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Interview Question of circuit breaker

1. What is the meaning of rupturing capacity of a circuit breaker?

Ans: Rupturing capacity of a circuit breaker means the maximum power a circuit breaker can interrupt under a fault. It is usually expressed in Mega volt Ampere (MVA) and it is then the product of the rated breaking current in kilo amperes and rated voltage expressed in kilo volts.

2. What do you mean by the term “Making Current” and “Breaking Current” of a circuit breaker?

Ans: (a) The ‘Making Current’ of a circuit breaker is the total maximum current peak which occurs during the first cycle immediately after the circuit is closed on a short circuit. (b) The ‘Breaking Current’ of a circuit breaker is the total maximum current peak that can be safely broken by the circuit breaker at the time of separation of the contacts at its rated voltage.

3. What do you understand by the term “Short Circuit MVA”? How s the breaking capacity specified?

Ans: “Short Circuit MVA” is the rupturing capacity or breaking capacity of a circuit breaker in case of a three phase symmetrical fault which determines the size of the circuit breaker. The breaking capacity of a circuit breaker is specified either in terms of symmetrical breaking current/MVA or asymmetrical breaking current/MVA.

4. Why is it necessary to know the fault level at the incoming point of supply? Who furnishes this information ?

Ans: To select the MVA rating of the circuit breaker of a high voltage installation the fault level at the incoming point of supply must be known. Power supply authority furnishes the fault level.

5. In what value the making current of a 3-phase 3000 MVA, 33 kV breaker?

Ans: the making current is normally specified in peak value.
Breaking current = 3000/(√3×33) KA = 52.48 KA
Making current = 2.55 × 52.48 = 133.82 KA.

6. What do you mean by “asymmetrical breaking current” and “symmetrical breaking current”?

Ans: The asymmetrical breaking current is the r.m.s. the value of the total current at the initial instant of contact separation and is equal to the square root of the sum of the squares of the symmetrical breaking current and of the direct component of the current at the same instant. The symmetrical breaking current is the r.m.s. the value of the symmetrical current (a.c. component of current) at the initial instant of contact separation. Actually, the rating of the breaker is specified in terms of symmetrical breaking current & voltage.

7. Why is the rating of a circuit breaker decided on the basis of symmetrical short circuit currents?

Ans: As the symmetrical fault is most severe in a three phase line and imposes the heavy duty on the control gear, therefore, the circuit breaker is rated on the basis of symmetrical short circuit currents.

8. Why is symmetrical fault more severe than the unsymmetrical fault in a 3-phase system?

Ans: In a symmetrical fault since all the three conductors are short circuit together causing equal fault currents to flow through the line then the symmetrical fault because more severe.

9. What is the importance of base kVA in short circuit calculations?

Ans: As the various equipment used in the power system have different KVA rating with different percentage reactances so for easy calculation it is necessary to find the percentage reactances of all equipment on a common KVA rating. This common KVA rating is known as base KVA.

10. Does the value of short circuit current vary for different values of base KVAs?

Ans: No. Whatever may be the value of base KVA the value of short circuit current remains unchanged.

11. What do you mean by the ‘recovery voltage’ of a circuit breaker?

Ans: The recovery voltage is defined to be the r.m.s. the value of the line voltage of service frequency that reappears across the poles of the circuit breaker short after arc extinction in all the poles and after damping of the transient phenomena.

12. What do you mean by the term ‘restriking voltage’ of a circuit breaker?

Ans: The restriking voltage or the transient recovery voltage is the voltage between the contacts of a pole of the circuit breaker after the extinction of the arcs while the transient state persists.

13. Why the circuit breakers to be used on 6.6 kV, 11 kV, 33 kV, 66 kV, and 132 kV lines should be of rated voltage 7.2 kV, 12 kV, 36 kV, 72 kV, and 145 kV respectively?

Ans: As the voltage rating of circuit breaker corresponds to highest system voltage so the circuit breaker to be used on 6.6 kV line should be of rated voltage 7.2 kV and so on.

14. On what factors does the rate of rising of restriking voltage depend?

Ans: The rate of rising of restriking voltage depends upon the inductance and capacitance of the system.

15. Why is current interruption easier in an a.c. circuit than in a d.c. circuit ?

Ans: Due to natural current zeros in alternating current waves interruption of an a.c. circuit becomes more easier.

16. Is an a.c. circuit breaker suitable for d.c. supply line?

Ans: Generally a.c. the circuit breaker is not suitable for d.c. supply line because of a.c. circuit breaker quenches arc easily at current zero of an alternating wave which is not possible in d.c. due to nonexistence of current zero.

17. (a) Is a 50 Hz circuit breaker suitable for 60 Hz supply? (b) Can a 12 kV circuit breaker be used for 6.6 kV application?

Ans: (a) It is possible only when the breaker is type tested at 60 Hz frequency, otherwise not.
(b) A 12 kV circuit breaker can use for any voltage up to its rated voltage under the condition that the breaking current is within its breaking capacity.

18. What is the difference between low voltage circuit breakers and high voltage circuit breakers?

Ans: i) Low voltage circuit breakers are used in indoor control gears for controlling power consuming devices of circuits below rated voltage 1000 volt a.c. or 1200 volts d.c. whereas high voltage circuit breakers are used in both indoor and outdoor controllers in high voltage supply system.
ii) Low voltage circuit breakers operate frequently than the high voltage circuit breaker.
iii)Low voltage circuit breakers are more compact than high voltage circuit breakers because of less phase to phase and phase to ground clearances.
iv)Testing requirements of a high voltage circuit breaker are quite different from those of low voltage circuit breakers.

19. How are the rating of a circuit breaker given?

Ans : The rating of a circuit breaker are given by (a) the maximum voltage in kV, (b) the maximum continuous current carrying capacity in amperes, (c) the maximum interrupting capacity in MVA and kilo amperes (kA), (d) the supply frequency and (e) the making capacity in amperes.

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