power protection. A reverse
power relay monitors the direction of power flowing between the generator and the load. If a
prime-mover failure occurred the generator would act as a motor. The reverse power relay detects
this fault and acts to trip the generator circuit-breaker.The Reverse power relay operation is easily
checked during a generator changeover. The outgoing generator is gradually throttled down so
that it motors causing the reverse power relay to trip its generator circuit-breaker.
Main Switchboard
The central section of the main switch-board is used for the control of the main
generators. The switchgear cubicles on either side of the generator panels are used for
essential services and flanking these are the grouped motor starter panels.
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Main switchboard layout
Handles for opening the doors on switchboard cubicles are usually linked (or interlocked)
to an isolating switch. This ensures that supplies to components in the cubicle are switched off
before the door can be opened. Fused isolators are isolating switches that incorporate fuses. The
action of opening the switch isolates the fuses so that they can be replaced safely. Fused isolators
can also be interlocked with the cubicle door handle. Motor starters frequently incorporate this
arrangement. One type of interlocked fused isolator can be completely withdrawn and removed to
ensure complete safety when carrying out maintenance on equipment. Maintenance on fused
isolators consists of periodically checking the operating mechanism. Contacts must be inspected
for damage and lightly greased with an electrical lubricant. The interlock mechanism (if fitted)
should also be examined for correct and safe operation.
A separate section switches the three phase 220 V a.c. low power and lighting services.
The alarms and insulation resistance (earth fault) monitors are fitted separately on both the 440 V
and the 220 V sections. The 440/220 V lighting transformers may be located inside the main
switchboard or, more likely, will be separately mounted nearby.
The main generator supply cables are connected directly to their respective circuitbreakers. Short copper bars from each generator circuit breaker connect it to the three bus-bars
which run through the length of the switchboard. The bus-bars may be seen if the rear doors of
the switchboard cubicle are opened, but they may be in a special enclosed bus-bar duct acting as
an internal fire barrier.
Switchboard instruments and controls for particular functions are grouped together. For
example, the generator synchronising panel has all the instruments, relays and switches necessary
for generator paralleling. Each generator panel has all the instruments, relays, switches, controls
and status lamps necessary for control of the generators. The instruments on panels of outgoing
circuits are usually limited to an ammeter, status lamps, function switches (e.g. manual/off/auto)
and push buttons.
Low power control and instrument wiring is of relatively small cross-section, with
multicoloured plastic insulation which is clearly identified against the larger main power cables.
The instrumentation and control wiring is supplied from fuses which are located behind the
appropriate panel. Green and yellow striped earth wiring from instruments and panel doors etc., is
connected to a common copper earth-bonding bar running the length of the switchboard at its rear.
This earth bar is electrically bonded to the ship’s steel hull.
Main Circuit Breakers
A circuit breaker is an automatically-operated electrical switch designed to protect an
electrical circuit from damage caused by overload or short circuit. Unlike a fuse, which operates
once and then has to be replaced, a circuit breaker can be reset (either manually or automatically)
to resume normal operation. When an alternating current is closed, the current reaches a steady
value after a transient process. The time depends upon the resistive, inductive and capacitive
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elements of the circuit. The highest switching current is achieved if switching is effected at zero
voltage (very high peak currents can develop if the switch is closed on short circuit conditions).
Opening
Below a threshold voltage, any circuit can be opened without any arc formation. In
practice, however, the commonly used switches do produce an arc while interrupting the current.
The arc must be either kept limited or extinguished at the earliest in order not to damage the
contacts.
Contacts
Copper is by far the most widely used contact material. But since non-conducting layers
are formed on copper contacts as a result of switching, a wiping action is provided while
designing copper contacts. These are also plated wih a layer of silver in many applictions. In low
voltage circuits, silver is also in use as contact material
Arc extinction
Since switching almost invariably gives rise to arcing, extinguishing such arcs assumes vital
importance to prolong contact life. The following methods are employed:
• a) Lengthening of the arc till it extinguishes
• b) Use of air
Insulation
The contacts need to be kept properly insulated from other metal parts including the body.
Different insulating materials are in use. The most commonly used material is cast epoxy. Besides,
PVC, polystyrene, polycarbonate and ceramics are also in use.
Operation
The circuit breaker must detect a fault condition. In low-voltage circuit breakers this is
usually done within the breaker enclosure. Once a fault is detected, contacts within the circuit
breaker must open to interrupt the circuit; some mechanically stored energy within the breaker is
used to separate the contacts, although some of the energy required may be obtained from the
fault current itself. When a current is interrupted, an arc is generated - this arc must be contained,
cooled, and extinguished in a controlled way, so that the gap between the contacts can again
withstand the voltage in the circuit. Finally, once the fault condition has been cleared, the contacts
must again be reclosed to restore power to the interrupted circuit.
Magnetic circuit breaker
Magnetic circuit breakers use a solenoid, whose pulling force increases with the current.
The circuit breaker contacts are held closed by a latch. As the current in the solenoid increases
beyond the rating of the circuit breaker, the solenoid's pull releases the latch which then allows
the contacts to open by spring action. Some types of magnetic breakers incorporate a hydraulic
time delay feature using a viscous fluid. The core is restrained by a spring until the current
exceeds the breaker rating. During an overload, the speed of the solenoid motion is restricted by
the fluid. The delay permits brief current surges beyond normal running current for motor starting,
energizing equipment, etc. Short circuit currents provide sufficient solenoid force to release the
latch regardless of core position thus bypassing the delay feature.
Thermal circuit breaker
Thermal breakers use a bimetallic strip, which heats and bends with increased current,
and is similarly arranged to release the latch. This type is commonly used with motor control
circuits. Thermal breakers often have a compensation element to reduce the effect of ambient
temperature on the device rating.
Thermomagnetic circuit breaker
Thermomagnetic circuit breakers, which are the type found in most distribution boards,
incorporate both techniques with the electromagnet responding instantaneously to large surges in
current (short circuits) and the bimetallic strip responding to less extreme but longer-term
overcurrent conditions.
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Arc interuption
Miniature low-voltage circuit breakers use air alone to extinguish the arc. Larger ratings
will have metal plates or non-metallic arc chutes to divide and cool the arc. Air circuit breakers
may use compressed air to blow out the arc, or alternatively, the contacts are rapidly swung into a
small sealed chamber, the escaping of the displaced air thus blowing out the arc. Circuit breakers
are usually able to terminate all current flow very quickly: typically the arc is extinguished
between 30 ms and 150 ms after the mechanisim has been tripped, depending upon the age and
construction of the device.
Protection
Under short-circuit conditions, a current many times greater than normal can flow. When
electrical contacts open to interrupt a large current, there is a tendency for an arc to form between
the opened contacts, which would allow the flow of current to continue. Therefore, circuit
breakers must incorporate various features to divide and extinguish the arc. In air-insulated and
miniature breakers an arc chute structure consisting of metal plates or ceramic ridges cools the
arc, and magnetic blowout coils deflect the arc into the arc chute.
Types of circuit breaker
Low voltage (less than 1000 V AC) include:
• MCB (Miniature Circuit Breaker)—rated current not more than 100 A. Trip
characteristics normally not adjustable. Thermal or thermal-magnetic operation.
• MCCB (Moulded Case Circuit Breaker)—rated current up to 1000 A. Thermal or
thermal-magnetic operation. Trip current may be adjustable in larger ratings.
• Low voltage power circuit breakers can be mounted in multi-tiers in LV switchboards.
Each main circuit breaker is mounted on guide rails inside a main switchboard cubicle from
which it must be withdrawn and isolated from the bus-bars for maintenance and testing. The
breaker and its guide rails are usually mounted in a special cassette bolted into the switchboard
cubicle and electrically connected to the bus-bars. If repair work demands that the breaker is to
be completely removed from its cassette then usually a special hoist or fork-lift is required for
large, heavy-duty units.
The action of withdrawing the circuit breaker causes a safety shutter to cover the live bus-bar
contacts at the rear of its cubicle. The mechanical linkage in a circuit-breaker is quite complex
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and should not be interfered with except for maintenance and lubrication as specified by the
manufacturer. The main fixed and moving contacts are of copper (sometimes of special arcresistant alloy or silver tipped) and usually silver-alloy coated.
Main contacts should not be scraped or filed. If the main contacts suffer severe burning they
will probably require realignment as specified by the manufacturer.
Arcing contacts normally suffer burning and may be dressed by a smooth file as recommended by
the manufacturer. Carborundum and emery should not h used - the hard particles can embed
themselves in the soft contacts and cause future trouble.
The arc chutes or arc splitter boxes confine and control the inevitable arc to rapidly
accelerate its extinction. These must be removed and inspected for broken parts and erosion of the
splitter plates.
Single phase a.c. motor
If the rotating magnet is able to keep up with the frequency of the alternating current
energizing the electromagnet
windings (coils), it will
continue to be pulled around
clockwise.
However, clockwise
is not the only valid direction
for this motor's shaft to spin.
It could just as easily be
powered in a counterclockwise direction by the
same AC voltage waveform.
With the exact same sequence
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of polarity cycles (voltage, current, and magnetic poles produced by the coils), the magnetic rotor
can spin in either direction.
These motors need a little help getting started. Once helped to spin in a particular
direction. they will continue to spin that way as long as AC power is maintained to the windings.
Where that help comes from for a single-phase AC motor to get going in one direction can vary.
Usually, it comes from an additional set of windings positioned differently from the main set, and
energized with an AC voltage that is out of phase with the main power.
These supplementary
coils are typically connected
in series with a capacitor to
introduce a phase shift in
current between the two sets
of windings. That phase shift
creates magnetic fields from
coils 2a and 2b that are
equally out of step with the
fields from coils 1a and 1b.
The result is a set of magnetic
fields with a definite phase rotation. It is this phase rotation that pulls the rotating magnet around
in a definite direction.
Capacitor phase shift adds second phase.
3 Phase AC induction motors
3 phase AC motors require no such trickery to spin in a definite direction. Because their
supply voltage waveforms already have a definite rotation sequence, so do the respective
magnetic fields generated by the motor's stationary windings. In fact, the combination of all three
phase winding sets working together creates what is often called a rotating magnetic field.
Three-phase AC motor: A phase sequence of 1-2-3 spins the magnet clockwise, 3-2-1spins the
magnet counterclockwise. The short explanation of the induction motor is that the rotating
magnetic field produced by the stator drags the rotor around with it.
The longer more correct explanation is that the stator's magnetic field induces an alternating
current into the rotor squirrel cage conductors which constitutes a transformer secondary. This
induced rotor current in turn creates a magnetic field. The rotating stator magnetic field interacts
with this rotor field. The rotor field attempts to align with the rotating stator field. The result is
rotation of the squirrel cage rotor.
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Three-phase AC motor: A phase sequence of 1-2-3
spins the magnet clockwise, 3-2-1 spins the
magnet counterclockwise.
Synchronous speed
• If there were no mechanical motor torque load, no
bearing, windage, or other losses, the rotor would
rotate at the synchronous speed.
• The synchronous speed is given by:
• Ns =f.60/P rpm
• Ns = synchronous speed
• f = frequency of applied power, Hz
• P = total number of pair of poles
• Example: The synchronous speed of 6-pole motor supplied at 60 Hz:
• Ns = 60x60/3 = 1200 rpm
However, the slip between the rotor and the synchronous speed stator field develops torque. It
is the magnetic flux cutting the rotor conductors as it slips which develops torque. Thus, a loaded
motor will slip in proportion to the mechanical load. If the rotor were to run at synchronous speed,
there would be no stator flux cutting the rotor, no current induced in the rotor, no torque.
Torque
When power is first applied to the motor, the rotor is at rest, while the stator magnetic
field rotates at the synchronous speed Ns. The stator field is cutting the rotor at the synchronous
speed Ns. The current induced in the rotor shorted turns is maximum, as is the frequency of the
current, the line frequency. As the rotor speeds up, the rate at which stator flux cuts the rotor is
the difference between synchronous speed Ns and actual rotor speed N, or (Ns - N). The ratio of
actual flux cutting the rotor to synchronous speed is defined as slip.
Slip
• Slip = (Ns - N)/Ns
• where: Ns = synchronous speed, N = rotor speed
• Slip at 100% torque is typically 5% or less in induction motors. Why is it so low? The
stator magnetic field rotates at 60 Hz. The rotor speed is 5% less. The rotating magnetic
field is only cutting the rotor at 3 Hz. The 3 Hz is the difference between the synchronous
speed and the actual rotor speed. If the rotor spins a little faster, at the synchronous speed,
no flux will cut the rotor at all.
Rotor direction reversal
By swapping over any two supply line connections at the terminal box, reverses the
direction of rotating magnetic field. This
reverses the direction of rotation of the
rotor.
Three-phase, three-wire “Y"
connection does not use the
neutral wire.
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•
•
•
•
Three-phase, three-wire Δ
connection has no common.
• The conductors connected to the
three points of a three-phase
source or load are called lines.
• The three components comprising
a three-phase source or load are
called phases.
• Line voltage is the voltage
measured between any two lines in
a
three-phase
circuit.
Phase voltage is the voltage measured across a single component in a three-phase source
or load.
Line current is the current through any one line between a three-phase source and load.
Phase current is the current through any one component comprising a three-phase source
or load.
In “star” circuits, Eline = √3 x Ephase and Iline = Iphase
In “delta” circuits, Eline = Ephase and Iline = √3 x Iphase
3 phase squirrel cage induction motor
An induction motor is composed of a rotor, known as an armature, and a stator containing
windings connected to a 3 phase energy source. The stator is wound with pairs of coils
corresponding to the phases of electrical energy available. The 3-phase induction motor stator
above has 3-pairs of coils, one pair for each of the three phases of AC. The individual coils of a
pair are connected in series and correspond to the opposite poles of an electromagnet. The
windings of a three-phase motor are installed in the stator slots. The ends of the stator windings
are terminated in the stator terminal box where they are connected to the incoming cable from the
three-phase a.c. power supply. The coils are wound on an external fixture, then worked into the
slots. Insulation wedged between the coil periphery and the slot protects against abrasion. The
coils are embedded into slots cut into the stator laminations.
The stator laminations are thin insulated rings with slots punched from sheets of electrical
grade steel. A stack of these is secured by end screws, which may also hold the end housings.
The key to the popularity of the AC induction motor is simplicity as evidenced by the simple
rotor. The rotor consists of a shaft, a steel laminated rotor punched from magnetic steel sheets and
including slot cutouts, are sandwiched on the rotor shaft. Conducting bars of copper or aluminum
are inserted in the slots and cast end rings form a low-resistance short circuit. No insulation is
required between the core and the bars because of the low voltages induced into the rotor bars.
The size of the air gap, between the rotor bars and stator windings, necessary to obtain the
maximum field strength is small.
.
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Stator frame showing slots for windings
3-Φ windings.
As compared to a DC motor armature,
there is no commutator. This eliminates the
brushes, arcing, sparking, graphite dust, brush
adjustment and replacement, and re-machining of
the commutator. The squirrel cage conductors
may be skewed, twisted, with respect to the shaft.
The misalignment with the stator slots reduces
torque pulsations. Both rotor and stator cores are
composed of a stack of insulated laminations.
The laminations are coated with insulating oxide
or varnish to minimize eddy current losses. The
alloy used in the laminations is selected for low
hysteresis losses.
Motor Starters
When an induction motor is connected directly to its three-phase a.c. supply voltage, a
very large stator current of 5 to 8 x full-load current (FLC) is taken. This is due to the maximum
rate of flux cutting (100%) in the rotor, creating large induced rotor currents. The corresponding
supply power factor at start-up is very low, typically about 0.2 lagging, which rises to about 0.5
lagging on no-load then to about 0.85 lagging on full-load. This starting surge current reduces as
the motor accelerates up to its running speed.
Most induction motors are Direct-on-Line (DOL) switch-started because such starters are
inexpensive and simple to operate and maintain. The high starting current surge will not cause
serious heating damage to the motor unless the motor is repeatedly started and stopped in a short
time period. When very large motors are started DOL they cause a significant disturbance of
voltage (voltage dip) on the supply lines due to the large starting current surge.
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DOL Starter
This voltage disturbance may result in the malfunction of other electrical equipment
connected to the supply e.g. lighting dip and flickering effects. To limit the starting current some
large induction motors are started at reduced voltage and then have the full supply voltage
reconnected when they have accelerated close to their rated speeds.
Reduced voltage starting is used for large motors driving loads like cargo pumps and bow
thrusters.
• Two methods of reduced voltage starting by switching are called
1. star-delta starting
2. autotransformer starting.
Star-Delta Starting
If a motor is direct-on-line started with the stator winding star connected, it will only take
one-third of the starting current that it would take if the windings were delta connected. The
starting current of a motor which is designed to run delta connected can be reduced in this way.
For large power motors, the phase windings are automatically switched using contactors
controlled by a timing relay. A choice of time delay relays are available whose action is governed
by thermal, pneumatic, mechanical or electronic control devices.
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Time delay is necessary between the KM2 auxiliary contacts to provide an electrical
interlock between contactors KM1 and KM3. This is to prevent a full short-circuit fault across the
supply lines during the’ changeover from star to delta.
At the instant of starting when the supply has just been switched on and the motor has not
yet started to rotate, there is no mechanical output from the motor. The only factors which
determine the current taken by the motor are the supply voltage (V) and the impedance of the
motor phase windings (ZPH).
•
•
•
•
•
For Delta : Phase current at start IPH = V/Z
Line starting current IL =√3. IPH = √3. V/Z
For Star : Phase current at start IPH = V/ √3 Z
Line starting current IL =IPH = V/ √3 Z
Line starting current Delta/ Line starting current Star
= √3. V/Z/ V/ √3 Z = 3
This shows that the starting current of a delta connected motor can be reduced to one
third if the motor is star connected for starting. The shaft torque is also reduced to one-third
which reduces the shaft acceleration and increases the run-up time for the drive but this is not
usually a problem. When an induction motor is running on load it is converting electrical energy
input to mechanical energy output. The input current is now determined by the load on the motor
shaft. An induction motor will run at the same speed when it is star connected as when it is delta
connected because the flux speed is the same in both cases being set by the supply frequency.
Remember that the motor copper losses are produced by the I2R heating effect so the motor will
run (√3)2 = 3 times hotter if left to run in the star connection when designed for delta running.
This malfunction may occur if the control timing sequence is not completed or the star contactor
remains closed while a mechanical interlock prevents the delta contactor from closing. For
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correct overcurrent protection, the overcurrent relays must be fitted in the phase connections and
not in the line connections.
Autotransformer Starting
Starting a large motor with a long run-up period will demand a very high current surge
from the supply generator for a few seconds. This causes a severe voltage dip which affects every
load on the system. Reduced voltage starting will limit the starting surge current. One way to
reduce the initial voltage supplied to the motor is to step it down using a transformer. Then, when
the motor has accelerated up to almost full speed, the reduced voltage is replaced by the full
mains voltage.
The transformer used in this starter is not the usual type with separate primary and
secondary windings. It is an autotransformer which uses only one winding for both input and
output. This arrangement is cheaper, smaller and lighter than an equivalent double-wound
transformer and it is only in operation during the short starting period. For induction motor
starting, the autotransformer is a 3-phase unit and because of expense, this method is only used
with large motor drives, e.g. electric cargo pumps.
The supply voltage is connected across the complete winding and the motor is connected to the
reduced voltage tapping. The autotransformer usually has a few tapping points to give a set of
reduced voltages (e.g. 40%, 50% and 65%) which help to match the motor current demand to the
supply capability. As with the star-delta starter, the autotransformer may use what is called an
open-transition switching sequence or a closed-transition switching sequence between the start
and run conditions. In the former, the reduced voltage is supplied to the motor at start then
disconnected and the full supply voltage rapidly reconnected to the motor. The problem with
open transition is that a very large surge current can flow after the transition from reduced to full
voltage.
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Contactors
When a relay is used to switch a large amount of electrical power through its contacts, it
is designated by a special name: contactor. Contactors typically have multiple contacts, and those
contacts are usually (but not always) normally-open, so that power to the load is shut off when the
coil is de-energized. Perhaps the most common industrial use for contactors is the control of
electric motors.
Construction
A contactor is composed of three different systems. The contact system is the current
carrying part of the contactor. This includes Power Contacts, Auxiliary Contacts, and Contact
Springs. The electromagnet system provides the driving force to close the contacts. The enclosure
system is a frame housing the contact and the electromagnet. Enclosures are made of insulating
materials like Bakelite, Nylon 6, and thermosetting plastics to protect and insulate the contacts
and to provide some measure of protection against personnel touching the contacts. Open-frame
contactors may have a further enclosure to protect against dust, oil, explosion hazards and
weather. Contactors used for starting electric motors are commonly fitted with overload
protection to prevent damage to their loads. When an overload is detected the contactor is tripped,
removing power downstream from the contactor.
A basic contactor will have a coil input (which may be driven by either an AC or DC
supply depending on the contactor design) and generally a minimum of two poles which are
controlled.
Operating Principle
Unlike general-purpose relays, contactors are designed to be directly connected to high-current
load devices, not other control devices. Relays tend to be of much lower capacity and are usually
designed for both Normally Closed and Normally Open applications. Devices switching more
than 15 amperes or in circuits rated more than a few kilowatts are usually called contactors. Apart
from optional auxiliary low current contacts, contactors are almost exclusively fitted with
Normally Open contacts.
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When current passes through the
electromagnet, a magnetic field is produced which
attracts ferrous objects, in this case the moving core
of the contactor is attracted to the stationary core.
Since there is an air gap initially, the electromagnet
coil draws more current initially until the cores
meet and reduct the gap, increasing the inductive
impedance of the circuit.
For contactors energized with alternating current, a
small part of the core is surrounded with a shading
coil, which slightly delays the magnetic flux in the
core. The effect is to average out the alternating
pull of the magnetic field and so prevent the core
from buzzing at twice line frequency.
Three-phase, 440 volt AC power comes in to the
three normally-open contacts at the top of the
contactor via screw terminals labeled "L1," "L2,"
and "L3". Power to the motor exits the overload assembly at the bottom of this device via screw
terminals labeled "T1," "T2," and "T3."
The main power circuit contacts for the motor are held
open by spring tension
When the coil becomes energized, the magnetic attraction between the armature and the
magnet overcomes spring tension, and the main contacts for the motor close. The motor now
operates.
Induction Motor Speed Control
Induction motor operates as an almost constant speed drive over its load range. This feature is
satisfactory for most of the ship’s auxiliary services supplying power to ventilation fans and
circulating pumps. Variable speed control is necessary for cranes, winches, windlass etc.
• Two main forms of speed change/control are available:
1. Pole-changing for induction motors to give two or more fixed speeds 3-speed winches
2. Continuously variable speed control e.g. smooth control of deck cranes, winches and
electric ship propulsion using variable frequency
Pole Changing
Fixed set speeds can be obtained from a induction motor by using a dual wound stator
winding, each winding being designed to create a different number of magnetic poles.
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A dual-wound induction motor is arranged to create 6 pole and 10 pole stator magnetic fields.
Estimate the rated speeds assuming that the rotor slips by 5% and the power supply is at a
frequency of 60 Hz.
• NS = f.60/p rpm
• For 6 poles: Ns = 60x60/3=1200 rpm
NR = 0.95x1200=1140 rpm
For 10 poles: Ns = 60x60/5=720 rpm
NR = 0.95x720=684 rpm
A 3-speed pole-change winch motor can be arranged by having two cage rotors mounted
on the same drive shaft. One stator winding (usually 24-pole) gives a low speed while the other is
dual wound to give medium speed 8 pole) and high speed (4 pole) outputs. Speed control and
drive direction are achieved by a set of switching and reversing contactors operated from the
winch control pedestal. Remember that to reverse the rotation of an induction motor it is
necessary to switch over two of the supply lines to the stator winding.
An alternative method giving two
fixed speeds in a 2:1 ratio from a
cage-rotor induction motor is to
use a single stator winding which
has
centre-tap
connections
available on each phase. This
method uses a starter with a set
of contactors to switch the phase windings into either single-star (low speed) or double-stars (high
speed).
Continuously variable speed
A continuously variable speed range of motor control involves more complication and
expense than that required to obtain a couple of set speeds. Various methods are available which
include:
1. Electro- hydraulic drive.
2. Wound-rotor resistance control of induction motors.
3. Ward-Leonard d.c. motor drive.
4. Variable-frequency induction motor control
Electro-hydraulic drive
The electro-hydraulic drive, often used for deck crane control, has a relatively simple
electrical section. This is a constant single-speed induction motor supplied from a DOL or stardelta starter. The motor runs continuously to maintain oil pressure to the variable-speed hydraulic
motors.
Wound-rotor resistance control
A crude form of speed control is provided by the wound rotor induction motor. The rotor
has a 3-phase winding (similar to its stator winding) which is connected to 3 slip rings mounted
on the shaft. An external 3-phase resistor bank is connected to brushes on the rotor slip rings. A
set of contactors or a slide wiper (for small motors) varies the amount of resistance added to the
rotor circuit. Increasing the value of external resistance decreases the rotor speed.
Ward-Leonard d.c. motor drive
A constant speed induction
motor drives a d.c. generator,
which in turn supplies one or
more d.c. motors. The
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generator output is controlled by adjusting its small excitation current via the speed regulator. The
d.c. motor speed is directly controlled by the generator voltage. The motor generator requires
more space and maintenance. An alternative is a static electronic thyristor controller, which is
supplied with constant a.c. voltage and delivers a variable d.c. output voltage to control the drive
motor.
Variable-frequency control
A static electronic transistor or thyristor (high power) controller can be used to generate
such a variable frequency output to directly control the speed of the motor.
Motor Protection
Protecting an electric motor basically involves preventing the motor from getting too hot.
Every 10°C above the maximum recommended temperature of the insulation can reduce its
working life by half. Obviously, the best way to protect a motor against overheating is to directly
monitor the temperature of the motor windings. If the temperature exceeds the maximum set
value for the motor insulation its contactor is tripped to stop the motor and allow it to cool down.
Temperature sensors
Three main types of direct temperature sensors can be used. These are:
1. Thermocouple
2. Resistance temperature device (RTD)
3. Thermistor
Thermistor protection
The Thermistor sensor’s thermal characteristic more closely matches that of a motor than
the other types. Thermistors are small pellets of semiconductor material , which are embedded
into all three of the motor stator windings during manunufacture. When a thermistor gets hot,
their resistance changes dramatically. They are connected so that if the motor temperature gets
too high, the starter contactor will be tripped by an electronic relay to stop the motor. Direct
thermistor protection is usually only fitted to large motors.
Over current relay (OCR)
Most motors are protected by monitoring the temperature indirectly by measuring the
current flowing in the supply lines. This method uses electronic, thermal or electromagnetic timedelayed overcurrent relays (OCRs) in the motor starter. The system is designed so that if the
motor takes too much current because it is mechanically overloaded, the OCR will trip out the
contactor coil, after a pre-set time delay, before severe overheating can occur. The largest
overcurrent possible is the current taken when the motor has stalled. This, of course, is the
starting current of the motor which will be about five times the full load current. The contactor is
capable of tripping this stalled current quickly and safely.
Short-circuit
If a short-circuit occurs in the motor, the starter, or the supply cable, then a huge fault
current will flow. If the contactor tries to open under short-circuit conditions, serious arcing will
occur at its contacts such that it may fail to interrupt the fault current. The prolonged short-circuit
current will cause serious damage to the motor, starter and cable with the attendant risk of an
electrical fire.
To prevent this, a set of fuses or a circuit breaker is fitted upstream of the contactor which
will trip out almost instantaneously thereby protecting the contactor during a short-circuit fault. It
is important that the tripping characteristics of the OCR and fuses/circuit breaker are co-ordinated
so that the contactor trips on thermal overcurrent, while the fuses/circuit breaker interrupt shortcircuit fault currents. This contactor-fuse arrangement is usually called back-up protection.
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It must be emphasised
that the motor fuses are not
chosen for their rated current but
for their inverse current/time (I/t)
characteristic. This means that the
current rating of fuses used to
protect a motor does not appear to
have any direct relationship to the
FLC rating of the motor. Fuses
used for back-up protection for
motor circuits have a special
time/current characteristic.
They are generally carrying steady
currents well below their rated
capacity to allow for short
duration DOL starting currents without blowing. Consequently they do not protect against normal
overloads but do protect the motor and supply system against short-circuit fault. A typical fuse
designation for motor circuits could be ‘32M63’, which indicates a continuous rating of 32 A but
a rating of 63A for starting period.
Types of OCR
• There are three types of OCR used for motor protection:
1. Electronic
2. Thermal
3. Electromagnetic
Electronic OCIT
Electronic overcurrent inverse time (OCIT) have largely superseded electromagnetic
types as they have no moving parts (except for their output trip relay) and their very reliable
tripping characteristics can be closely matched to the motor circuit. Such relays are robust,
smaller and lighter than the equivalent electromagnetic type.
Electromagnetic OCR
Although electromagnetic devices with time delays can give adequate protection against
large, sustained overloads to motors which are operated well below their maximum output and
temperature, they have been found to be inadequate for continuous maximum rated (CMR)
motors.
Thermal OCR
Most motors are protected by less expensive thermal OCRs. Inverse-time thermal OCRs
usually work with bi-metal strips. The strips are heated by the motor current and bend depending
on the temperature. If the motor takes an overload current, the strips operate a normally-closed
(NC) contact which trips out the line contactor to stop the motor. The minimum tripping current
of such a device can be adjusted over a small range. This adjustment alters the distance the strips
have to bend before operating the trip contact. For larger motors, the heaters do not carry the full
motor current. They are supplied from current transformers (CTs) which proportionally stepdown, the motor current so that smaller heater components may be used.
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Bimetallic overcurrent action.
Undervoltage protection
Undervoltage protection is necessary in a distribution system that supplies motors. If
there is a total voltage loss or black-out, all the motors must be disconnected from the supply.
This is to prevent all the motors restarting together which would result in a huge current surge,
tripping out the generator again. Motors must be restarted in a controlled sequence after a supply
failure.
Undervoltage protection for motors is simply provided by the spring-loaded motor
contactor because it will drop out when the supply voltage is lost. When the supply voltage
becomes available, the motor will not restart until its contactor coil is energised. This will usually
require the operator to press the stop/reset button before initiating the start sequence.
For essential loads, the restart may be performed automatically by a sequence restart
system. This system ensures that essential services are restarted automatically on restoration of
supply following a blackout. Timer relays in the starters of essential motor circuits are set to
initiate start-up in a controlled sequence.
Single-phasing
To operate correctly, induction motors must be connected to a three phase a.c. supply.
Once started they may continue to run even if one of the three supply lines becomes disconnected.
This is called single-phasing and can result in motor burn-out.
Single-phasing, is usually caused when one of the three back-up fuses blows or if one of
the contactor contacts is open-circuited. The effect of single-phasing is to increase the current in
the two remaining lines and cause the motor to become very noisy due to the uneven torque
produced in the rotor. An increase in line current due to single-phasing will be detected by the
protective OCR.
The three thermal elements of an OCR are arranged in such a way that unequal heating of
the bi-metal strips causes differential movement, which operates the OCR switch contacts to trip
the motor contactor.
For star connected motor windings the phase current and line currents are equal so the
line connected OCR is correctly sensing the winding current. If the overcurrent setting is
exceeded during a single-phase fault, the motor will be tripped off. In delta connected motor, the
phase current is about half of line current (IPH =IL/√3 = 0.577 IL). When one of the lines becomes
open-circuited a balanced three phase condition no longer exists. Now the sets of line and phase
currents are no longer balanced..
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Healthy condition (balanced)
% of rated FLC
60
70
100
Single phasing – fault condition (unbalanced)
% of rated FLC
IL2 and IL3
IA and IB
IC
102
62
131
130
79
161
243
129
185
The current in winding C is considerably higher than that in the other two windings.
When the motor is at 60% of full load and single-phasing occurs: the line currents are 102% of
the full load value but the current in winding C is 131% of Its full load value, The 102% line
current will probably not activate a line connected OCR and the motor remains connected.
However the local overheating in winding C of the motor will quickly result in damage. Motors
can be protected against this condition by using a differential type relay which trips out with
unbalanced currents.
In fact, most modern thermal OCRs for motors have this protection against singlephasing incorporated as a normal feature. If single-phasing occurs when in operation on light load,
the motor keeps on running unless the protection trips the contactor.
If the motor is stopped, it will not restart. When the contactor is closed, the motor will take a large
starting current but develop no rotating torque. The OCR is set to allow the starting current to
flow long enough for the motor, under normal conditions, to run up to speed. With no ventilation
on the stationary motor, this time delay will result in rapid and severe over-heating. Worse still, if
the operator makes several attempts to restart the motor, it will burn out. If a motor fails to start
after two attempts, you must investigate the cause.
3 bi-metals - cold position
3 bi-metals – hot (balanced)
2 bi-metals hot,1 cold (differential)
Motor enclosures
Enclosure protection for electrical equipment is defined in terms of its opposition to the
ingress of solid particles and liquids. The enclosure protection is defined by the Ingress
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Protection (IP’) Code where a two-figure number is used to indicate the degree of protection
against the ingress of solids and liquids. First digit indicates protection against ingress of solids
and the second digit indicates protection against ingress of liquids. Higher the numerals, greater
the degree of protection.
Drip-proof open ventilated motors are used where the risk of liquids leaking from
overhead pipes and valves may be a problem. Air is drawn into the machine by an internal fan to
provide cooling. The ventilation ducts are fitted with mesh screens to prevent any objects from
entering the motor and causing damage. These screens must always be kept clean and free from
dust otherwise the motor will overheat due to inadequate ventilation.
TEFV motor enclosure..
When a greater degree of protection is
required the enclosure is made Totally
Enclosed Fan Ventilated (TEFV) and jet-proof.
No external air is allowed inside the motor. To
improve heat transfer the motor casing is finned
to increase the surface area, and airflow across
the fins is achieved by means of an external fan
and cowl arrangement.
Deck motors for tankers must have a
flameproof (Exd) enclosure if they are within
3m (4.5m for some ships) of an oil tank outlet.
Motor nameplate
1. Rated Full Load Current (FLC)
2. Rated Voltage
3. Rated frequency
4. Power rating
5. Rated speed
6. Ingress Protection number (IP)
7. Insulation class
Insulation class
All electrical equipment heats up when carrying load current with the consequent rise in
temperature. This temperature rise is above that of the ambient cooling air temperature. All
marine electrical equipment is constructed and rated to work satisfactorily in a maximum ambient
air temperature of 45°C. Under these conditions the expected temperature rise will not exceed the
permitted temperature limit set for the insulation material. It is therefore the insulation material
that dictates the maximum permitted operating temperature of the electrical equipment.
For this purpose, insulation is classified according to the maximum temperature at which it is safe
to operate. Various classes of insulation are listed. A machine operating continuously with these
hot-spot temperatures would have an expected life of 15 to 20 years before the insulation failed
completely. However, the life expectancy would be halved for every 10°C above these allowed
hot-spot temperatures.
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Insulation
class
Maximum
temperature
allowed (0C)
Allowable
Typical material
temperature
rise at full
load (0C)
A
B
105
120
55
70
E
130
80
F
155
105
H
180
130
Cotton, natural silk, synthetic silk, presspan
Wire enamels with a base of polyvinyl acetyl, epoxy
or polyamide resins
Mica products, wire enamels with a base of
polyterephthalate, laminated glass fibre materials
Mica products, glass fibre, wire enamels with a base
of imide-polyester and esterimide
Mica products, glass fibre, wire enamels with a base
of pure polyimide
Electrical Practice for Tankers
Hazardous Zones ashore
Hazardous areas ashore are classified into zones which indicate the probability of an
explosive gas-air mixture being present and, therefore, the likelihood of an explosion occurring.
• Zone 0
In which an explosive gas-air mixture is continuously present, or present for long periods.
• Zone 1
In which an explosive gas-air mixture is likely to occur in normal operation.
• Zone 2
In which an explosive gas-air mixture is not likely to occur in normal operation and, if it
occurs, will exist for only a short time.
Hazardous Zones in tankers
• An area which is not classified Zone 0, 1 or 2 is assumed to be a non-hazardous or safe
area. Examples of this zoning applied to ships could be:
• Zone 0
Interior spaces of oil cargo tanks, pipes, pumps, etc.
• Zone 1
Enclosed or semi-enclosed spaces on the deck of a tanker, the boiler firing area on a LNG
gas carrier using methane boil-off as a fuel and battery rooms.
• Zone 2
1. Open spaces on the deck of a tanker
2. The cargo pump rooms of tankers are, at present, considered as falling somewhere
between Zone 0 and Zone 1.
Types of Explosion Protection
There are a number of different constructional techniques employed in preventing
electrical equipment causing explosions in hazardous areas. Some techniques, such as flameproof
enclosures, have long been established but others, such as intrinsic safety and increased safety,
are the result of developments in material and electrical/electronic‘ circuit design.
It has been internationally agreed that explosion protected equipment be identified by the symbol
“Ex” followed by a letter indicating the type of protection employed.
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Symbol Type of Protection
Exd
Flameproof enclosure
Exi
Intrinsic safety
Exe
Increased safety
Exn
Non-sparking
Exq
Powder filled (non-applicable to ships)
Exo
Oil immersed (non-applicable to ships)
Exp
Pressurisation
Exs
Special protection
Some equipment may use more than one of these types of protection in its construction.
In this case, the primary type of protection is quoted first. For example, an increased Safety motor
with a flameproof terminal box would be marked Exe d. Equipment may also be marked with a
prefix ‘E” which denotes compliance with European Standards e.g. EExe d.
Exd Flameproof Enclosure
Exd flamepaths.
Type ‘d’ protection, code EExd, uses a flameproof enclosure to contain the electrical
apparatus. The internal apparatus may include parts which arc and, surfaces which become hot.
Gas may be inside the enclosure so it must fulfill three conditions:
1. The enclosure must be strong enough to withstand an internal explosion without suffering
damage.
2. The enclosure must prevent the flame and hot gases from being transmitted to the
external flammable atmosphere.
3. The external surface temperature of the enclosure must remain below the ignition
temperature of the surrounding gas under all operating conditions.
The transmission of flame and hot gases from a flameproof enclosure is prevented because all
joints, such as flanges, spigots, shafts and bearings are closely machined to achieve a small gap
which is less than a defined maximum. The pressure of an internal explosion is then released
through the small gap between machined faces which cools the gas sufficiently to prevent it from
igniting any external flammable atmosphere.
• The maximum permitted gap depends upon three factors:
1. The type of gas with which the apparatus is safe for use. This is indicated by Apparatus
Group.
2. The width of the joint (L).
3. The volume of the’ enclosure (V).
Exi Intrinsic Safety
These are circuits in which no spark nor any thermal effect produced under prescribed
test conditions (which include normal operation and specified fault conditions) is capable of
causing ignition of a given explosive atmosphere. Generally, this means limiting the circuit
conditions to less than 30 V and 50 mA. Naturally, this restricts the use of Exi protection to low
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power instrumentation, alarm and communication circuits. The design of the circuit will depend
on the type of gas present (gas grouping).
In the UK, two grades of intrinsic safety are recognised based on the safety factor of the
equipment involved:
• Exia - the highest category based on a safety factor of 1.5 with two faults on the circuit.
• Exib - based on a safety factor of 1.5 with one fault on the circuit.
Exe Increased Safety
Increased safety equipment is based primarily on the elimination of open sparking as at
relay and switch contacts or on the commutators or slip-rings of motors and generators, and on
the close control of surface temperatures. Also, the construction of the equipment is to a very high
standard to prevent faults developing.
Extra insulation is used, creepage distances between bare terminals are made longer and
special enclosures to protect against damage due to entry of moisture and mechanical damage are
also specified.
Creepage and clearance distances.
The enclosure is made to withstand impact and to prevent ingress of solids and liquids.
Applications include cage-rotor induction motors, luminaires and connection boxes. Special Exe
cable glands, metal or plastic, are used with Exe apparatus. Power and intrinsically safe cable
runs should be separately identified. i.e. by labels or by using cables with a distinctive colour
(typically blue for Exi).
Exn Non sparking
Similar to Exe, the designation Exn applies to equipment which has no arcing contacts or
hot surfaces which could cause ignition. The Exn requirements are less stringent than for Exe,
and designs are very close to that of normal electrical apparatus. The main consideration is extra
care to ensure locking of terminal connections to avoid any risk of electric sparking or flashover.
Exp Pressurised Enclosure
Clean, dry air or an inert gas is supplied to the equipment slightly above atmospheric
pressure to prevent entry of the external flammable gas. This method is sometimes used for
motors, instrumentation enclosures and lighting. The diagrams in Fig. 6.9 show that the internal
pressure may be maintained by leakage compensation or by continuous circulation. A
pressurisation system requires a purge flow before the internal electrical equipment is permitted
to operate. Also, the pressurised enclosure must be fitted with alarm and trip signaling for a
reduction of pressure which in turn will switch-off the enclosed electrical Circuits.
Exs Special Protection
This includes precautions taken to prevent explosions which are not specifically covered
by the previous designations. The table below shows the type of protection which is allowed in
the three hazardous zones:
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Zone
0
Type of Protection
Exia
Exs (Specially certified for use in Zone 0)
1
Any type of protection suitable for Zone 0 and:
Exd; Exib; Exp; Exe and Exs
2
Any type of protection suitable for Zone 0 or 1 and:
Exn; Exo and Exq
Electrical Testing in Hazardous Areas
All electrical apparatus and associated circuits are required to be tested periodically in
accordance with a definite testing routine with recorded test results. Insulation resistance, earth
loop resistance and earth continuity resistance tests are required to be made, the last two in
relation to the setting or rating of the protective devices associated with the apparatus and its
circuitry. It is important that insulation resistance tests are NOT made in such a way that the
safety devices and insulation used in intrinsically safe apparatus and circuits are damaged by
excess test voltages.
No apparatus should be opened in a danger area until it has been made dead and effective
measures (e.g. locking-off the isolating switch) have been taken to prevent its being made live
again inadvertently. Where, for the purpose of electrical testing, it is necessary to restore the
power supply before the apparatus is reassembled, tests should be made using a suitable gas
detector and continued during the operation to ensure that the combustible does not approach the
explosive limit. Unless the hazardous area can be made gas-free or otherwise safe, or the
electrical equipment is removed from the area, then insulation resistance testing should be carried
out using a 500 V d.c. tester of certified intrinsically safe design.
The testing and maintenance of flameproof or intrinsically safe equipment should be
entrusted only to competent persons who have received instruction in the special techniques
involved. The body material of instruments and tools required for maintenance purposes should
be designed so that they will not make a hot spark when dropped. The energy output of all
intrinsically safe instruments should be so small that they do not produce hot sparks. An
insulation tester has a drooping characteristic to prevent high currents and may be intrinsically
safe when applied to circuits of small inductance or capacitance but a risk may arise when such
energy-storing properties of a circuit have an appreciable value. Where such instruments are used
the test leads should be firmly connected throughout and on completion of the test they should not
be detached until the circuit has been discharged through the testing instrument (leave the tester
for one minute after test is finished).
Stroboscopic effect of light
The normal sinusoidal a.c. voltage wave-form causes discharge lamps to extinguish at the
end of every half cycle, i.e. every 10 ms at 50 Hz or every 6.7 ms at 60 Hz. Although this rapid
light fluctuation is not detectable by the human eye, it can cause a stroboscopic effect whereby
rotating shafts in the vicinity of discharge lamps may appear stationary or rotating slowly which
could be a dangerous illusion to operators.
Methods to alleviate a stroboscopic problem
1. Use a combination of incandescent and discharge lighting in the same area.
2. Use twin discharge lamp fittings with each lamp wired as a lead-lag circuit, i.e. the lamp
currents are phase displaced so that they go through zero at different times, hence the
overall light output is never fully extinguished.
3. ‘Where a 3-phase supply is available, connect adjacent discharge luminaires to different
phases (Red, Yellow, Blue) so the light in a given area is never extinguished.
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Navigation and Signal Lights
The number, position and visible range of navigation lights aboard ships are prescribed
by the International Maritime Organization (IMO) in their “International Regulations for
Preventing Collisions at Sea”. By far the most common arrangement is to have five specially
designed navigation running lights referred to as Foremast, Mainmast (or Aftmast), Port,
Starboard and Stem.
Two anchor lights, fitted forward and aft, may also be switched from the Navigation
Light Panel on the bridge. The side lights are red for Port and green for Starboard while the other
lights are white. For vessels length more than 50 metres, the masthead light(s) must be visible
from a range of six nautical miles and the other navigation lights from three nautical miles.
To achieve such visibility, special incandescent filament lamps are used each with a typical
power rating of 65 W but 60 W and 40 W ratings are also permitted in some cases.
Due to the essential safety requirement for navigation lights it is common practice to have two
fittings at each position, or two lamps and lamp holders within a special dual fitting.
Each light is separately .supplied, switched, fused and monitored from a Navigation Light Panel
in the wheelhouse. The electric power is provided usually at 220 V a.c. with a main supply fed
from the essential services section of the main switchboard. An alternative or standby power
supply is fed from the emergency switchboard. A changeover switch on the Navigation Light
Panel selects the main or standby power supply. The Navigation Light Panel has indicator lamps
and an audible alarm to warn of any lamp or lamp-circuit failure.
Navigation light Panel
Each lamp circuit has an alarm relay which monitors the lamp current. The relay may be
electromagnetic or electronic.
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Navigation light panel.
Signal light arrangement
Various signal lights with
red, green, white and blue colours
are arranged on the signal mast.
These lights are switched to give
particular combinations to signal
states
relating
to
various
international
and
national
regulations. Pilotage requirements,
health, dangerous cargo conditions,
etc. are signalled with these lights.
White Morse-Code flashing lights
may also be fitted on the signal mast.
The NUC (Not Under Command)
state is signalled using two all-round
red Iights vertically mounted at least
2m apart. Such important lights are
fed from 24V dc supply.
Electrical Diagrams
There are various types of diagram which attempt to show how an electrical circuit
operates.
1. Block Diagram
2. System diagram
3. Circuit diagram
4. Wiring diagram
Symbols are used to represent the various items of equipment.
Block Diagram
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Diesel
Generator
(440V)
Main
Switchboard
Auxiliary
Switchboard
Emergency
Generator
(440V)
Emergency
Switchboard
Motors
(440V)
Motors
(440V)
Change
over
switch
Motors
(440V)
A block diagram shows in simplified form , the main inter-relationships of the
elements in a system, and how the system works or may be operated. Such diagrams are often
used to depict control systems and other complex relationships.
System diagram
A system diagram shows the main features of a system and its bounds, without necessarily
showing cause-to-effect. Its main use is to illustrate the ways of operating the system. Detail is
omitted in order to make the diagram as clear as possible, and so, easily understood.
Circuit diagram
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A circuit diagram shows, in full, the functioning of a circuit. All essential parts and
connections are depicted by means of graphical symbols arranged to show the operation as clearly
as possible but without regard to the physical layout of the various items, their parts or
connections. (See Star-Delta Starter Circuit Diagram).
A most important point is that no attempt is made to show the moving contacts of a relay
or contactor alongside the coil that operates them (where they are actually physically located).
Instead, the coil and its related contacts are identified by a common number or letter. For example,
contactor coil ‘CC’ controls two auxiliary contacts identified as ‘CC-1’ and ‘CC-2’ in addition to
the main contacts identified as ‘CC’.
The use of a circuit diagram is to enable the reader to understand the operation of the
circuit, to follow each sequence in the operation from the moment of initiating the operation (e.g.
by pressing a start button) to the final act (e.g. starting of the motor). If the equipment fails to
operate correctly, then you can follow the sequence of operations until you come to the operation
that has failed.
The components involved in that faulty operation can then be examined to locate the
suspect item. There is no need to examine other components that are known to function correctly
and have no influence on the fault, so the work is simplified. A circuit diagram is an essential tool
for fault finding.
Salient features of circuit diagram
• Every sequence is drawn from left to right and from top to bottom.
• Each stage is in order of occurrence from left to right.
• All contacts and components which are in series is drawn in a straight line with the
component they control.
• All contacts and components which are in parallel are drawn side by side and at the same
level to emphasise their parallel function.
• All major components operating at bus-bar voltage are drawn at the same level to help
identify the required components quickly.
• All contacts should be shown open or closed as in their normal or de-energised condition.
Wiring diagram
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A wiring diagram shows the detailed connections between components or items of
equipment, and in some cases the routeing of these connections. An equipment wiring diagram
shows the components in their approximate positions occupied ,within the actual enclosure. The
component may be shown complete (e.g. a contactor coil together with all the contacts it drives)
or may be simply represented by a block with the necessary terminals clearly marked.
A different thickness of line can be used to differentiate between power and control
circuit connections. A wiring diagram may be of a fairly simple circuit, but its layout makes it
quite difficult to use and to understand the sequential operation of the circuit. The purpose of a
wiring diagram is mainly to instruct the wiring installer how to construct and connect the
equipment. It is of little use in trouble shooting apart from identifying the exact position of
suspect components, terminals and wires.
Battery
Atoms bound together by electrons are called molecules. Ionic bonds are molecular
unions formed when an electron deficient atom (a positive ion) joins with an electron-excessive
atom (a negative ion). Chemical reactions involving ionic bonds result in the transfer of electrons
between atoms. This transfer can be harnessed to form an electric current. A cell is a device
constructed to harness such chemical reactions to generate electric current. A cell is said to be
discharged when its internal chemical reserves have been depleted through use.
Types of batteries
The two main types of rechargeable battery cell are:
• Lead-acid
• Alkaline
The nominal cell voltages of each type are 2 V for lead-acid and 1.2 V for alkaline. Hence,
twelve lead-acid cells or twenty alkaline cells must be connected in series to produce a nominal
24 V. More cells may be connected in parallel to increase the battery capacity which is rated in
Ampere-hours (Ah). The battery capacity is usually rated in terms of its discharge at the 10 hour
rate. A 350 Ah battery would be expected to provide 35 A for 10 hours. However, the battery will
generally have a lower capacity at a shorter discharge rate. After a 10 hour discharge a lead-acid
cell voltage will have fallen to approximately 1.73 V. The equivalent figure for an alkaline cell is
1.14 V.
Lead acid battery
In the common "lead-acid" cell the negative
electrode is made of lead (Pb) and the positive is made of
lead peroxide (PbO2), both metallic substances. The
electrolyte solution is a dilute sulfuric acid (H2SO4 + H2O).
If the electrodes of the cell are connected to an external
circuit, such that electrons have a place to flow from one
to the other, negatively charged oxygen ions (O-) from the
positive electrode (PbO2) will ionically bond with
positively charged hydrogen ions (H+) to form molecules
of water (H2O).
This creates a deficiency of electrons in the lead
peroxide (PbO2) electrode, giving it a positive electrical
charge. The sulfate ions (SO4-) left over from the
disassociation of the hydrogen ions (H+) from the sulfuric
acid (H2SO4) will join with the lead (Pb) in each electrode
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to form lead sulfate (PbSO4). This process of the cell providing electrical energy to supply a load
is called discharging, since it is depleting its internal chemical reserves.
Theoretically, after all of the sulfuric acid has been exhausted, the result will be two
electrodes of lead sulfate (PbSO4) and an electrolyte solution of pure water (H2O), leaving no
more capacity for additional ionic bonding. In this state, the cell is said to be fully discharged. In
a lead-acid cell, the state of charge can be determined by an analysis of acid strength. This is
easily accomplished with a device called a hydrometer, which measures the specific gravity
(density) of the electrolyte. Sulfuric acid is denser than water, so the greater the charge of a cell,
the greater the acid concentration, and thus a denser electrolyte solution.
There is no single chemical reaction representative of all voltaic cells, so any detailed
discussion of chemistry is bound to have limited application. The important thing to understand is
that electrons are motivated to and/or from the cell's electrodes via ionic reactions between the
electrode molecules and the electrolyte molecules. The reaction is enabled when there is an
external path for electric current, and ceases when that path is broken. Being that the motivation
for electrons to move through a cell is chemical in nature, the amount of voltage (electromotive
force) generated by any cell will be specific to the particular chemical reaction for that cell type.
For instance, the lead-acid cell just described has a nominal voltage of 2.2 volts per cell, based on
a fully "charged" cell (acid concentration strong) in good physical condition. There are other
types of cells with different specific voltage outputs. The Edison cell, for example, with a positive
electrode made of nickel oxide, a negative electrode made of iron, and an electrolyte solution of
potassium hydroxide (a caustic, not acid, substance) generates a nominal voltage of only 1.2 volts,
due to the specific differences in chemical reaction with those electrode and electrolyte
substances.
The chemical reactions of some types of
cells can be reversed by forcing electric current
backwards through the cell (in the negative
electrode and out the positive electrode). This
process is called charging. Any such (rechargeable)
cell is called a secondary cell. A cell whose
chemistry cannot be reversed by a reverse current
is called a primary cell. When a lead-acid cell is
charged by an external current source, the
chemical reactions experienced during discharge
are reversed:
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Lead acid battery
The state of charge held by a lead-acid battery is best indicated by a test on the
electrolyte specific gravity (SG) by using a hydrometer as shown in Fig. 5.26. A fully charged
lead-acid cell has an SG of about 1.27-1.285 (often written as 1270-1285) which falls to about 1.1
(or 1100) when fully discharged. The cell voltage also falls during discharge and its value can
also be used as an indication of the state of charge. A lead-acid battery may be safely discharged
until the cell voltage drops to approximately 1.73 V (measured while delivering load current).
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Alkaline battery
The state of charge of an alkaline battery cell cannot be determined from its SG value.
The electrolyte density does not change during charge/discharge cycles but gradually falls during
the lifetime of the battery. New alkaline cells have an SG of around 1190. When this reduces to
about 1145 (which may take 5-10 years depending on the duty cycle) the electrolyte must be
completely renewed or the battery replaced. Discharge of alkaline cells should be discontinued
when the cell voltage has fallen to about 1.1V. Battery charging equipment uses a
transformer/rectifier arrangement to supply the required d.c. voltage to the cells. The size of
voltage depends on the battery type (lead-acid or alkaline) and the mode of charging, e.g.
charge/discharge cycle, boost charge, trickle or float charge. Check the manufacturer’s
instructions for details of the required charging voltages. Do not allow electrolyte temperatures to
exceed about 45°C during charging. A lead-acid cell will gas freely when fully charged but an
alkaline cell gases throughout the charging period. The only indication of a fully charged alkaline
cell is when its voltage remains at a steady maximum value of about 1.6-1.8 V. Generally,
alkaline cells are more robust, mechanically and electrically, than lead- acid cells.
Nickel cadmium cells will hold their charge for long periods without recharging so are ideal for
standby duties. Also they operate well with a float-charge to provide a reliable emergency supply
when the main power fails. For all rechargeable batteries (other than the sealed type) it is
essential to replace lost water (caused during gassing and by normal evaporation) with the
addition of distilled water to the correct level above the plates. Exposure of the cell plates to air
will rapidly reduce the life of the battery.
Battery installations for both types of battery are similar in that the battery room should be well
ventilated, clean and dry. Both types generate hydrogen gas during charging so smoking and
naked flames must be prohibited in the vicinity of the batteries. Steelwork and decks adjacent to
lead-acid batteries should be covered with acid-resisting paint and alkali resisting paint used near
Ni-cad cells. Acid cells must never be placed near alkaline cells otherwise rapid electrolytic
Corrosion to metalwork and damage to both batteries is certain.
For similar reasons, never use lead-acid battery maintenance gear (e.g. hydrometer, topping up
bottles, etc.) on an alkaline installation or vice-versa. Battery maintenance includes keeping the
cell tops clean and dry, checking the tightness of terminal nuts and applying a smear of petroleum
jelly to such connections to prevent corrosion. Be most careful when handling the battery
electrolyte (e.g. when using a hydrometer to check its specific gravity). Use protective rubber
gloves and eye goggles when handling electrolyte. Insulated spanners should be available for use
on cell connections to prevent accidental short-circuiting of battery terminals. Such a short-circuit
across the terminals of just one cell of a battery will cause a blinding flash with the probability of
the cell being seriously damaged.
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Dated: 01.02.2012
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