Switchgear and Protection MCQ With Answers

1. Which of the following results in a symmetrical fault ?

(a) Single-phase-to earth.
(b) Phase-to-phase.
(c) All the three phases-to earth.
(d) Two phases-to earth.

2. Which portion of the transmission system is more prone to faults?

(a) Alternator.
(b) Transformer.
(c) Overhead lines.
(d) Underground cable.

3. Which portion of the power system is least prone to faults ?

(a) Alternators.
(b) Switchgear.
(c) Transformers.
(d) Overhead lines.

4. The magnitude of fault current depends upon

(a) total impedance up to fault.
(b) voltage at the fault point.
(c) load current being supplied before occurrence of fault.
(d) both (a) and (b).

5. The most common type of fault is

(a) single-phase-to ground.
(b) phase-to-phase.
(c) two-phase-to ground.
(d) three-phase to ground.

6. The maximum short-circuit current occurs in the case of:

(a) three-phase bolted fault.
(b) double-line-to-ground fault.
(c) line-to-line fault.
(d) single-line-to-ground fault.

7. For a fault at the terminals of synchronous generator, the fault current is maximum for a

(a) 3-phase fault.
(b) 3-phase to ground fault.
(c) line-to-ground fault.
(d) line-to-line fault.

7. The load currents in short-circuit calculation are neglected because

1. short-circuit currents are much larger than load currents.
2. short-circuit currents are greatly out of phase with load currents.

Which of these statement(s) is/are correct ?
(a) neither 1 nor 2
(b) 2 alone
(c) 1 alone
(d) 1 and 2

8. Series reactors are used to

(a) improve the transmission efficiency.
(b) improve the power factor of the power system.
(c) improve the voltage regulation.
(d) bring down the fault level within the capacity of the switchgear.

9. Current limiting reactors may be

(a) air-cored air-cooled.
(b) oil immersed magnetically shielded.
(c) oil immersed non-magnetically shielded.
(d) any of the above.

10. The symmetrical components are used in the fault analysis because

(a) the number of equations becomes smaller.
(b) the sequence networks do not have mutual couplings.
(c) the results are required in terms of symmetrical components.

11. In a star-connected system without neutral grounding, zero-sequence currents are

(a) zero.
(b) phasor sum of phase currents.
(c) same as rms value of phase currents.
(d) same as peak value of phase currents.

12. A balanced 3-phase system consists of

(a) zero-sequence currents only.
(b) positive-sequence currents only.
(c) negative and zero-sequence currents.
(d) zero, negative and positive sequence currents.

13. The positive sequence current of a transmission line is:

(a) always zero.
(b) 1/3 of negative sequence current.
(c) equal to negative sequence current.
(d) 3 times negative sequence current.

14. In case of an unbalanced star-connected load supplied from an unbalanced 3-phase, 3-wire system, load currents will consist of

(a) positive-sequence components.
(b) negative-sequence components.
(c) zero-sequence components.
(d) only (a) and (b).

15. For an unbalanced fault, with paths for zero-sequence currents, at the point of fault

(a) the negative and zero-sequence voltages are minimum.
(b) the negative and zero-sequence voltages are maximum.
(c) the negative-sequence voltage is minimum and zero-sequence voltage is maximum.
(d) the negative-sequence voltage is maximum and zero-sequence voltage is minimum.

16. When a line-to-ground fault occurs, the current in the faulted phase is 100 A. The zero-sequence current in this case will be:

(a) zero
(b) 33.3 A
(c) 66.6 A
(d) 100 A

17. Zero-sequence fault current is absent when fault is

(a) single-line-to ground fault.
(b) line-to-line ground fault.
(c) double-line-to ground fault.
(d ) line-to-line.

18. The current of a single-phase load drawn from a 3-phase system has:

(a) Zero-sequence component of current as zero.
(b) Negative-sequence component of current more than positive sequence component.
(c) Positive, negative and zero-sequence components equal.
(d) Negative-sequence component of current less than positive-sequence component.

19. Zero-sequence currents can flow from a line to transformer bank if the windings are in

(a) grounded star/delta.
(b) delta/star.
(c) star/grounded star.
(d) delta/delta.

20. If the positive, negative and zero-sequence reactances of an element of a power system are 0.3, 0.3 and 0.8 pu respectively, then the element would be a

(a) synchronous generator.
(b) synchronous motor.
(c) static load.
(d) transmission line.