Switchgear and Protection MCQ With Answers

41. Interrupting medium in a contactor may be

(a) air at atmospheric pressure.
(b) SF6 gas.
(c) oil.
(d) compressed air.
(e) any of these.

42. The fluids used in circuit breakers should be of

(a) high dielectric strength and thermal stability.
(b) non-inflammability.
(c) arc extinguishing ability.
(d) all of the above.

43. Sparking between contacts can be reduced by inserting

(a) a capacitor in parallel with the contacts.
(b) a capacitor in series with the contacts.
(c) a resistor in the line.
(d) a reactor in the line.

44. For magnetic blow out of arc the magnetic field is produced

(a) in the load circuit.
(b) at right angles to the axis of the arc.
(c) in line with the axis of the arc.
(d) any of the above.

45. In circuit breakers the contact space is ionized by

(a) field emission from the contact surface.
(b) thermal emission from the contact surface.
(c) thermal ionization of gas.
(d) any of the above.

46. In a circuit breaker, ionization is facilitated by

(a) increase in field strength.
(b) increase of mean free length.
(c) high temperature of surrounding medium.
(d) all of the above.

47. The stability of arc in vacuum depends on

(a) The contact material only.
(b) The contact material and its vapor pressure.
(c) The circuit parameters only.
(d) The combination of (b) and (c).

48. The interrupting time of a circuit breaker is the period between the instant of

(a) initiation of short circuit and the arc extinction on an opening operation.
(b) energizing of the trip circuit and the arc extinction on an opening operation.
(c) initiation of short circuit and the parting of primary arc contacts.
(d) energizing of the trip circuit and the parting of primary arc contacts.

49. For a high speed circuit breaker the total clearing time is around

(a) few minutes.
(b) few seconds.
(c) 1 to 2 cycles.
(d) 5 to 20 cycles.

50. The resistance of an electric arc can be increased by

(a) increasing the concentration of ionized particles.
(b) reducing the arc length.
(c) splitting the arc.
(d) increasing the arc x-section.

51. The arc voltage in a circuit breaker is

(a) in the phase with the arc current.
(b) lagging the arc current by 90°.
(c) leading the arc current by 90°.
(d) lagging the arc current by 180°.

52. In a circuit breaker, the active recovery voltage depends upon

(a) circuit conditions.
(b) power factor.
(c) armature reaction.
(d) all of these.

53. In a circuit breaker the current that exists at the instant of contact separation is called the………current.

(a) restriking
(b) breaking
(c) arc
(d) recovery

54. The rate of rise of restriking voltage (RRRV) depends upon

(a) system voltage.
(b) circuit pf only.
(c) switching condition only.
(d) both (b) and (c).

55. The RRRV depends upon the

(a) type of the circuit breaker.
(b) capacitance of the system only.
(c) inductance of the system only.
(d) inductance and capacitance of the system.

56. It is difficult to interrupt a capacitive circuit because

(a) the current has a leading power factor.
(b) the restriking voltage can be high.
(c) current magnitude is very small.
(d) stored energy in the capacitor is very high.

57. In circuit breakers, oil, as quenching medium, has the advantage(s) of

(a) good cooling properties.
(b) absorption of arc energy.
(c) high dielectric strength.
(d) acting as an insulator.
(e) all of the above.

58. In circuit breakers, oil, as quenching medium, has the disadvantage(s) of

(a) need of periodical reconditioning or replacement.
(b) risk of formation of explosive mixture with air.
(c) possibility of causing fire hazards.
(d) all of the above.

59. Main purpose of oil in OCB is to

(a) provide insulation.
(b) provide cooling of contacts.
(c) quenching arc.
(d) none of above.

60. Arc interruption in an oil circuit breaker takes place when

(a) contacts apart.
(b) voltage goes through zero.
(c) current goes through zero.