Circuit Breakers MCQs
1. A circuit breaker is
a. power factor correcting device
b. a device to neutralize the effect of transients
c. a waveform correcting device
d. a current interrupting device
Answer
Answer. d
2. Low voltage circuit breakers have rated voltage of less than
a. 220 V
b. 400V
c. 1000 V
d. 10,000 V
Answer
Answer. c
3. The medium employed for the extinction of arc in air circuit breaker is
a. SF6
b. Oil
c. Air
Answer
d. Water
Answer. C
4. For high voltage, ac circuit breakers, the rated short circuit current is
passed for
a. 0.01 sec
b. 0.1 sec
c. 3 sec
d. 30 sec
Answer
Answer. c
5. Interrupting medium in a contactor may be
a. air
b. oil
c. SF6 gas
d. any of the above
Answer
Answer. d
6. SF6 gas is
a. sulphur fluoride
b. sulphur difluoride
c. sulphur hexafluoride
Answer
d. sulphur hexafluoride
Answer. d
7. SF6 gas
a. is lighter than hydrogen
b. is lighter than air
c. has density 2-times as compared to that of air
Answer
d. has density 5-times as compared to that of air
Answer. d
8. While selecting a gas for circuit breaker, the property of gas that should
be considered is
a. high dielectric strength
b. non-inflammability
c. non-toxicity
Answer
d. all of the above
Answer. d
9. Which of the following circuit breaker can be installed on 400 kV line
a. Tank type oil circuit breaker
b. Miniature circuit breaker
c. Vacuum circuit breaker
Answer
d. Air blast circuit breaker
Answer. d
10. In modern EHV circuit breakers, the operating time between the instant
of receiving trip signal and final contact separation is, of the order of
a. 0.001 sec
b. 0.015 sec
c. 0.003 sec
Answer
d. 0.03 sec
Answer. D
11. In Extra voltage lines which of the following Circuit breakers are most
suitable?
a. SF6 circuit breakers
b. Air circuit breakers
c. Oil circuit breakers
d. Vacuum circuit breakers
Answer
Answer. a
12. _______ is not a method of high resistance interruption of arc
extinction
a. Blast effect
b. Arc lengthening
c. Arc constraining
Answer
d. Arc cooling
Answer. a
13. _______ does not have any repairable component
a. Air Blast CB
b. MCB
c. OCB
Answer
d. Air Break CB
Answer. b
14. Resisting capacity of Vaccum circuit breaker is:
a. Zero
b. Moderate
c. Lower as compared to others
d. High
Answer
Answer. d
15. Air Blast Circuit breaker is employed among which of the following?
i. Arc furnace duty
ii. Traction system
a. only i
b. only ii
c. i and ii both
Answer
d. neither i nor ii
Answer. b
16. Number of immobile contacts in Double Break Oil Circuit Breaker is:
a. 1
b. 2
c. 3
Answer
d. 4
Answer. b
17. During current chopping the current actually breaks
a. During the instant of natural current zero
b. Before the instant of natural current zero
c. after the instant of natural current zero
Answer
d. irrespective of the instant of natural zero
Answer. a
18. In which of the following type of relay, the solenoid is used to move in
iron core or plunger
a. electrodynamic type
b. solenoid type
c. induction type
Answer
d. attracted armature type
Answer. a
19. The rate of rise of restriking voltage is
a. The type of circuit breaker
b. The inductance of the system only
c. The capacitance of the system only
Answer
d. The inductance and capacitance of the system only
Answer. d
20. Arcing contacts in a circuit breaker is usually made of
a. Copper tungsten alloy
b. Zinc mercury amalgam
c. Porcelain
Answer
d. Graphite
Answer. A
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21. A 3 phase circuit is rated at 1200 A, 2000 MVA, 100 kV, 4sec. The
rated symmetrical breaking current is
a. 1.67 kA
b. 20 kA
c. 11.55 kA
Answer
d. 300 kA
Answer. c
22. Bulk oil circuit breakers are suitable for:
a. High voltage systems
b. Low voltage systems
c. Extra high voltage systems
Answer
d. Medium voltage systems
Answer. a
23. In circuit breakers arcing is desirable:
a. Only in oil circuit breaker before current zero
b. After contact separation till current zero and also after current
zero
c. Only after contact separation till current zero
Answer
d. Only in the post current zero period
Answer. c
24. Choose the correct statement about circuit breakers:
a. The making capability of circuit breaker should be 2.55 times
than breaking capability
b. The making capability of circuit breaker should be equal to
breaking capability
c. The making capability of circuit breaker be
breaking capability
times than
d. The breaking capability of circuit should be 2.55 times than
making capability
Answer
Answer. a
25. The resistance of are between the contacts of circuit breaker is
increased by
a. Increasing the cross-section area of the arc
b. Splitting the arc in number of arcs in parallel
c. Shortening the arc
Answer
d. Splitting the arc in number of arcs in series
Answer. d
26. Which of the following circuit breakers has the highest dielectric
strength?
a. Bulk oil circuit breakers
b. Air blast circuit breakers
c. Minimum oil circuit breakers
d. Vacuum circuit breakers
Answer
Answer. b
27. A 3-phase circuit breaker is rated at 1250 A, 2000 MVA, 33 kV. The
making current will be
a. 35 kA
b. 49.44 kA
c. 89 kA
Answer
d. 69 kA
Answer. c
28. Why does minimum oil circuit breaker (CB) has less volume of oil?
a. There is insulating between contacts
b. The oil between the breaker contacts has greater strength
c. Solid insulating material is provided
Answer
d. None of these
Answer. a
29. The breaking current of a circuit breaker is
a. Less than its making current
b. More than its making current
c. Equal to its making current
Answer
d. None of the other options
Answer. a
30. The main purpose of using oil in the oil circuit breaker is to
a. provide insulation between oil circuit breaker contacts
b. provide cooling of contacts
c. quenching arc
d. none of these
Answer
Answer. c
31. The making current of 3-phase breaker with rating 2000 MVA, 33 kV
will be
a. 35 A
b. 50 A
c. 70 kA
d. 89 A
Answer
Answer. d
32. An air blast circuit breaker is usually employed for
a. Instantaneous voltage
b. Intermittent duty
c. Repeated duty
Answer
d. Short duty
Answer. c
33. The recovery voltage that appears across the circuit breaker contacts
will be maximum for power factor of
a. Zero
b. 0.5
c. 0.707
Answer
d. Unity
Answer. a
34. Air used in air blast circuit breaker must
a. Be free from moisture
b. Have least CO2
c. Be ionized
d. Have oil mist
Answer
Answer. a
35. Breaking capacity of a circuit breaker is usually expressed in
a. Volts
b. Amperes
c. MVA
Answer
d. MW
Answer. c
36. In the event of a fault on the connected circuit, a circuit breaker will
operate
a. Automatically
b. Manually
c. Manually through a control switch
Answer
d. Depending on the design
Answer. a
37. Which of the following is not a part of the circuit breaker?
a. Operating mechanism
b. Conservator
c. Explosion pot
d. Fixed and moving contacts
Answer
Answer. b
38. In a circuit breaker, ionization is not facilitated by
a. Material of contacts
b. Increase of mean path
c. Increase of field strength
d. High temperature of surrounding medium
Answer
Answer. a
39. At electric breakdown
a. Conductor become insulator
b. Insulator becomes conductor
c. Semiconductor becomes conductor
Answer
d. Conductor becomes superconductor
Answer. b
40. In the case of circuit breakers if the length of arc increases, its
resistance is
a. Decreases
b. Increases
c. Remain same
Answer
d. Some times increases or decreases
Answer. b
41. A three-phase, 33 kV oil circuit breaker is rated for 1200 A, 2000
MVA, 3s. The symmetrical breaking current is
a. 1200 A
b. 3600 A
c. 35 kA
d. 104.8 kA
Answer
Answer. c
42. In a circuit breaker, the time duration from the instant of the fault of
the extinction of arc is known as
a. Total clearing time
b. Operation time
c. Protection time
Answer
d. Recovery time
Answer. a
43. In an indoor substation for 220 kV, the circuit breakers will be of the
type
a. Oil
b. Air break
c. SF6
Answer
d. Vaccum
Answer. c
44. Resistance switching is normally employed in
a. All breakers
b. Bulk oil breakers
c. Minimum oil breakers
Answer
d. Air blast breakers
Answer. d
45. Which is the preferred type of C.B. to be installed in the EHV system?
a. Vacuum C.B.
b. Air blast C.B.
c. Bulk oil C.B.
Answer
d. None of these
Answer. b
46. Circuit breaker is used for which of the following
a. Voltage control
b. Power control
c. Frequency control
Answer
d. Over control protection
Answer. d
47. Which one of the following is not a routine test of a circuit breaker?
a. Short circuit test
b. Dielectric test
c. Thermal test
Answer
d. DC resistance voltage drop test
Answer. a
48. The most suitable circuit breaker for interrupting short line fault without
resistance switching is
a. Minimum oil circuit breaker
b. SF6 circuit breaker
c. Bulk oil circuit breaker
d. Air blast circuit breaker
Answer
Answer. b
49. The restriking voltage in circuit breaker is measured in
a. RMS value
b. Peak value
c. Instantaneous value
Answer
d. Average value
Answer. c
50. The Air Blast circuit breakers for 400 kV system are designed to
operate
a. 0.1 s
b. 0.5 s
c. 50 ms
Answer
d. 100 μs
Answer. c
51. In a circuit breaker……..
a. The current chopping is associated with low inductive current
b. The current chopping is associated with the low inductive
current before its natural zero
c. The current chopping gives rise to low voltage
d. The current chopping gives rise to high voltage cause the
restriking of arc
Answer
Answer. d
52. In a circuit breaker
a. Both restriking and recovery voltages are transient in nature
b. Both restriking and recovery voltages are RMS in nature
c. Restriking voltage is transient while recovery voltage is RMS in
nature
d. Recovery voltage is transient while restriking voltage is RMS in
nature
Answer
Answer. c
53. The rate of rise of restriking voltage in a circuit breaker depends upon
the
a. Both on the inductance and capacitance of the system
b. Type of circuit breaker
c. Inductance of the system only
Answer
d. Capacitance of the system only
Answer. a
54. Temperature of the generated heat in the maximum oil circuit breaker
is
a. 550 °C
b. 2500 °C
c. 5000 °C
Answer
d. 10000 °C
Answer. c
55. When using air blast circuit breaker, current chopping is a
phenomenon which is observed, when
a. a long overhead line is Switched off.
b. a bank of capacitors is switched off.
c. a transformer on no load is switched off.
Answer
d. a heavy load is switched off.
Answer. c
56. In a Circuit breaker, the re-striking voltage is measured in ……….
a. RMS value
b. Peak value
c. Instantaneous value
Answer
d. Average value
Answer. b
57. There will be voltage across the gap in circuit breakers when ……….
a. R = ∞
b. R = 0
c.
Answer
d.
Answer. c
58. For extra high voltage lines, which circuit breaker is used?
a. Bulk oil circuit breaker
b. Vacuum circuit breaker
c. Minimum oil circuit breaker
Answer
d. SF6 circuit breaker
Answer. d
59. For average values of load current, current chopping occurs more
frequently in
a. ACB’s
b. SF6 CB’s
c. VCB’s
Answer
d. OCB’s
Answer. c
60. An isolator is used in series with Air blast Circuit Breaker employed at
UHV lines because
a. gap between CB contacts is small so an isolator is used to
switch off voltage
b. gap between CB poles the use of isolator
c. CB life is enhanced with the use of isolator
Answer
d. current to be interrupted will be large.
Answer. a
61. In order to switch-off and EHV circuit for maintenance, the following
sequence is adopted
a. Open the circuit breaker, open the isolator, operate the earth
switch
b. Operate the earth switch, open the isolator, open the circuit
breaker
c. Open the isolator, operate the earth switch, open the circuit
breaker
d. Open the isolator, open the circuit breaker, operate the earth
switch
Answer
Answer. a
62. Making current of a circuit breaker refers to
a. Steady-state rated current
b. Average value of transient short circuit current
c. R.M.S. value of the short circuit current at the time of major
peak
Answer
d. Equal to the fault current
Answer. c
63. Out of the following, which one is correct in case of a circuit breaker?
a. Making capacity = 2.55 x braking capacity
b. Braking capacity = 2.55 x making capacity
c. Making capacity = 1.8 x braking capacity
Answer
d. Braking capacity =1.8 x making capacity
Answer. a
64. Which of the following is incorrect?
a. Double break oil circuit breaker has 3 breakers.
b. There are 2 stationary contacts in double break oil circuit
breaker.
c. Arc generated in most of the circuit breakers absorb the
energy oil.
Answer
d. Circuit breaker is a mechanical switching.
Answer. a
65. The breaking capacity of a 3-phase circuit breaker is
a. 3 times the product of service voltage and rated symmetrical
current
b.
c.
times the product of service voltage and rated symmetrical
current
times the product of service voltage and rated symmetrical
current
d. 2 times the product of service voltage and rated symmetrical
current
Answer
Answer. b
66. The insulating fluids that are commonly used in circuit breakers is/are
a. SF6
b. Air at atmospheric pressure
c. Compressed air
d. All of the above
Answer
Answer. d
67. Which of the following circuit breaker is generally used in railway
electrification?
a. Air Blast circuit breaker
b. Minimum Oil circuit breaker
c. Bulk Oil circuit breaker
d. SF6 circuit breaker
Answer
Answer. a
68. The main purpose of oil in OCB is to
a. provide insulation
b. provide cooling of contacts
c. quenching arc
Answer
d. none of the above
Answer. c
69. A 3-phase circuit breaker is rated at 1250 A, 2000 MVA, 33 kV, 4s.
Its making current capacity will be
a. 35 A
b. 89 A
c. 69 A
Answer
d. 79 A
Answer. b
70. A circuit breaker is rated as follows: 1500 A, 33 kV, 3 sec, 3-phase
oil circuit breaker. Determine the making current?
a. 110 A
b. 35 A
c. 15 A
Answer
d. 89 A
Answer. d
71. 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
Answer
D. Whenever fault occurs in the line
72. In a circuit breaker contact wipe is necessary
A. To decrease the speed of opening of the circuit breaker
B. To increase the speed of opening and reduce the arcing of contacts
C. To increase the dielectric strength across the circuit breaker contacts
D. None of the above
Answer
C. To increase the dielectric strength across the circuit breaker contacts
73. The fluids used in circuit breaker should be of
A. High dielectric strength and thermal stability
B. Non inflammability
C. Arc extinguishing availity
D. All of the above
Answer
D. All of the above
74. In circuit breakers the contact space is ionised 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
Answer
B. Thermal emission from the contact surface
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75. For 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
Answer
C. 1 to 2 cycles
76. Circuit breakers usually operates under
A. Transient state of short circuit current
B. Sub transient state of short circuit current
C. Steady state of short circuit current
D. After DC component has ceased
Answer
A. Transient state of short circuit current
77. The voltage appearing across the contacts after opening of the circuit
breaker is called..............voltage
A. Recovery
B. Surge
C. Operating
D. Arc
Answer
A. Recovery
78. In a circuit breaker the basic problem is to
A. Maintain the arc
B. Extinguish the arc
C. Transmit large power
D. Emit the ionizing electrons
Answer
B. Extinguish the arc
79. The arc voltage produced in ac circuit breaker is always
A. In phase with arc current
B. Legging the arc current by 90o
C. Leading the arc current by 90o
D. None of the above
Answer
A. In phase with arc current
80. In railway applications............circuit breaker is used
A. Sf6
B. Bulk oil
C. Minimum oil
D. Air brake
Answer
D. Air brake
81. Arc in a circuit breaker is interrupted at
A. Zero current
B. Maximum current
C. Minimum voltage
D. Maximum voltage
Answer
A. Zero current
82. The information to the circuit breaker under fault conditions is
provided by
A. Relay
B. Rewirable fuse
C. H.R.C. only
D. All of the above
Answer
A. Relay
83. In a circuit breaker the time duration from the instant of the fault to
extinction of arc is known as
A. Clearing time
B. Operation time
C. Lag time
D. Lead time
Answer
A. Clearing time
84. Breaking capacity of a circuit breaker is usually expressed in terms of
A. Volts
B. Amperes
C. MVA
D. MW
Answer
C. MVA
85. Which of the following is not a part of the circuit breaker
A. Operating mechanism
B. Conservator
C. Explosion pot
D. Fixed and moving contacts
Answer
B. Conservator
86. The rating of a circuit breaker is generally determined on the basis of
A. Line-to-line fault current
B. Symmetrical fault current
C. Single line to ground fault current
D. Double line to ground fault current
Answer
B. Symmetrical fault current
87. In circuit breakers, the advantages of using oil as quenching medium
is
A. Oil act as insulator
B. Oil has high dielectric strength
C. Absorption of arc energy
D. All of the above
Answer
D. All of the above
88. In the event of a fault on connected circuit a circuit breaker will
operate
A. Automatically
B. Manually
C. Manually through control switch
D. Depends on the design
Answer
A. Automatically
89. An arc in a circuit behaves like
A. An inductive reactance
B. A capacitive reactance
C. Infinite resistance
D. A resistance decreasing with voltage rise across the arc
Answer
D. A resistance decreasing with voltage rise across the arc
90. In a circuit breaker the active recovery voltage depends upon
A. Circuit conditions
B. Power factor
C. Armature reaction
D. All of the above
Answer
D. All of the above
91. The making to breaking current ratio for an EHV circuit breaker is
A. More than 1
B. Equal to 1
C. Less than 1
D. A negative number
Answer
A. More than 1
92. The making capacity of a circuit breaker is
A. Less than the asymmetrical breaking capacity of the breaker
B. Greater than the asymmetrical breaking capacity of the breaker
C. Equal to the symmetrical breaking capacity of the breaker
D. Equal to the asymmetrical breaking capacity
Answer
B. Greater than the asymmetrical breaking capacity of the breaker
93. The probable cause(s) for failure of a circuit breaker on electrical
compound could be
A. Trip circuit open
B. Trip latch defective
C. Spring defective
D. Any of the above
Answer
D. Any of the above
94. Routine tests are conducted on
A. Bulk oil circuit breakers
B. Minimum oil circuit breakers
C. Air-blast circuit breakers
D. All of the above
Answer
D. All of the above
95. The air blast circuit breakers for 400 KV system are designed to
operate in
A.0.1 sec
B. 0.5 sec
C. 50 msec
D. 200 μsec
Answer
C. 50 msec
96. Air used in air blast circuit breaker must
A. Be ionised
B. Be free from moisture
C. Have least CO2
D. Have oil mist
Answer
B. Be free from moisture
97. In air blast circuit breaker resistance switching is used to
A. Read use the magnitude of the fault current
B. Control the CB operating time
C. Damp out the fast transient
D. Change the fault current power factor
Answer
C. Damp out the fast transient
98. While using air blast circuit breaker current, chopping is a
phenomenon often observed when
A. A long overhead line is switched off
B. A bank of capacitors is switched off
C. A transformer on no-load is switched off
D. A heavy load is switched off
Answer
C. A transformer on no-load is switched off
99. The chances of arc interruption in subsequent current zeros
A. Increase in case of ABCB but decrease in OCB
B. Decrease in case of ABCB but increase in OCB
C. Decrease in both cases
D. Increase in both cases
Answer
B. Decrease in case of ABCB but increase in OCB
100. Main purpose of oil in oil circuit breaker is to
A. Provide insulation
B. Provide cooling of contacts
C. Quenching arc
D. All of the above
Answer
C. Quenching arc
101. In a minimum oil circuit breaker the oil use used
A. To act as a circuit breaking medium only
B. For circuit breaking and providing insulation
C. For providing insulation only
D. For none of the above purposes
Answer
A. To act as a circuit breaking medium only
102. Arc interruption in an oil circuit breaker takes place when
A. Contacts apart
B. Voltage goes through zero
C. Current goes through zero
D. None of the above
Answer
C. Current goes through zero
103. The minimum oil circuit breaker has less volume of oil because
A. The oil between the breaker contacts has greater strength
B. There is insulation between the breaker contacts
C. Solid insulation is provided for insulating the contacts from the earth
D. Any of the above
Answer
C. Solid insulation is provided for insulating the contacts from the earth
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104. To limit current shopping in vacuum circuit breaker is the contact
material employed should have the properties of
A. Low conductivity and high vapour pressure
B. Low conductivity and low vapour pressure
C. High conductivity and high vapour pressure
D. High conductivity and low vapour pressure
Answer
A. Low conductivity and high vapour pressure
105. SF6 gas has excellent heat transfer properties because of its
A. Low gaseous viscosity
B. High dielectric strength
C. Higher molecular weight
D. Both A and C
Answer
D. Both A and C
106. SF6 gas is transported in
A. Air cylinders
B. Gas cylinders
C. Liquid form in cylinders
D. Solid form
Answer
C. Liquid form in cylinders
107. During arc extension SF6 gas gets
A. Decomposed into SF4 and SF2
B. Decomposed into S and F ions
C. Reduced to SF2
D. Oxidised
Answer
A. Decomposed into SF4 and SF2
108. The single most important property that makes SF6 a very efficiant
medium for circuit breaking is
A. It is non-toxic and non-inflammable
B. It has high dielectric constant
C. It has high break down strength
D. It is highly electronegative in character
Answer
D. It is highly electronegative in character
109. The current chopping tendency is minimised by using SF6 gas at
relatively
A. Low pressure and high viscosity
B. Low pressure and low viscosity
C. High pressure and low viscosity
D. High pressure and high viscosity
Answer
B. Low pressure and low viscosity