All electrical equipment needs to be tested for assurance of their satisfactory performance. However, Circuit breaker testing is more difficult than the testing of other electrical equipment because the short circuit currents are very high.
Testing of circuit breakers can be classified into two main groups.
• Type test
• Routine test
Type tests are conducted on the 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 the testing process.
Type tests can be classified as
1 Mechanical performance test
2 Thermal test
3 Dielectric or insulation test
4 Short circuit 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.
The 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 the conductor cross-section using suitable low-resistivity material. In such tests, the contact drops or the contact resistances are also measured as these contact surfaces are responsible for the generation of heat and subsequent temperature rise.
A circuit breaker connected to the system is subjected to high voltage transients due to switching and lightning.
The insulation of the 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 the 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 the terminal with circuit breaker open
The voltage is gradually increased and maintained at the test value for 1 min. Power frequency tests are conducted under normal atmospheric conditions and are termed as dry withstand tests. A wet withstand test is performed by spraying the external insulation with water for 2 minutes while the rated service voltage is applied. The test overvoltage is maintained for 1 minute. This test is not required for indoor breakers.
Impulse voltage withstands the test
This test is not mandatory for indoor breakers since indoor breakers are not usually electrically exposed to impulse surges produced by lightning. In the 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 the 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 a non-effectively grounded system. The breaker should not puncture or flash over for at least ten applications of such a voltage.
Power frequency withstands voltage of circuit breaker: it is r.m.s. value of alternating voltage wave of power frequency (50 c/s) which the insulation of the circuit breaker should withstand under specified conditions of the 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-circuit test laboratories and oscillograms 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 is studied with particular reference to the making and breaking currents, both symmetrical and asymmetrical re-striking voltages, and the switchgear is tested several 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
Once type tests are conducted and a particular design is found to be satisfactory the product becomes a prototype and a large number of circuit breakers of similar design are manufactured.
However, 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 the main circuit of each pole
– Power frequency voltage withstand the 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
Testing station and equipment
There are three types of testing stations
– Field type
– Laboratory type
– Composite 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 to the system. Though this type of testing provides the most convincing method of testing HV circuit breakers it suffers from the drawback of limited available flexibility of the system. It is difficult to set the system for the specified RRRV for HV breakers.
In the laboratory-type testing station, special generators, called short circuit generators provide the power for testing. In this type of testing station, it is possible to vary the test conditions at will. The establishment of a short circuit testing plant particularly of the laboratory type is an exceedingly costly project, and all switchgear manufacturers can’t have such a facility of their own.
A composite testing station is a combination of the field-type testing station and laboratory-type testing station.