Switchgear and Protection MCQ With Answers

21. Negative-sequence reactance of a transformer is

(a) equal to the positive-sequence reactance.
(b) larger than the positive-sequence reactance.
(c) smaller than the positive-sequence reactance.

22. If all the sequence voltages at the fault point in a power system are equal, then the fault is a

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

23. The zero-sequence current of a generator for line-to ground fault is j2.4 pu. Then the current through the neutral during the fault is

(a) j2.4 pu
(b) j0.8 pu
(c) j7.2 pu
(d) j0.24 pu

24. One current transformer (CT) is mounted over a 3-phase 3-core cable with its sheath and armour removed from the portion covered by the CT. An ammeter placed in the CT secondary would measure

(a) the positive-sequence current.
(b) the negative-sequence current.
(c) the zero-sequence current.
(d) three times the zero-sequence current.

25. Consider the following statements regarding the fault analysis :

1. The neutral grounding impedance Z_n appears as 3Z_n in zero-sequence equivalent circuit.
2. For faults on transmission lines, 3-phase fault is the least severe amongst other faults.
3. The positive and negative-sequence networks are not affected by method of neutral grounding.

Which of the statements given above are correct ?
(a) 2 and 3
(b) 1 and 2
(c) 1 and 3
(d) 1, 2 and 3

26. An alternator (star connected) with star neutral earthed. On the star side of the transformer a single line-to-ground fault occurs. This is equivalent to :

(a) A line-to-ground fault on the generator side of the transformer.
(b) A line-to-line fault on the generator side of the transformer.
(c) A 3-phase fault on the generator side of the transformer.
(d) A single-line-to-ground fault on the generator side of the transformer.

27. Fuses have got advantages of

(a) cheapest-type of protection.
(b) inverse time-current characteristic.
(c) no maintenance.
(d) current limiting effect under short-circuit conditions.
(e) all of the above.

28. The primary function of a fuse is to

(a) open the circuit.
(b) protect the appliance.
(c) protect the line.
(d) prevent excessive currents from flow through the circuit.

29. The material used in liquid fuses is

(a) SF6
(b) distilled water.
(c) carbon tetra chloride.
(d) transformer oil.

30. In comparison to rewirable fuses HRC fuses have the advantage(s) of

(a) high speed operation.
(b) high rupturing capacity.
(c) no ageing effect.
(d) all of the above.

31. In HRC fuse the time between cut-off and final current zero is called the

(a) pre-arcing time.
(b) arcing time.
(c) total operating time.
(d) none of these.

32. If a combination of HRC fuse and a circuit breaker is employed, the circuit breaker operates for

(a) short-circuit current.
(b) low overload currents.
(c) under all abnormal currents.
(d) the combination is newer employed in practice.

33. The least expensive protection for over-current in low-voltage system is

(a) rewirable fuse.
(b) isolator.
(c) circuit breaker.
(d) air-break switch.

34. A circuit breaker normally operates

(a) when the power is to be supplied.
(b) when the line is to be tested.
(c) when the switch is to be put on.
(d) whenever fault occurs in the line.

35. When a fault occurs in a high voltage transmission line, first the

(a) circuit breaker operates then the relay.
(b) relay operates then the circuit breaker.
(c) relay operates, then successively the isolator and the circuit breaker.
(d) isolator operates, then successively the relay and the circuit breaker.

36. The initiation of electric arc at the instant of contact separation is caused by

(a) thermionic emission of electrons.
(b) field emission of electrons.
(c) (a) and (b) both.
(d) none of the above.

37. Arcing voltage will be the least in case of

(a) carbon.
(b) copper.
(c) silver.
(d) tungsten.

38. The heat produced at the contact point owing to flow of electric current is least affected by

(a) temperature of the surrounding medium.
(b) contact resistance.
(c) magnitude of electric current flowing.
(d) duration of flow of current.

39. Which of the following should have low value for the contacts and their material ?

(a) Thermal capacity.
(b) Contact resistance.
(c) Thermal conductivity.
(d) None of these.

40. In a CB contact wipe is necessary

(a) to decrease the speed of opening of the CB.
(b) to increase the speed of opening and reduce the arcing of contacts.
(c) to increase the dielectric strength across the CB contacts.
(d) none of the above.