Download A circuit is a complete path by which electricity can flow

ELECTRICAL CIRCUITS
An electrical circuit is a complete path through which electricity can flow.
In an electrical circuit, an open circuit is caused intentionally when a user
opens a switch or unintentionally when vibration or mechanical damage cuts
a wire. In electrical circuits, closing a switch creates a closed loop for the
electrons to flow through.
OPEN CIRCUIT: When there is a break in the electrical circuit it is called
open circuit.
CLOSE CIRCUIT: When the path of the current flow is complete, it is
called close circuit.
SHORT CIRCUIT: When the path of the current flow is completed through
not the intended path but through some leakage / damaged path to earth, it is
called short circuit.
Table of Electrical Symbols
Symbol
Component name
Meaning
Bulb
Supply
Switch
1-, 230V,AC
Open Circuit—when switch is open
Closed Circuit – when switch is closed
Electrical Wire
Conductor of electrical current
Connected Wires
Connected crossing
Not connected Wires
Wires are not connected
SPST Toggle Switch
(Single Pole Single Throw)
Disconnects current when open
SPDT Toggle Switch
(Single Pole Double Throw)
Selects between two connections
Pushbutton Switch (N.O)
(Normally Open)
Momentary switch - normally open
Pushbutton Switch (N.C)
(Normally Closed)
Momentary switch - normally closed
Earth Ground
Used for zero potential reference and electrical
shock protection.
Chassis Ground
Connected to the chassis of the circuit
Resistor
Resistor reduces the current flow.
Variable Resistor / Rheostat
Adjustable resistor - has 2 terminals.
Capacitor
Inductor
Capacitor is used to store electric charge. It acts as
short circuit with AC and open circuit with DC.
Coil / solenoid that generates magnetic field
AC Voltage Source
AC voltage source
Generator
Electrical voltage is generated by mechanical
rotation of the generator
Battery
Generates constant voltage
Voltmeter
Measures voltage. Has very high resistance.
Connected in parallel.
Ammeter
Measures electric current. Has near zero
resistance. Connected serially.
Ohmmeter
Measures resistance
Wattmeter
Measures electric power
Lamp / light bulb
Generates light when current flows through
Diode
Diode allows current flow in one direction only
(left to right).
Motor
Electric motor
Transformer
Change AC voltage from high to low or low to
high.
Electric bell
Rings when activated
Buzzer
Produces buzzing sound
Fuse
Loudspeaker
The fuse disconnects when current above threshold.
Used to protect circuit from high currents.
Converts electrical signal to sound waves
Series and Parallel connections
Here, we have three resistors (labeled R1,
R2, and R3), connected in a long chain
from one terminal of the battery to the
other. The defining characteristic of a
series circuit is that there is only one path
for electrons to flow. In this circuit the
electrons flow in a counter-clockwise
direction, from point 4 to point 3 to point 2
to point 1 and back around to 4.
We have three resistors, but this time they form more than one
continuous path for electrons to flow. There's one path from 8 to 7 to 2
to 1 and back to 8 again. There's another from 8 to 7 to 6 to 3 to 2 to 1
and back to 8 again. And then there's a third path from 8 to 7 to 6 to 5 to
4 to 3 to 2 to 1 and back to 8 again. Each individual path (through R1,
R2, and R3) is called a branch. The defining characteristic of a parallel
circuit is that all components are connected between the same set of
electrically common points. Looking at the schematic diagram, we see
that points 1, 2, 3, and 4 are all electrically common. So are points 8, 7,
6, and 5. Note that all resistors as well as the battery are connected
between these two sets of points.
SHORT CIRCUIT:
A short circuit is simply a low resistance connection between the two conductors supplying electrical power to any circuit.
This results in excessive current flow in the power source through the 'short,' and may even cause the power source to be
destroyed. If a fuse is in the supply circuit, it will do its job and blow out, opening the circuit and stopping the current flow
The two basic types of Over-current Protective Devices
(OCPDs):
Fuse—An over-current protective device with a circuit-opening fusible
part that is heated and severed by the passage of over-current through it.
Kit-Kat fuse or Porcelain re-wireable fuse
A fuse interrupts excessive current (blows) so that further damage by
overheating or fire is prevented. Overcurrent protection devices are
essential in electrical systems to limit threats to human life and property
damage. Fuses are selected to allow passage of normal current and of
excessive current only for short periods. A fuse (Kit-Kat fuse)consists of
a metal strip or wire fuse element, of small cross-section compared to the
circuit conductors, mounted between a pair of electrical terminals, and
(usually) enclosed by a non-conducting and non-combustible housing.
The fuse is arranged in series to carry all the current passing through the
protected circuit. The resistance of the element generates heat due to the
current flow. If too high a current flows, the element rises to a higher temperature and either directly melts, or else melts a
soldered joint within the fuse, opening the circuit. Fuses act as a weak link in a circuit. They reliably rupture and isolate the
faulty circuit so that equipment and personnel are protected. Following fault clearance they must be manually replaced
before that circuit may be put back into operation.
High Rupturing Capacity (HRC) Fuse
A high rupturing capacity (HRC) fuse is a fuse that has a high breaking capacity (higher kA Rating). The minimum fault
value for an HRC fuse is 80kA. A fuse should be selected with a rating just above the normal operating current of the
device to be protected. A general approach is that it should operate at 1.2 times the rated current. A typical fuse is made of
silver-coated copper strips and granular quartz.
Circuit breaker—A device designed to open and close a circuit by non-automatic means and to open the circuit
automatically on a predetermined over-current without damage to itself when properly applied within its rating. Miniature
circuit breakers (MCBs) or moulded case circuit breakers (MCCBs) are also over-current protection devices often with
thermal and magnetic elements for overload and short circuit fault protection. As a switch they allow isolation of the supply
from the load. Normally the MCB requires manual resetting after a trip situation but solenoid or motor driven closing is
also possible for remote control. The MCB is an automatic, electrically operated switching device that was designed to
automatically protect an electric circuit from overload currents and short circuit currents. It is a complicated construction
made up of almost 100 individual parts. It has the ability to respond within milliseconds when a fault has been detected.
Westinghouse Electric introduced the world’s first MCB and it initially had a porcelain base and cover mounted in a metal
housing.
Fuse Advantages over MCBs
The advantages of the fuse can be summarized
as follows:
# Cheaper when compared to MCBs
# It is easy to identify where the fault is due to
the open air gap
# It can cut-off fault current long before it
reaches its first peak
# Hence, very little energy “let through” (I2t)
due to the low cut-off value
MCB Advantages over fuses
The advantages of the MCB can be
summarized as
follows:
# Closed overload protection compared to HRC
fuses
# Common tripping of all the phases of a motor
# Instant re-closing of the circuit after a fault
has been
cleared
# Safety disconnect features for circuit isolation
# Terminal insulation for operator safety
# Ampere ratings that can be fixed and
modified
compared to the possibility of introducing
overrated
fuses
# It is reusable, hence very little maintenance
and
replacement costs
Fuse Disadvantages
The disadvantages of the fuse can be summarized as follows:
* The abrupt introduction of high resistance in the circuit by a
badly designed and assembled fuse can create unwanted effects
while clearing the fault
* Although this is very rare, fuses are likely to produce high
peak voltage which is much higher than the system voltage and
can puncture the insulation of the rest of the circuit.
in the form of continuously monitoring the state of the fuse;
and replacement after each and every fault
* The cut-off current increases with the fuse rating
* Fuse of incorrect ratings can easily be installed in the fuse
holders
* In a three phase power circuit, if one fuse blows, all the fuses
must be replaced at the same time
MCB Disadvantages
The disadvantages of the MCB can be summarized as follows:
* More expensive than the fuse
* Difficult to identify where the fault occurred
* Fault can be cleared in any time up to 10 cycles of
the current waveform
* Large amount of energy “let through” (10 times that released
by the fuse)
# Simplicity of mounting and wiring
# Lower space requirements
# Provision of accessories e.g. auxiliary switch
# Stable arc interruption
Earthing: The earthing is made up of materials that is electrically conductive. A fault current will flow to 'earth' through
the live
conductor,
provided it is earthed . This is to prevent a potentially live conductor from rising above the safe level .
based
on
All exposed
metal
of an electrical installation or electrical appliance must be earthed . The main objectives of the
current or basedparts
on time
earthing are to :
1) Provide an alternative path for the fault current to flow so that it will not endanger the user
2) Ensure that all exposed conductive parts do not reach a dangerous potential
3) Maintain the voltage at any part of an electrical system at a known value so as to prevent over current or excessive
voltage on the appliances or equipment .
The qualities of a good earthing system are :
1) Must be of low electrical resistance
2) Must be of good corrosion resistance
3) Must be able to dissipate high fault current repeatedly
Switch on live wire
A switch or fuse is always placed on the live wire because if you break the neutral with the switch instead of the hot i.e.
live, then you can not de-energize the circuit by turning off the switch. And as you can not see those electrons, it will hurt
when you try to work on such a system. To eliminate the risk of electrocution (i.e. the flow of electrons from live wire to
the earth via a human body) switches and fuses are always be placed in the live wire.
SAFETY
Before removing any fuse from a circuit, be sure the switch for the
SAFETY TIPS
circuit is open or disconnected. When replacing fuses, install the fuse
1.
Do
not
touch
an
electric switch or appliance
first into the load side of the fuse clip, then into the line side.
when
hands
are
wet.
Electricity Boards or Supply Companies generally give single phase
2. Be alert while replacing fuse/inserting plugs.
supply upto 5 KW connected load and 3 phase supply to consumers
3. Do not use copper wire as a substitute for
having load more than 5KW.
fuse wire.
1. Always use copper wires/cables of adequate size or one size above
4. Do not use wires with poor insulation.
the load requirement. Increase in size of conductor reduces resistance
5. Do not replace fuse unless cause is detected.
thereby heat generation and fire hazard is minimised.
6. Do not hang wet clothes on electrical
2. Electrical power circuits and communication circuits e.g. telephone
fittings/conducts.
should run in separate conduits/casing capings.
7. Use 3 pin plugs and ensure that earth
3. The wiring for high power equipment viz. air-conditioner, geyser etc.
connection is proper.
should be run separately with separate neutral brought from supply
8. Take help of qualified electrician for any
terminal. This reduces voltage fluctuation in other loads.
alterations / modifications in wiring.
4. Normally, in wiring single pole switches are used which ensure that
9. Do not replace bulb or any appliance with
switches are in the "live" or "phase" wire of the circuit appliance. If the
switch in "ON" condition.
switch is connected in "neutral" wire, the equipment/circuit can give
10. Do not shift any appliance with switch in
shock even though switch is in "off" position.
"ON" condition.
5. Proper earthing is must in house wiring. Only proper earthing will
11. Check earthing frequently for physical
guarantee safety to you and your family (hence, earthing must be
damage, if any.
checked periodically).
12. In case minor shock is felt anywhere, do
6. All appliances are provided with 3 pin plugs. Please ensure that
not neglect it - contact licensed electrician.
earthing wire is connected to all such plugs.
13. Check the electrical specifications of
7. Always ensure that connections to sockets are made by 3 pin plugs
equipment before it is switched "ON" viz.
and not by inserting loose wires or 2 pin plugs.
110V, 230V, 110V, 440V etc.
8. To prevent shocks & protection against fault in appliances, Earth
Leakage Circuit Breaker (ELCB) needs to be installed.
9. In case of repeated tripping of ELCB, locate the fault and eliminate the same instead of by-passing ELCB.
10. Before adding heavy power consuming equipment viz. geyser, window AC etc. ensure that wiring is of adequate
capacity to take this additional load.
11. Earthing wire should never be used as a return wire of any electrical circuit and it's use should be restricted only for the
purpose of equipment body earthing.
12. (a) Standard practice of connecting wires for 3-pin sockets & plug top should be followed.
(b) While connecting 3 core cable to plug top, earthing wire should be little longer than P&N wires so that mechanical
forces act equally on all the 3 wires.
Electrical Shock: Here are some electromedical facts that should make you think twice before taking chances. It's not the
voltage but the current that kills. People have been killed by 100 volts AC in the home and with as little as 42 volts DC.
The real measure of a shock's intensity is in the amount of current (milliamperes) forced through the body. Any electrical
device on a house wiring circuit can, under certain conditions, transmit a fatal amount of current. Currents between 100 and
200 milliamperes (0.1 ampere and 0.2 ampere) are fatal. Anything in the neighborhood of 10 milliamperes (0.01) is capable
of producing painful to severe shock.
Clothing and Personal Protective Equipment:
See Figure below
Clothing should fit snugly to avoid danger of becoming
entangled in moving machinery or creating a tripping or
stumbling hazard.
Thick-soled work shoes for protection against sharp
objects such as nails. Make sure the soles are oil resistant
if the shoes are subject to oils and grease.
Wear rubber boots for damp locations
Wear an approved safety helmet, if the job requires.
Confine long hair or keep hair trimmed and avoid placing
the head near to rotating machinery. Do not wear jewelry.
Gold and silver are excellent conductors of electricity.
FIRE SAFETY:
The chance of fire is greatly decreased by good housekeeping. Keep rags containing oil, gasoline, alcohol, paint, varnish
etc. in a covered metal container. Keep debris in a designated area away from the building. Sound an alarm if a fire occurs.
Alert all workers on the job and then call the fire department. Make a reasonable effort to contain the fire.
Fire Extinguishers:
Always read instructions before using a fire extinguisher. Always use the correct fire extinguisher for the class of fire. See
Figure. Fire extinguishers are normally red. Fire extinguishers may be placed on a red background so they can be easily
located. Always use the correct fire extinguisher for the class of fire. Be ready to direct firefighters to the fire Inform them
of any special problems or conditions that exist, such as downed electrical wires or leaks in gas lines. Report any
accumulations of rubbish or unsafe conditions that could be fire hazards. Also, if a portable tool bin is used on the job, a
good practice is to store a C02 extinguisher in it.
Lighting Schemes and Lighting Lamps: (i) Direct lighting : In this, maximum light is thrown towards the ceiling from
where it is diverted to the room through diffused reflection. This is suitable for drawing offices, workshops. (ii) Semi-direct
lighting: Here, 50% of the light is sent from the source directly on the reading plane and rest is sent upwards. This provides
uniform distributed lighting. (iii) Semi-indirect lighting: Here 40% of the light is sent upwards and 40% is sent directly to
the surface.
Lighting Lamps: The six varieties of lamps are incandescent quartz, Fluorescent, Mercury vapour, Metal halide, High
pressure sodium and low pressure sodium.
Lamp
Colour
Efficiency
Intensity
Life
Cost
Initial
Operating
Incansescent
High
Low
High
Short
Low
High
Quartz
Fluorescent
Moderate
High
Moderate
Moderate
Moderate
Low
cool white,
warm white
Mercury
High
Moderate
Moderate
Moderate
High
Moderate
vapour
High pressure
Low
High
High
Long
Low
Low
sodium