MCQs on Switchgear and Protection

Switchgear & Protection - Electrical Engineering

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1)   A three phase circuit breaker is rated 2000 MVA, 33 kV. What will be its making current?
a. 35 kA

b. 49 kA

c. 70 kA

d. 89 kA

2) Which of the following circuit breakers has the lowest operating voltage?
a. SF₆ circuit breaker

b. Air break

c. Air blast

d. Minimum oil circuit breaker

3) The most efficient torque producing actuating structure for the induction type relays is

a. Shaded pole structure

b. Watt hour meter structure

c. Induction cup structure

d. Single induction loop structure

4)   Plug setting of a 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

5)   In the following figure, the tripping circuit is__________. 

a. AC

b. DC

c. Either AC or DC

d. None of these

6) In the following figure, which component ensures the safety of the line from damage?

a. Relay

b. Circuit breaker

c. Bus bar

d. Current transformer

In the following figure, the relay circuit is divided into three parts. What does the third part consist of?

a. Primary winding of a current CT which is connected in series with the line to be protected.

b. Secondary of the CT and the operating coil.

c. Tripping circuit.

d. None of these

8)   In the following figure, the relay circuit is divided into three parts. What does the first part consist of?

a. Primary winding of a current CT which is connected in series with the line to be protected.

b. Secondary of the CT and the operating coil.

c. Tripping circuit.

d. None of these

9)   On what factor does the operating speed of the relay depend?

a. Rate of flux built up

b. Armature core air gap

c. Spring tension

d. All of these

10)   Protective relays can be designed to respond to ___________.

a. Light intensity, impedance

b. Temperature, resistance, reactance

c. Voltage and current

d. All of these

11)   What is the actuating quantity for the relays?

a. Magnitude

b. Frequency

c. Phase angle

d. All of these

12)   What is the purpose of back up protection?

a. To increase the speed

b. To increase the reach

c. To leave no blind spot

d. To guard against failure of primary

13)   What is the major cause of the failure of the circuit breaker?

a. Trip circuit open

b. Trip latch defective

c. Spring defective

d. All of these

14)   Why are the isolators used?

a. Break abnormal current

b. Making under fault conditions

c. Breaking the circuit under no load condition

d. None of the above

15)   The isolators used in the transmission lines are capable of breaking

a. Fault current

b. No current

c. Charging current

d. Load current

16)   For which among the following the current ratings are not required?

a. Circuit breakers

b. Relays

c. Isolators

d. Load break switch

17)   Why is an isolator installed?

a. To isolate one portion of the circuit from another

b. As an substitute for the circuit breaker

c. It used on either sides of the circuit breaker

d. Both (a) and (c)

e. None of these

18)   What is the making to breaking current ratio for an extra high voltage circuit breaker?

a. More than 1

b. Equal to 1

c. Less than 1

d. A negative value

19)   Assertion (A): In comparison to making capacity of a circuit breaker its breaking capacity is normally higher.
Reason (R): The breaking capacity of a CB is expressed as √3 * VI * 10^-6 MVA

a. Both A and R are true and R is the correct explanation of A

b. Both A and R are true and R is not the explanation of A

c. A is true but R is false

d. A is false but R is true.

20)   The breaking capacity of a three phase circuit breaker is given by

a. Service line voltage * rated symmetrical current in amperes * 10^-6 MVA

b. √3 * Service line voltage * rated symmetrical current in amperes * 10^-6 MVA

c. 1.1* Service line voltage * rated symmetrical current in amperes * 10^-6 MVA

d. √2 * Service line voltage * rated symmetrical current in amperes * 10^-6 MVA

21)   What is the making capacity of the circuit breaker?

a. Less than the asymmetrical breaking capacity of the breaker

b. Greater than the asymmetrical breaking capacity of the breaker

c. Equal to the asymmetrical breaking capacity of the breaker

d. Equal to the symmetrical breaking capacity of the breaker

22)   Circuit breakers usually operate under

a. Steady short circuit current

b. Sub transient state of short circuit current

c. Transient state of short circuit current

d. None of these

23)   Which of the following circuit breaker is highly reliable and has a least maintenance?

a. Oil circuit breakers

b. Air blast

c. Vacuum circuit breakers

d. SF₆ circuit breakers

24)   The rating of the circuit breaker is usually determined on the basis of ____________ fault.

a. Symmetrical

b. Line to line

c. Single line to ground

d. Double line to ground

25)   Which among these circuit breakers produce the least arc energy?

a. Plain oil

b. Minimum oil

c. Air blast

d. Air break

26)   Which circuit breaker is preferred to be installed in extra high voltage AC system?

a. Bulk oil type circuit breaker

b. Air blast circuit breaker

c. SF₆ circuit breaker

d. Vacuum circuit breaker

27)   Which of the following circuit breakers is used for the railway electrification?

a. Air blast circuit breaker

b. SF₆ circuit breaker

c. Bulk oil circuit breaker

d. Minimum oil circuit breaker

28)   A thermal protection switch provides protection against what?

a. Overload

b. Temperature

c. Short circuit

d. Over voltage

29)   A three phase, 33 kV oil circuit breaker is rated 1200 A, 2000 MVA, 3s. What is its symmetrical breaking current?

a. 1200 A

b. 3600 A

c. 35 kA

d. 104.8 kA

30)   What is / are the main disadvantage / s of using oil as the quenching medium in the circuit breakers?

a. Need periodical replacement.

b. Risk of formation of explosive mixture with air.

c. Possibility of causing fire hazards.

d. All of the above.

31)   What does protective relay provide?

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 or ground any stray voltage.

32)   What is the main purpose of oil in oil circuit breakers?

a. Provide insulation

b. Quenching arc.

c. Provide cooling of contacts.

d. None of the above

33)   What is the advantage of using oil as the arc quenching medium?

a. Good cooling properties.

b. High dielectric strength.

c. Acts as an insulator.

d. All of these

34)   When does the arc interruption in oil circuit breaker take place?

a. Contacts apart.

b. Voltages becomes zero

c. Current goes through zero

d. All of the above

35)   For rural electrification in India, which circuit breaker is generally used?

a. Oil

b. SF₆

c. Vacuum

d. Air blast

36)   Keeping in view the cost and the overall effectiveness, which of the following circuit breaker is best suited for capacitor bank switching?

a. Vacuum circuit breaker

b. Air blast CB

c. SF₆

d. Oil CB

37)   To limit current chopping in vacuum circuit breakers, 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.

38)   SF₆ gas is imported in _____________.

a. Air cylinders

b. Gas cylinders

c. Liquid form in cylinders

d. Solid form.

39)   During arc extinction SF₆ gas gets converted to which among these?

a. Gets decomposed to SF₄ and SF₂

b. Gets decomposed to S and F

c. Gets reduced to SF₆

d. Gets oxidized

40)   What is the most important property which makes the SF₆ very efficient medium for circuit breaking?

a. Is non toxic and non inflammable.

b. Has a high dielectric constant.

c. Has a high breakdown strength

d. Is highly electronegative gas

41)   What is the normal pressure at which the SF₆ gas is maintained in the closed position of the breaker?

a. 2 kg / cm²

b. 2.5 kg /cm²

c. 2.8 kg / cm²

d. 3 kg / cm²

42)   SF₆ is which type of gas?

a. Electro positive

b. Electro negative

c. Both (a) and (b)

d. None of these

43)   What is the major drawback of using SF₆ circuit breakers?

a. Sealing problems of the gas.

b. Ingress of moisture in the gas system – dangerous.

c. Deterioration of SF₆ gas with time.

d. Both (a) and (b)

e. None of these

44)   Why do the SF₆ gases have an excellent heat transfer property?

a. Low gaseous viscosity.

b. High dielectric strength.

c. Higher molecular weight.

d. Both (a) and (c)

e. None of these

45)   What is the breaking capacity of the air blast circuit breaker?

a. 5000 MVA

b. 6000 MVA

c. 7000 MVA

d. 10000 MVA

46)   In axial blast type of CB, expansion of air takes place from ______________.

a. High pressure to low pressure.

b. Low pressure to high pressure.

c. Always in high pressure.

d. Always in low pressure.

47)   The air blast circuit breakers for 400 kV systems are designed to operate in how much time?

a. 0.1 s

b. 0.5 s

c. 50 ms

d. 100µs

48)   What is the type of air blast in cross blast type CB?

a. The blast of air is along the arc.

b. The blast of air cuts across the arc.

c. Both (a) and (b)

d. None of the above.

49)   Why is the resistance switching used in a air blast circuit breaker?

a. Reduce the magnitude of fault current.

b. Control the CB operating time.

c. Damp out the fast transient.

d. Change the fault current power factor.

50)   When using air blast circuit breaker, current chopping is a phenomenon 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.

51)   Which type of air is used in air blast circuit breaker? a. Ionised air b. Air free from moisture c. Air should have least CO2 d. Air must have oil mist.

52)   For the following figure shows a CB circuit with L - C introducing a damping circuit. For the critical damping what is the value of r? a. √(LC) b. 0.5*√(C/L) c. 0.5*√(L/C) d. 2π*√(L/C)

53)   A 50 Hz 3 phase synchronous generator has inductance per phase of 15mH. The capacitance of generator and the circuit breaker is 0.002μ F. What is the natural frequency of oscillation? a. 29 kHz b. 2.9 kHz c. 290 kHz d. 29 MHz

54)   What is the average rate of rise of restriking voltage upto the first peak? a. 525 * 103kV / sec b. 453 * 103kV / sec c. 582 * 103 kV / sec d. 467 * 103 kV / sec

55)   Refer to the following facts to answer the question A 50 Hz, 11 kV, 3 phase alternator with earthed neutral having a reactance of 3 ohms per phase and is connected to a bus bar through a circuit breaker, if the distributed capacitance upto CB between the phase and neutral is 0.01 μ F. What is the frequency of oscillations? a. 10000 Hz b. 12500 Hz c. 12628 Hz d. 13265 Hz

56)   Refer to the following facts to answer the question A 50 Hz, 11 kV, 3 phase alternator with earthed neutral having a reactance of 3 ohms per phase and is connected to a bus bar through a circuit breaker, if the distributed capacitance upto CB between the phase and neutral is 0.01 μ F. What is the peak re striking voltage? a. 18.36 kV b. 17.96 kV c. 15.96 kV d. 12.65 kV

57)   In a short circuit test on a circuit breaker, the following readings were obtained on single frequency transients a. Time to reach the peak re striking voltage - 50μ sec b. The peak re striking voltage, 100 kV 58) what will be the frequency of oscillations? a. 100 Hz b. 1,000 Hz c. 10,000 Hz d. 10 Hz

58)   In a short circuit test on a circuit breaker, the following readings were obtained on single frequency transients a. Time to reach the peak re striking voltage - 50μ sec b. The peak re striking voltage, 100 kV 59) What is its average RRRV? a. 2*106kV/sec b. 3*105kV/sec c. 2*105kV/sec d. 3*106kV/sec

59)   The making and breaking currents of a 3 phase ac circuit breakers in power systems are respectively in ________form. a. rms value, rms value b. instantaneous value, rms value c. rms value, instantaneous value d. instantaneous value, instantaneous value

60)   How is the restriking voltage measured? a. RMS value b. Peak value c. Instantaneous value d. Average value

61)   What is the interrupting medium in the contactor?

a. Air at atmospheric pressure.

b. SF₆ gas

c. Oil

d. All of the above.

62)   Recovery voltage is the value of rms voltage that reappears across the poles of a circuit breaker before_______________

a. Restriking voltage

b. Final arc extinction

c. Rise of voltage

d. All of the above

63)   Why is it difficult to interrupt a capacitive circuit?

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.

64)   The stability of arc in vacuum depends on _________.

a. The contact material only.

b. The circuit parameters only.

c. The contact materials and its vapour pressure.

d. Both (b) and (c)

e. None of these

65)   Rate of rise of restriking voltage depend on_____________?

a. Type of circuit breaker.

b. Capacitance of the system.

c. Inductance of the system.

d. Both (b) and (c)

e. None of these

66)   On what factor does the rate of rise of restriking voltage (RRRV) depend?

a. System voltage.

b. Circuit power factor only

c. Switching conditions

d. Both (b) and (c).

67)   In a circuit breaker, the active recovery voltage depends upon which among these?

a. Circuit conditions.

b. Power factor.

c. Both (a) and (b)

d. None of the above

68)   What is restriking voltage?

a. The transient voltage that appears across the circuit breaker contacts at the instant of arc extinction.

b. The transient voltage that appears across the circuit breaker contacts at the end of arc extinction.

c. Both (a) and (b)

d. None of the above

69)   For a high speed circuit breaker what will the total clearing time?

a. Few minutes.

b. Few seconds.

c. 1 to 2 cycles.

d. 5 to 20 cycles.

70)   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 the trip circuit and the arc extension 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.

71)   In a circuit breaker the contact space is ionised by what?

a. Field emission from the contact surface.

b. Thermal emission from the contact surface.

c. Thermal ionisation of gas.

d. All of above.

72)   In a circuit breaker, ionisation is facilitated by_________________.

a. Increase in field strength.

b. Increase in mean free length.

c. High temperature of the surrounding medium.

d. All of the above.

73)   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 above.

74)   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.

75)   The resistance of an electric arc can be increased by

a. Increasing the concentration of ionised particles.

b. Reducing the arc length.

c. Splitting the arc.

d. Increasing the arc cross section.

76)   By using low resistance method for arc extension what is the value of arc resistance?

a. Arc resistance is zero.

b. Arc resistance is high

c. Arc resistance is low

d. Arc resistance is very high

77)   What happens in the arc extinction using high resistance method?

a. Arc resistance is decreased with time.

b. Arc resistance is increased with time.

c. No change it remains same.

d. Arc resistance is kept zero.

78)   What is the arc voltage in a circuit breaker?

a. In phase with the arc current.

b. Lagging the arc current by 90°

c. Leading the arc current by 90°

d. Lagging the arcing current by 180°

79)   Arcing voltage will be the least in case of

a. Carbon

b. Copper

c. Silver

d. Tungsten

80)   How is the initiation of electric arc at the instant of contact separation caused?

a. Thermionic emission of electrons.

b. Field emission of electrons.

c. Both (a) and (b)

d. None of these.

81)   The arc resistance depends on which among the following factors?

a. Cross section of the arc.

b. Length of the arc.

c. Degree of ionization

d. All of the above.

82)   Which type of tripping is generally preferred for the circuit breaker?

a. Manual

b. Automatically

c. Depending on the voltage level

d. None of the above

83)   Which part of the circuit breaker is helpful in breaking the current?

a. Trip coil.

b. Contacts.

c. Medium

d. Handle.

84)   When a fault occurs in a high voltage transmission line, what happens first?

a. Circuit breaker operates then the relay.

b. Relay operates and 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.

85)   Upto what voltage can the liquid type HRC fuses be used?

a. 33 kV

b. 132 kV

c. 66 kV

d. 220 kV

86)   What is the main advantage of using a fuse?

a. Cheapest type of protection.

b. Inverse time current characteristics.

c. Current limiting effect under short circuit conditions.

d. All of above.

87)   On what basis is the selection of fuse done?

a. Steady load.

b. Fluctuating load.

c. Both (a) and (b)

d. None of these.

88)   Upto what voltage a cartridge type of fuse can be used?

a. 400 V

b. 11 kV

c. 20 kV

d. 33 kV

89)   In a HRC fuse what is the time between the cut off and the final current zero called?

a. Pre - arcing time.

b. Arcing time.

c. Total operating time.

d. None of these.

90)   What is the cut off current in the fuse?

a. Maximum value actually reached.

b. Rms value actually reached.

c. Average value actually reached.

d. None of the above

91)   If the strands of the fusing wire are twisted, what happens to the fusing current?

a. Increases

b. Reduces.

c. No change/ remains same

d. Depends on the value of current, increases or decreases.

92)   Which among these tests are performed to check the nation or international standards?

a. Type test.

b. Production tests.

c. Site checks.

d. All of the above.

93)   A fuse wire of circular cross section has a radius of 0.8mm. The wire blows off at a current of 9A. What will be the radius of the wire that will blow off at a current of 1A?

a. 0.2 mm

b. 0.18 mm

c. 0.28 mm

d. 0.3 mm

94)   What is the typical value of the pre arcing time?

a. 0.1

b. 0.01

c. 0.001

d. 0.0001

95)   What is the maximum current upto which fuses can be used?

a. 25 A

b. 50 A

c. 75 A

d. 100 A

96)   What should be the value of fusing factor?

a. Equal to zero

b. Equal to one

c. Less than one

d. More than one

97)   What is fusing factor?

a. The ratio of current rating of the fuse to the minimum fusing current

b. The ratio of minimum fusing current to the current rating of the fuse

c. The ratio of maximum fusing current to the current rating of the fuse

d. The ratio of minimum fusing current to the voltage rating of the fuse

98)   What is the relation between the fusing current and the diameter of the wire?

a. I = k d³

b. I = k d^3/2

c. I = k d²

d. I = k d^2/3

99)   For a current upto 10A which material is used as the fusing element?

a. Copper

b. Silver

c. Alloy of lead and tin

d. Zinc

100)   Which among these is the least expensive protection for over current in low voltage system?

a. Rewirable fuse.

b. Isolator.

c. Circuit breaker.

d. Air breaker switch.

101)   Which among the following statement is true related to fuse?

a. Greater the current smaller is the time taken by the fuse to blow out.

b. Greater the current greater is the time taken by the fuse to blow out.

c. The current is directly proportion to the blow out time of fuse.

d. Is dependent on the temperature and atmospheric conditions.

102)   Which among these are the main characteristics of a fuse element?

a. Low melting point

b. High conductivity

c. Least deterioration due to oxidation

d. All of the above

103)   How is the bus bars rated?

a. Current only.

b. Voltage only.

c. Current, voltage and frequency.

d. Current, voltage, frequency and short circuit current

104)   Which device sends the information to the circuit breaker to interrupt the circuit in case of fault?

a. Switch

b. Relay

c. Circuit breaker itself does this function.

d. Fuse.

105)   When does the circuit breaker operate in the line?

a. When power is to be supplied.

b. When the line is to be tested.

c. Whenever the fault occurs in the line.

d. Whenever the switch and the relay has to be operated.

106)   What is the primary principle of a fuse?

a. Open the circuit.

b. Protect the appliance.

c. Protect the line.

d. Prevent excess current from flowing into the line.

107)   What is switchgear?

a. An apparatus used for switching, controlling and protecting the electrical circuits and equipments.

b. It detects the faults only.

c. It corrects the faults only.

d. All of the above.