Why "Testing of Circuit Breaker" is Necessary?
A Circuit Breaker should be capable of carrying, making, and
breaking under normal and abnormal conditions. In any power
system circuit breaker has to withstand power frequency over
voltages and transient over voltages due to switching and
The performance of a circuit breaker under normal and abnormal
conditions can be verified by performing different type of tests
on circuit breakers. The main purpose of testing of circuit
breakers is to confirm if circuit breaker is able to work on
particular voltage and current ratings or not.
There are mainly two tests classified:
1) Type test
2) Routine Test
1) Type Tests:
The purpose of type tests is to prove design
features and the quality of circuit breaker. Type tests
are not conducted on each circuit breaker. This is
done to prove the capabilities and to confirm the
rated characteristics of the circuit breakers.
Routine test is performed before circuit
breaker dispatch to ensure the product.
This gives result about defects in
materials and construction of circuit
breaker. We can check quality of material
of circuit breaker by performing Routine
Mechanical endurance tests
Measurement of resistance of the main circuits
Short Circuit tests
In this test, the C.B.. is open and closed 500 times or
other value as agreed to between the purchaser and the
supplier.the test are carried out without current through
the main circuit of the C.B.Out of the total number of
tests, 10% should be closed-open operation,that is with
the tripping,mechanism energized by the closing of
main contacts.During the tests,occasional
lubrication,but no mechanical adjustments are
permissible.after the tests,all parts including contacts
should be in good condition and there should be no
permanent distortion and undue wear of the parts.
This test determines the maximum normal current that the
circuit breaker can carry without exceeding the maximum
allowable temperature rise.In this test the rated normal
current of normal frequency is passed through the current
carrying parts of circuit breaker.
Method are recognized by Indian standards for measuring
temperature rise of parts:-
1) Thermometer method
2) Thermocouple method
3) Self resistance method
1) Breaking capacity Test:-
• Sequence of performing this tests is as follows:-
First of all,the master circuit breaker (MB)and the breaker under test
The s.c.current is passed by closing the make switch.
The circuit breaker under test(TB) is opened to interrupt the s.c.current at
• The following measurements related to the breaking capacity
performance are taken from the oscillogram during the test:-
Symmetrical breaking current
Asymmetrical breaking current
Natural frequency of oscillations and RRRV(RATE OF RISE OF
Sequence of Performing this test :-
First of all,the master circuit breaker (MB)and the
make switch(MS) are closed.
Then,the short circuit current is initiated by closing
the test breaker (TB).
The rated short circuit making current i.e.the peak
value of the first major loop of the short circuit current
envelope is measured from the oscillorgram.
3(Short Time Withstand Current Capacity
In this test,the rated short-time withstand current is
applied to the circuit breaker
under test for the specified duration of the time.
The rated short time withstand current is equal to be
rated short circuit breaking current and standarad
value of rated duration of short circuit current is 1
second or 3 seconds.
The current is measured by taking an oscillograph of
the short circuit current wave.
After the test,there should be no mechanical or
insulation damage and any contact welding.
Routine Test Listed Below:-
1(Mechanical Operational test
2(One-Minute power frequency
voltage dry withstand test
3(Measurement of resistance of main
This test should be made before temperature rise
test.The measurement of resistance is made by
measuring dc voltage drop or resistance across the
terminals of each pole of circuit breaker.The current
during the test shall have any convenient value
between 100 A and the rated current of the circuit
breaker.The d.c. resistance of main contact of each
pole of a circuit breaker is of the order of a few tens
Operational tests are made to ensure that the circuit breakers
comply with prescribed operating conditions within the specified
voltage and supply pressure limits of their opening and closing
During routine test five opening and five closing operations should
carry out at
a. Minimum supply voltage and pressure
b. Maximum supply voltage and pressure
After the test,all parts should be in good condition and should not
The preliminary preparations of circuit breaker testing include
Connect the equipment.
Adjust the correct values of resistance and reactors to set
the required magnitude of short circuit.
Connect and set the transformer to get desired test voltage.
The contacts on sequence switch are adjusted to get
Oscillographs are adjusted and calibrated.
• 1) Short Circuit generator:-
It is a specially designed generator having very low
leakage reactance to provide short power for testing c.b.
It is driven by 3-phse induction motor.A flywheel is
provided on shaft of generator to supply kinetic energy
during short circuit and helps in spped regulation.
• 2)Short Circuit Transformer:-
It is connected between TB and power source.It is used
to allow for flexibility to test at different voltage levels and
also provide isolation between the power source and test
It is designed to have low leakage reactance and to
withstand repeated short circuit.
It is used as a back up c.b. to interrupt the short circuit
current in event of c.b.under test(TB) fails to operate.It
has higher capacity than the circuit breaker under
test.After every test,it is also isolates the test circuit
breaker(TB) from the supply source.It is set to operate at
a predetermined time after the initiation of short circuit
4) Make Switch(MS):-
It is very high speed switch used for initiation of short
circuit test current at desired moment.Recently,SF6 gas
make switch are used in short circuit testing station.
It is connected in series.It is used to limit or control the
magnitude of the short circuit or test current to its
6) Resistors and Capacitors (R and C):-
TRV shaping circuit(R and C) is connected across the
circuit breaker under test(TB).This circuit is used to control
transient recovery voltage(TRV).Capacitors are also used
for voltage measurement.
7) Measuring Equipments:-
For the measurements of voltage,voltage
dividers are used and for cureent measurements
shunts or CTs are used in the short circuit testing