Circuit Breaking Interview Questions and Answers:

1. What is a circuit breaker ?

Ans. Circuit breaker is a mechanical device designed to close or open contact members, thus closing or opening an electrical circuit under normal or abnormal conditions.

2. How does a circuit breaker differ from a switch ?

Ans. A circuit breaker is a device that interrupts the abnormal or fault currents and in addition performs the function of a switch.

3. Which of the gases are employed in commercial gas-blast circuit breakers ?

Ans. SF6 and air are used in commercial gas-blast circuit beakers.

4. What is meant by current chopping ?

Ans. Current chopping is a phenomenon of current interruption before the natural current zero is reached.

5. Why is current chopping considered a serious drawback in a circuit breaker ?

Ans. The current chopping is considered a serious drawback because it sets up high voltage transient across the breaker contacts.

6. How is the effect of overvoltage developed due to current chopping overcome ?

Ans. By using resistance switching i.e., by connecting a resistance across the circuit breaker contacts.

7. Why current chopping is not common in oil circuit breakers ?

Ans. Current chopping is not common in oil circuit breakers because in most of the oil circuit breakers the are extinguishing power is proportional to the magnitude of current to be interrupted.

8. What is meant by kilometric fault ?

Ans. The fault occurring between a distance of a few km to few tens km from the circuit breaker are is called kilometric fault.

9. What is meant by breaking or rupturing capacity of a circuit breaker ?

Ans. Breaking capacity of a circuit breaker is the highest rms value of current that it is capable of breaking under specified conditions of transient recovery voltage and power frequency voltage. It is expressed in kArms at contact separation.

10. Why asymmetrical breaking current is higher than symmetrical breaking current in a circuit breaker ?

Ans. Asymmetrical breaking current is the rms value of the combined sum of ac and dc components of short-circuit current at the instant of separation of breaker contacts while symmetrical breaking current is the rms value of the ac component alone. Thus asymmetrical breaking current is higher than symmetrical breaking current.

11. What is meant by making capacity of a circuit breaker ?

Ans. The making capacity of a circuit breaker, when closed on a short circuit, is the peak value of the maximum current wave (including dc component) in the first cycle of the current after the circuit is closed by the circuit breaker.

Rated making capacity = 2.55 x symmetrical rated breaking capacity

12. What is meant by rated voltage of a circuit breaker ?

Ans. The rated maximum voltage of a circuit breaker is the highest rms voltage, above nominal system voltage, for which the circuit breaker is designed and is the upper limit for operation.

13. Why testing of a circuit breaker is more difficult than the testing of other electrical equipment like transformer or machine ?

Ans. Testing of circuit breaker is more difficult than the testing of other electrical equipment like transformer or machine because the short-circuit currents are very large. Also there is no satisfactory method of testing circuit breaker at reduced power.

14. What are type tests which are conducted on circuit breakers ?

Ans. Type tests are conducted on first few prototype circuit breakers of each type for the purpose of proving the capabilities and confirming the rated characteristics of the circuit breaker of that design. Such tests are conducted in specially built testing laboratories. Type tests are performed as per recommendations of standards (IEC) or (IS).

Type tests can be broadly classified as (i) mechanical performance tests (ii) thermal tests (iii) dielectric or insulation tests and (iv) short-circuit tests in order to check making capacity, breaking capacity, short-time rating current and operating duty.

15. What are routine tests which are conducted on circuit breakers ?

Ans. Once type tests are conducted and a 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). These tests are performed in the manufacturer’s premises. Routine tests confirm the proper functioning of the circuit breaker.

The routine tests include (i) power frequency voltage test (ii) millivolt drop test and (iii) operational tests.

16. What are the types of testing stations for CBs ?

Ans. There are three types of testing stations viz. field type testing station, laboratory type testing station; composite testing station.

17. List the main equipment required in the testing stations for circuit breakers.

Ans. Equipment used in testing stations are : (i) Short-circuit generator and drive motor (ii) Impulse exciter (iii) Pilot generator, (iv) Short-circuit transformer (v) Master circuit breaker, (vi) Resistors and reactors (vii) Make switch (viii) Capacitors (ix) Test cubicles (x) Test control room (xi) Safety and signaling system.

18. What is direct testing in case of circuit breakers ?

Ans. The direct testing of CBs in a plant enables us to test it under conditions largely representing those in actual network as well as tests of greater severity. The circuit breaker under test is subjected to the value of transient restriking voltage to which it is expected to be put in practice rather than testing it under most severe conditions.

19. Why does indirect testing of CBs become necessary ?

Ans. The short-circuit power available in earlier testing stations (of the order of 4,000 MVA in laboratory type station) is not sufficient for testing a complete circuit breaker (which is of rated breaking capacity of the order of 10,000 MVA at 245 V). Even single pole of a EHV circuit breaker cannot be tested by direct means. It is, therefore, necessary to utilize some form of indirect testing. The important indirect methods of testing are : Unit testing and synthetic testing.

20. What is unit testing and synthetic testing ?

Ans. Almost all modern EHV circuit breakers, minimum oil, air blast, SF6 etc. consists of two or more identical units (or interrupters) per pole. These interrupters operate simultaneously and share the voltage across the pole almost equally. The breaking capacity in MVA is also shared equally. Hence a test on one unit can be accepted as proof for all units. Such tests are called the unit tests. Unit testing is an internationally accepted method.

Synthetic testing is a practical and economical solution for testing of CBs of high rupturing capacities, without actually using the corresponding short-circuit capacity of the testing station. The synthetic circuit is designed to simulate as accurately as possible the electrical stresses impressed on the circuit breaker during the interruption of fault current under system conditions.

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