Download Earth shock

What is mains
supply??
Referred by several names
household power, household electricity,
power line, domestic power, wall power, line
power, AC power, city power, street power,
grid power etc.
• Currents for mains supply is produced
by dynamos at power stations
• EMF is induced by movement of
conductor & magnetic field in relation
to each other
• Mechanical energy must be available
• Water power, coal, nuclear energy
Dynamos can be constructed to produce either
alternating current or direct current
First electricity was produced as DC
But now it’s replaced by AC
Flow of Electrons:
Alternating Current
Direct Current
Electrons keep switching
directions - forward and
backward
Electrons move steadily in one
direction or 'forward'
Alternating Current
Cause of the
direction
Flow
of Electrons:
of flow of electrons:
Cause of the direction of
Electrons keep switching directions
Rotating
magnet along the
- forward and backward
wire
flow of electrons:
Amount of energy that can
be carried:
Rotating magnet along the wire
Safer to transfer over longer city
distances and can provide more
power
Direct Current
Steady magnetism along the
direction or 'forward'
wire
Electrons move steadily in one
Steady magnetism along the wire
Voltage of DC cannot travel very far
until it begins to lose energy
Safer to transfer over longer Voltage of DC cannot travel
Amount of energy that
city distances and can provide very far until it begins to lose
can be carried:
more power
energy
Mains supply
Why AC not DC?
– Greater voltage can be produced with AC than DC.
– Voltage of AC can be alter with the transformers which
is more suitable for long distance transmission.
– Thin cables which cost less can be used for transmission
of current because EMF is stepped up to several thousand
volts and reduce the current. When necessary EMF can
be stepped down.
– If higher voltage is carrying the lost in voltage can be
negligible when compared to the total and original level
can be restored by using step-up transformers.
– The construction of modern apparatus is such that they
work only on AC supply.
Distribution and grid system
1.
2.
3.
4.
Each dynamo has three coils of wire
One end of each coil is connected to live distribution line
Other ends are connected together and earth
Distribution of current is by three live cables & one neutral
cable
5. These four cables are observed on the pylons
6. Each of the consumer receives one of the live wires & the
neutral wire
• Advantage of the grid system
– All areas supplied receive the same voltage and type of current.
– Large demand in one area do not put an excess load on any
particular power station.
– Breakdown of one power station does not cut off the supply to
any area.
– It is not necessary for all generators to be in operation all time.
Wiring of the houses
Distribution in a house
Current on entering the house passes
through the main fuses & the meter
Next comes the main switch
& the house main fuses, then the
various circuits which are parallel to
each other
In this method each circuit receives
the full voltage of the supply
Light & power circuits
• Circuits in the house are divided into two
1.Light circuits
2.Power circuits
Power circuits may be arranged in different ways
e.g.Similar to light circuits
Ring main
Sub circuits
Distribution in a house – Ring main
•Complete loop is taken from
each of the two supply cables
•Supply points are wired in
parallel with each other
between loops
•Fused plugs are used
•No fuses are incorporated,
but 30A fuse is placed on the
live wire
•Each wire carries at least 15A
Fuses
•Designed to a weak point in a circuit
•It blows if a current of too great
intensity is passed
•It consist of a short length wire of
low melting point
•If current passing through it exceeds
a certain value the heat generated
melts the wire preventing further
current flow
•It prevents damage to another part &
gives warning of the defect
Fuses cont.
There are different types of fuses
1. Cartridge fuse
Fusible element made of silver
wire
Runs between metal caps through
tube of glass
It has color cords
5 A –white
15 A – blue
20 A – yellow
30 A – red
45 A – green
2. Normal fuse
Fuses cont.
In many cases there are fuses on both wires of the circuit
But if only one is provided it must be on live wire
In physiotherapy departments fuses are included in circuit of
each apparatus
The blowing of fuse is due to passage of too great current
It may arise from
1. Too low resistance
2. Too high voltage
3. When several parallel circuits are taken from one supply
Power Plugs
•Apparatus working on a power
circuit should be connected to the
supply by three pin wall plug.
•Pins are arranged in a triangle
•Two similar pins are for connect
apparatus to the circuit
•Marked “L” and “N’: live & neutral
•The wire connected to pin marked
“E” is to connect apparatus casing to
earth.
Brown wire to L
Blue wire to N
Yellow wire to E
Earthing
Earthing an apparatus casing is a precaution against earth shock
Sometimes insulation on live wire become worn the wire come in contact
with casing
If apparatus casing is not earthed connection between casing & earth
complete a circuit
If the connection is through a person he receives an electric shock
By correct earthing
When live wire gets contacted with casing current passes by the earth wire
This sis a pathway of low resistance
The current flow is great
Fuse on live wire blows
This stops current flow giving a warning of defect
Switches
•Current is turned on & off by a switch
•Switches vary type according to
currents that pass through
•Commonly used in houses & physio
depts. has two metal blades which fit into
metal sockets
The principle is
When switch is on, the blades are
gripped on the socket & circuit is
completed
When circuit is broken a spring ensures
the sudden separation of socket & blades
Type of Switch
ON-OFF
Single Pole, Single Throw
(ON)-OFF
Push-to-make
ON-ON
Single Pole, Double
Throw
Dual ON-OFF
Double Pole, Single
Throw
Circuit Symbol
Example
Dangers of electric current
Electric shocks: painful
stimulation of sensory nerves
caused by a sudden flow,
cessation or variation in the
current passing through the
body.
Severity of shocks
1. Lower the resistance of the skin greater the
current passes
e.g. – if circuit touched with wet hands shock is
severe than when hands are dry
2. Greater the current the shock is severe
3. Path taken by the current
e.g. – strong current through head, neck or heart
is fatal
4. Type of current
e.g. - shocks are severe with alternating current
than with direct current
Current
Reaction
1 Milliampere
Perception level
5 Milliamperes
Slight shock felt; not
Reaction
painful
but disturbing
Current
6-30 Milliamperes
1 Milliampere
Perception level
5 Milliamperes
Slight shock felt; not
painful but disturbing
50-150 Milliamperes
6-30 Milliamperes
Extreme
pain,
Painful
shock;
"letgo"
range
respiratory
arrest,
50-150 Milliamperes
1000-4,300 Milliamperes
1000-4,300 Milliamperes
10,000+ Milliamperes
10,000+ Milliamperes
Painful shock
Extreme
severe pain,
muscular
respiratory arrest,
contraction
severe
muscular
contraction
Ventricular fibrillation
Ventricular fibrillation
Cardiac
arrest, severe
Cardiac
arrest,
severe
and
burnsburns
and probable
probable death
death
Effects of electric shock
Minor electric shock
Victim gets frightened & distressed
No loss of consciousness
Major or severe electric shock
There is fall of blood pressure
Patient may become unconsciuos
There could be cessation of respiration followed by
ventricular fibrillation & cardiac arrest
Treatment of electric shock
1. Current should be switched off immediately
2. Victim should be disconnected from the source of supply
3. If there is no switch, the victim must be removed from contact
with the conductor
4. Following minor shock patient must be reassured that every thing
is alright
5. Water may be given to drink, but hot drinks avoided
6. Tight clothing loosened
7. If respiration has ceased the airway must be cleaned and artificial
ventilation commenced immediately
8. CPR may also be given
9. Oxygen therapy
10.Patient shifted to a hospital after primary care
Preventing Electrical Hazards
1. Inspect wiring of equipment before each use. Replace damaged or
frayed electrical cords immediately.
2. Use safe work practices every time electrical equipment is used.
3. Know the location and how to operate shut-off switches and/or
circuit breaker panels. Use these devices to shut off equipment in
the event of a fire or electrocution.
4. Limit the use of extension cords. Use only for temporary
operations. In all other cases, request installation of a new
electrical outlet.
5. Use only multi-plug adapters equipped with circuit breakers or
fuses.
6. Minimize the potential for water or chemical spills on or near
electrical equipment.
`
Dangers of electric
current
Earth shock
when a shock is due to a connection between the
live wire of the main and earth.
– How the patients and the therapists can get earth shocks?
– How can we prevent the earth shocks?
Causes of earth shock
1. Connection to live wire
when wire is not properly insulated
live wire is touched to metal casing
live wire is touched to any wet thing
2. Connection to earth
if the floor is made up of stone
if the conductor is touching any radiated metal casing
or metal wire
Precaution for earth shock
1.Proper arrangement of physiotherapy
dept.
2.Proper flooring done with rexin
3.Proper insulation
4.While on treatment patient should not
touch any machine part
5.Metal casing should be connected to
earth
6.Floor kept dry
Clothing and Personal Protective Equipment
Type of Switch
ON-OFF
Single Pole,
Single Throw =
SPST A simple onoff switch. This type
can be used to
switch the power
supply to a circuit.
When used with
mains electricity this
type of switch must
be in the live wire,
but it is better to
use a DPST switch
to isolate both live
and neutral.
Photograph ©
Rapid Electronics
(ON)-OFF
Push-to-make =
SPST Momentary
A push-to-make
switch returns to its
normally open (off)
position when you
release the button,
this is shown by the
brackets around
ON. This is the
standard doorbell
switch.
Photograph ©
Rapid Electronics
ON-(OFF)
Push-to-break =
SPST Momentary
A push-to-break
switch returns to its
normally closed (on)
position when you
release the button.
Photograph ©
Rapid Electronics
ON-ON
Single Pole,
Double Throw =
SPDT This switch
can be on in both
positions, switching
on a separate
device in each
case. It is often
called a
changeover
switch. For
example, a SPDT
switch can be used
to switch on a red
lamp in one position
and a green lamp in
the other position.
A SPDT toggle
switch may be used
as a simple on-off
switch by
connecting to COM
and one of the A or
B terminals shown
in the diagram. A
and B are
interchangeable so
switches are usually
not labelled.
ON-OFF-ON
SPDT Centre Off
A special version of
the standard SPDT
switch. It has a third
switching position in
the centre which is
off. Momentary
(ON)-OFF-(ON)
versions are also
available where the
switch returns to
the central off
position when
released.
Photographs ©
Rapid Electronics
Circuit Symbol
Example
SPST toggle switch
Push-to-make
switch
Push-to-break
switch
1. Thick-soled work shoes for protection against sharp objects such as nails.
2. Wear work shoes with safety toes if the job requires.
3. Make sure the soles are oil resistant if the shoes are subject to oils and
grease
Rubber boots for damp locations
A hat or cap. Wear an approved safety helmet (hard hat) if the job
requires
SPDT toggle switch
SPDT slide switch
(PCB mounting)
Thank You..