Switchgear and Protection MCQ

1. Purpose of backup protection is

(a) to increase the speed.
(b) to increase a reach.
(c) to leave no blind spot.
(d) to guard against failure of primary.

2. Protective relays are the devices that detect abnormal conditions in electrical circuits by measuring

(a) voltage.
(b) current.
(c) constantly the electrical quantities which differ during normal and abnormal conditions.
(d) none of the above.

3. The actuating quantity for the relays may be

(a) magnitude.
(b) frequency.
(c) phase angle.
(d) any of these.

4. Protective relays can be designed to respond to

(a) light intensity.
(b) temperature.
(c) resistance, reactance or impedance.
(d) voltage or current.
(e) all of the above.

5. The protective relay is provided to

(a) provide additional safety to the circuit breaker in its operation.
(b) close the contacts when the actuating quantity attains a certain predetermined value.
(c) limit the arcing current during the circuit breaker operation.
(d) earth any stray voltage.

7. The operating speed of a relay depends upon the

(a) rate of flux built up.
(b) armature core air-gap.
(c) spring tension.
(d) all of these.

8. Plug setting of an electromagnetic relay can be altered by varying

(a) number of ampere turns.
(b) air gap of magnetic path.
(c) adjustable back stop.
(d) none of these.

9. The torque produced in shaded pole structure induction type relay is

(a) proportional to the square of the current.
(b) proportional to the current.
(c) inversely proportional to the current.
(d) inversely proportional to the square of the current.

10. The most efficient torque-producing actuating structure for induction-type relays is

(a) shaded pole structure.
(b) watt-hour meter structure.
(c) induction-cup structure.
(d) single induction loop structure.

11. A bimetal strip, in a thermal relay, consists of two metal strips of different

(a) specific heat.
(b) thermal conductivity.
(c) coefficients of expansion.
(d) all of the above.

12. Thermal relays are used for protection of motors against over-current owing to

(a) short circuits.
(b) heavy loads.
(c) earth fault.
(d) all of these.

13. Buchholz relay is

(a) located in the conservator tank.
(b) located in the transformer tank itself.
(c) connected in the pipe connecting main tank of transformer and conservator.
(d) installed in the circuit breaker.

14. Buchholz relays are used on

(a) air cooled transformers.
(b) instrument transformers.
(c) distribution transformers.
(d) oil immersed power transformers of ratings above 500 kVA.

15. The Buchholz relay protects a transformer from

(a) types of internal faults.
(b) a turn to turn fault.
(c) winding to winding fault.
(d) none of these.

16. Which of the following relays is/are overload relays?

(a) Thermal.
(b) Electromagnetic.
(c) Induction.
(d) Solid state.
(e) All of the above.

17. In an overcurrent protection the setting of the earth fault relay is

(a) more than the phase fault relay.
(b) equal to the phase fault relay.
(c) less than the phase fault relay.
(d) the two settings are unrelated to each other.

18. Inverse time-current relays are used for the protection of

(a) feeders.
(b) transformers.
(c) alternators.
(d) both (a) and (b).

19. IDMT relays are used to protect the power transformers against

(a) external short circuits.
(b) overloads.
(c) internal short circuits.
(d) both (a) and (b).

20. Consider the following statements with reference to protective relays :

1. The minimum relay coil current at which the relay operates is called pick-up value.
2. The pick-up value of a relay is 7.5 A and fault current is 30 A. Therefore, its plug setting multiplier is 5.
3. An earth fault current is generally lesser than the short-circuit current.
4. Induction relays are used with both ac and dc quantities.

Which of these statements are correct ?
(a) 1 and 2.
(b) 2 and 3.
(c) 1 and 3.
(d) 1, 2 and 4.