Circuit Breaker Tester Classification:
Circuit Breaker Tester can be classified into two main groups, viz. (i) type tests and (ii) routine tests.
These are performed solely for the purpose of proving the correctness of general design and the results of such tests conducted on a randomly selected breaker, are applicable to all others of identical construction.
Under this class fall mostly the short-circuit tests, which again can be performed either by (a) direct testing or (b) indirect testing methods.
The routine tests are performed on each individual circuit breaker of proven design, the purpose is to prove the correctness of the assembly and the material used.
The routine tests include the following tests:
- Operational tests.
- Measurement of resistance of the main circuits.
- One minute power frequency voltage dry withstand tests.
The test procedures, methods of finding results, tests to be performed, etc. are specified by Standards Organizations. The information regarding the tests given here is based on the recommendations of the International Electrotechnical Commission (IEC) vide publication No. 56-1 on the testing of a.c. circuit breakers.
It is commonly accepted and provided in various standards that the certificates of type tests shall be considered as evidence of the compliance of the circuit breakers with the relevant clauses of the specifications and the manufacturer shall make available such certificates together with the detailed drawings of the Circuit Breaker Tester and a record of any alteration that may have been made in them subsequent to the type tests.
Type tests can be broadly classified as :
- Mechanical tests.
- Thermal tests.
- Dielectric tests.
- Short-circuit tests: (i) making capacity test, (ii) breaking capacity tests, (iii) duty cycle tests and (iv) short time current tests.
Short circuit tests are of utmost importance and will be dealt in detail in the next section. However, a brief account of other tests is given below.
Mechanical Tests: These are mechanical endurance type-tests involving repeated opening and closing of the breaker. The circuit breaker is opened and closed several times (500) without any current in the current carrying parts. Some operations (50) are by energizing the relay, remaining are by closing the trip circuit by other means. No adjustments are permissible during testing. After the test there should be no distortion or wear of the parts.
Thermal Tests: These are conducted by passing alternating current of normal frequency through the current carrying parts of the Circuit Breaker Tester. The temperature is measured by thermometers or thermocouples with temperature indicators or self-resistance methods.
The temperature rise for rated current should not exceed 40°C for currents less than 800 A normal current and 50°C for normal value of current 800 A and above.
An additional requirement hi the type-test is the measurement of contact resistances between the isolating contacts and between the moving and the fixed contacts. These points are generally the main sources of excessive heat generation. The voltage drop across the breaker pole is measured for different values of d.c. currents. The voltage drop gives a measure of resistance of current carrying parts, hence that of contacts.
Dielectric Tests: These are conducted for testing the insulators of the These can be classified as:
Power frequency tests: 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: (i) between poles with circuit breaker closed, (ii) between poles and earth with circuit breaker open, and (iii) across terminals with circuit breaker open.
The voltage is gradually increased and maintained at test value for 1 minute.
Impulse tests: In this test impulse voltage of specified shape and magnitude is applied to the breaker. For outdoor circuit dry and wet tests are conducted.
Short-Circuit Test: These [are conducted in short-circuit testing stations and are mainly meant to prove the ratings of the circuit breaker. Before discussing the tests proper it would be worth while to discuss about the short-circuit testing stations.
Short Circuit Testing Stations:
There are two types of short-circuit testing stations:
(a) The Field Type Testing Station: In this the tests are conducted taking power directly from the system. While it provides the most convincing method of testing high voltage Circuit Breaker Tester, it suffers from the drawback that flexibility of the system available is limited. It is difficult to set the system for the specified RRRV for breakers of high voltages. It is also not suitable for research and development work as it is always not possible to repeat the test again and again without disturbing the networks. The power available in the field testing station varies according to the machinery running on the network and the layout of the network.
(b) Laboratory Type Testing Station: It has special generators to supply power for the short circuit testing. In this it is possible to vary the test conditions at will. Such a station is most suited from the point of view of the design engineer and supply engineer. The designer can study the behavior Of arc rupture and make improvements for higher voltages and higher breaking capacities. He can repeat his tests again and again in order to show the reproducibility of his results which is very important. The supply engineer is mostly interested in the operation of the breaker ready for delivery. He wants to be satisfied whether his breaker fulfills the requirements enunciated by certain specifications.
To establish a short circuit 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.
The testing station at Bhopal is of laboratory type and barring Japan it is likely to remain the only switchgear testing station in Asia for some time to come. Another small but a unique short-circuit testing laboratory is available at the Indian Institute of Technology (IIT) Bombay.