The document discusses various types of switchgear used in electrical power systems, including their purposes and basic operating principles. It describes isolators, main switches with fuses, molded case circuit breakers, oil or air circuit breakers, earth leakage circuit breakers, contactors, and changeovers. Protection devices like relays and circuit breakers are used to isolate power when faults occur. The summary provides an overview of the key switchgear components and their functions discussed in the document.
2. Switchgear
• Any device or combination of devices, capable of
making or breaking circuit on all conditions including fault
is termed as switch-gear.
• Switchgear is needed to isolate the power supply when
necessary
• It is used to have some control over the electrical power
flow
4. • Isolators
• Main switch with fuse
• Module Case Circuit Breakers (MCCB)
• Oil or air Circuit Breaker (OCB/ACB)
• Earth Leakage Circuit Breakers (ELCB)
• Contactors
• Changeovers
5. • Manually operated
switch to isolate the
power supply at no load
condition.
• Used for disconnecting
feeders, circuit
breakers, transformer
and bus bar for regular
maintenance and repair
works.
6. • Simplest and
cheapest device used
for interrupting an
electrical circuit under
short circuit or
excessive overload,
current magnitudes.
• used for overload
and/or short circuit
protection in high
voltage (up to 66 KV)
and low voltage (up to
400 V)
installations/circuits.
7. • A main switch is a
central cut-off
switch that controls
the smaller cut-off
switches and
machines of a
building.
• The main switch
can be cut off by a
human or a
computerized
system to control
the flow of power in
the building.
8. • Protection against overload –
currents above the rated value that
last longer than what is normal for
the application.
• Protection against electrical faults –
During a fault such as a short circuit
or line fault, there are extremely high
currents that must be interrupted
immediately.
• Switching a circuit on and off – This
is a less common function of circuit
breakers, but they can be used for
that purpose if there isn’t an
adequate manual switch.
• current ratings that range from low
values such as 15 amperes, to
industrial ratings such as 2,500
9. • Oldest type of circuit breakers.
• Employed for high voltage application up to 11 KV with a
current rating up to 2400 A.
• Main contacts are immersed in oil and the oil acts as the
ionizing medium between the contacts.
• The oil is mineral type, with high dielectric strength to
withstand the voltage across the contacts under normal
conditions.
• Used for voltage application up to 11 KV with a current
rating up to 2400 A. Major application are in power
system transmission and distribution networks between
11 KV to 66 KV rated voltage and fault level of 150 MVA
to 1000 MVA.
11. • Air Circuit Breaker (ACBs) are used for medium and low
voltages. In air circuit breakers, the interrupting contacts
are situated in air instead of any other medium.
• The arc produced is chopped into number of small arcs
by the arc chute as it rises due to heat and magnetic
force.
• In such CB high resistance principle is employed for
effective arc extinction.
• The arc is lengthened by means of the arc runners and
arc chutes thus increasing the resistance.
12.
13. • Phase, neutral and earth wire connected
through ELCB. ELCB is working based on earth
leakage current.
• ELCB measures the voltage on the earth
conductor, if this voltage is not zero, this indicate
a current leakage to earth.
14. • A contactor is an automatic switching device which
operates under the action of an electromagnet for
repeatedly breaking and making of electric circuit.
• A contactor has three components.
• This includes power contacts, auxiliary contacts, and
contact springs. The electromagnet or coil provides the
driving force to close the contacts.
15.
16. • Trips when current exceeds beyond preset value
• Over current can damage generator winding, switches,
cables and other equipment due to excess heat
generated.
• Fuse, MCCB, PTC thermistor (Positive temperature
coefficient) etc. are usually used for the purpose
17. • Causes of OC:
• Excess load to generator
• Faulty equipment being connected to generator
• Lagging power factor
• Short circuit
• Incorrect frequency
18. • Trips when voltage drops or exceeds beyond a preset
value.
• Electric motors and alternators will be damaged if UV
persists for a long time.
• OV damages lamps, heating element insulators etc.
• Causes:
Under Voltage Over voltage
-Defective AVR
-Machine overloaded
-Bad power factor
-Poor regulation of
transmission line
-Defective AVR
-Leading power factor
load
-Poor regulation of
transmission line
-Generator run at
overspeed
-
19. • Trips when frequency drops or exceeds beyond a preset
value.
• Electric motors and alternators may be damaged due to
incorrect frequency
• Causes:
Under frequency Over frequency
-Defective governor
-Machine overloaded
-Insufficient water
-Belt slip
-Defective ELC
--Excess water
-Generator run at over
speed due to sudden
decrease in load
20. • Trips when temperature violates its set limit
• Probes are generally embedded in winding and wired to
trip or give alarm
• Causes:
• Overloading of generator
• Incorrect frequency
• Excess current due to low power factor
• Wave form distortion
• Defective bearings
• Bad vantillation
• Ambient temperature too high
• Altitude not within rated value
21. • Transmission lines are subjected to direct stroke which
are protected by running earth wire
• Indirect strokes may damage electrical and electronic
equipment and installations which is protected by
installing lightening arresters(LAs).
• LAs have high resistance at normal voltage and low
resistance at high voltage as insulating material breaks at
high voltage.
22. • All equipments are earthed which gives rise to zero
voltage reference to all other voltages. All metal works
are earthed by electrically connecting them to earth
electrodes which may be a GI pipe or copper plate
burried in the ground. Earth resistance of these earth
electrode must be less than 1 ohm as measured by earth
megger.
• One of the popular earthing system is TT system (used in
Srilanka) in which neutral at source is earthed in
conjunction with earth fault protection.
• Earth fault protection consists of an earth fault relay
associated with earth leakage circuit breaker to isolate
the power system if earth fault is detected.
23. Voltmeter
• A voltmeter is an instrument used
for measuring electrical
potential difference between two
points in an electric circuit.
• Analog voltmeters move a pointer
across a scale in proportion to the
voltage of the circuit; digital
voltmeters give a numerical display
of voltage by use of an analog to
digital converter.
24. Ammeter
• An ammeter (from Ampere Meter)
is a measuring instrument used to
measure the current in a circuit.
• The main principle of ammeter is
that it must have a very
low resistance and also inductive
reactance.
• Connected in series.
25. • DC Ammeter
• AC Ammeter.
• DC Ammeter are mainly PMMC (Permanent magnet
moving coil) instruments, MI(Moving Iron) can measure
both AC and DC currents, also Electrodynamometer type
thermal instrument can measure DC and AC, induction
meters are not generally used for ammeter construction
due to their higher cost, inaccuracy in measurement.
26. • Watt hour meter or energy meter is an instrument which
measures amount of electrical energy used by the
consumers.
• Utilities install these instruments at every place like
homes, industries, organizations to charge the electricity
consumption by loads such as lights, fans and other
appliances
3 Basic types of Energy meters
Energy meter or watt hour meter is classified in accordance
with several factors such as:
• Type of display like analog or digital electric meter.
• Type of metering point like grid, secondary transmission,
primary and local distribution.
• End applications like domestic, commercial and
industrial.
27.
28.
29. • A frequency meter is an instrument that displays
the frequency of a periodic electrical signal.
• Many are instruments of the deflection type, ordinarily
used for measuring low frequencies but capable of being
used for frequencies as high as 900 Hz.
• These operate by balancing two opposing forces.
Changes in the frequency to be measured cause a
change in this balance that can be measured by the
deflection of a pointer on a scale.
• Deflection-type meters are of two types,
electrically resonant circuits and ratiometers.
30.
31. • Used to measure and display pressure in an integral unit
• A manometer is a good example as it uses a column of
liquid to both measure and indicate pressure.
• Two common reasons for gauge failure are pipe vibration
and water condensation.
• The pressure gauge must be located for better operator
visibility.
Editor's Notes
In normal operating conditions, when the current flowing through the circuit is within the safe limits, the heat developed in the fuse element carrying this current is readily dissipated into the surrounding air and therefore fuse element remains at a temperature below its melting point.
