Circuit Breakers MCQs

61. In order to switch-off and EHV circuit for maintenance, the following sequence is adopted

1. Open the circuit breaker, open the isolator, operate the earth switch
2. Operate the earth switch, open the isolator, open the circuit breaker
3. Open the isolator, operate the earth switch, open the circuit breaker
4. Open the isolator, open the circuit breaker, operate the earth switch

62. Making current of a circuit breaker refers to

1. Steady-state rated current
2. Average value of transient short circuit current
3. R.M.S. value of the short circuit current at the time of major peak
4. Equal to the fault current

63. Out of the following, which one is correct in case of a circuit breaker?

1. Making capacity = 2.55 x braking capacity
2. Braking capacity = 2.55 x making capacity
3. Making capacity = 1.8 x braking capacity
4. Braking capacity =1.8 x making capacity

64. Which of the following is incorrect?

1. Double break oil circuit breaker has 3 breakers.
2. There are 2 stationary contacts in double break oil circuit breaker.
3. Arc generated in most of the circuit breakers absorb the energy oil.
4. Circuit breaker is a mechanical switching.

65. The breaking capacity of a 3-phase circuit breaker is

1. 3 times the product of service voltage and rated symmetrical current
2. $\sqrt{3}$ times the product of service voltage and rated symmetrical current
3. $\sqrt{2}$ times the product of service voltage and rated symmetrical current
4. 2 times the product of service voltage and rated symmetrical current

66. The insulating fluids that are commonly used in circuit breakers is/are

1. SF₆
2. Air at atmospheric pressure
3. Compressed air
4. All of the above

67. Which of the following circuit breaker is generally used in railway electrification?

1. Air Blast circuit breaker
2. Minimum Oil circuit breaker
3. Bulk Oil circuit breaker
4. SF₆ circuit breaker

68. The main purpose of oil in OCB is to 

1. provide insulation
2. provide cooling of contacts
3. quenching arc
4. none of the above

69. A 3-phase circuit breaker is rated at 1250 A, 2000 MVA, 33 kV, 4s. Its making current capacity will be

1. 35 A
2. 89 A
3. 69 A
4. 79 A

70. A circuit breaker is rated as follows: 1500 A, 33 kV, 3 sec, 3-phase oil circuit breaker. Determine the making current?

1. 110 A
2. 35 A
3. 15 A
4. 89 A