Circuit Breaker Testing

Circuit breaker testing

CB testing

All electrical equipment’s need to be tested for assurance of their satisfactory performance. However, testing of circuit breakers is more difficult than the testing of other electrical equipment’s because the short circuit currents
are very high.

Testing of circuit breaker can be classified into two main groups.
• Type test
• Routine test

Type test

Type tests are conducted on first few prototype circuit breakers to determine whether the units are manufactured as per design or not.These tests are not conducted on every circuit breaker.The tests are conducted in specially built
testing laboratories. IEC standard is thoroughly followed in testing process.

Type test can be classified as

1 Mechanical performance test
2 Thermal test
3 Dielectric or insulation test
4 Short circuit test

Mechanical test

These are mechanical endurance type tests involving repeated opening and closing of the breaker. A circuit breaker must open and close at the correct speed and perform its designated duty and operation without mechanical failure.
After the tests, the contacts and all the other parts should be in good condition and should not show any permanent deformation or distortion.

Thermal test

Alternating current of rated value and rated frequency is passed through a closed circuit breaker continuously till a steady temperature is attended. When the steady temperature is reached, the max temperature rise of each part should be less than the permissible limit. The temperature rise for rated current should not exceed 40°C for
current less than 800 A and 50°C for current 800 A and above. When a circuit breaker in closed condition carries normal current, the heat is generated in current carrying parts due to I2Rt loss. To maintain the temperature rise within specified limits, the I2Rt losses should be reduced by increasing conductor cross section using suitable low resistivity material. In such test the contact drops or the contact resistances are also measured as these contacts surfaces are responsible for generation of heat and subsequent temperature rise.

Dielectric test

A circuit breaker connected in the system is subjected to high voltage transients due to switching and lightning.
The insulation of circuit breaker should not fail due to such voltage surges. These tests are performed to check power
frequency and impulse voltage withstand capacity.

Power frequency test

– One minute dry withstand test
– One minute wet withstand test
Power frequency tests are conducted on a clean new circuit breaker, the test voltage varies with circuit breaker rated voltage. The test voltage with a frequency between 15 – 100 Hz is applied as follows.
– between poles with circuit breaker closed
– between poles and earth with circuit breaker open
– across terminal with circuit breaker open
The voltage is gradually increased and maintained at test value for 1 min.  Power frequency tests are conducted under normal atmospheric conditions and termed as dry withstand test. Wet withstand test is performed by spraying the external insulation by water for 2 minutes while the rated service voltage is applied. The test over voltage is maintained for 1 minute. This test is not required for indoor breakers.

Impulse voltage withstand test

This test is not mandatory for indoor breakers since the indoor breakers are not usually electrically exposed to
impulse surges produced by lightning. In case of outdoor breakers the effect of impulse voltage is very serious and hence the breaker is tested to prove its immunity to impulse voltage conditions. Depending on grounding system, where the breaker is to be installed, the peak value of the impulse wave is selected.  The magnitude of the selected peak of the impulse voltage is high for non-effectively grounded system. The breaker should not puncture or flash over for at least ten applications of such a voltage.

 Power frequency withstand voltage of circuit breaker: it is r.m.s. value of alternating voltage wave of power frequency (50 c/s) which the insulation of circuit breaker should withstand under specified conditions of test.
Impulse withstand voltage: it is the amplitude of the standard voltage which the insulation of the circuit breaker can withstand under specified conditions.

Short circuit tests

Circuit breakers are subjected to sudden short-circuits in short-circuits test laboratories and oscillogram are taken to
know the behavior of the circuit breakers at the time of switching-in, during contact breaking, and after arc
extinction . The oscillogram are studied with particular reference to the making and breaking currents, both symmetrical and asymmetrical re-striking voltages, and switchgears are tested a number of times at rated conditions.
• The various tests carried out for circuit breaker testing are as follows;
– Making capacity test
– Breaking capacity test
– Duty cycle test
– Short time current test

Routine tests

Once type tests are conducted and particular design is found to be satisfactory the product becomes prototype and a large number of circuit breakers of similar design are manufactured.  However, each and every circuit breaker is still
subjected to a few more tests before commissioning. These tests are called routine tests. Routine tests are also performed as per recommendations of the standards (IEC/IS). Routine tests confirm the proper functioning of
the circuit breaker.

The routine tests include;
– Mechanical operation test
– Measurement of resistance of main circuit of each pole
– Power frequency voltage withstand test
– Voltage withstand test on auxiliary circuits
– Measurement of insulation resistance of main circuits
– Measurement of insulation resistance of auxiliary circuits
– Tests and checks after mechanical operation tests

CB testing

Testing station and equipments

There are three types of testing stations
– Field type
– Laboratory type
– Composite type


Field type

In the field type of testing, the power required for testing is directly drawn from a large power system.  The breaker being tested is connected in the system. Though this type of testing provides the most convincing method of testing HV circuit breakers but it suffer from the drawback of limited available flexibility of the system. It is difficult to set the system for the specified RRRV for HV breakers.

Laboratory type

In the laboratory type testing station, special generators, called the short circuit generators provide the power for testing.  In this type of testing station, it is possible to vary the test conditions at will. Establishment of a short circuit testing plant particularly of the laboratory type is an exceedingly costly project, and it is not possible for all switchgear manufacturers to have such a facility of their own.

Composite type

A composite testing station is a combination of the field type testing station and laboratory type testing station.


Read more:

1.What is vacuum circuit breaker?

2.Circuit breaker ratings




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