Circuit Breaker Multiple Choice Questions

1. Which of the following statement about SF6 gas is incorrect?

(a) it is a non-toxic gas
(b) it is non-inflammable
(c) it has density 5 times that of air at 20^oC
(d) it has dark yellow color.

2. SF6 gas is transported in

(a) gas cylinders
(b) liquid form in cylinders
(c) solid form in boxes
(d) air cylinders.

3. During arc extinction SF₆ gas

(a) decomposes into S and F ions
(b) decomposes into SF₄ and SF₂
(c) gets oxidized
(d) reduces to SF₃.

4. Dielectric strength of SF₆ is

(a) less than that of air at atmospheric pressure
(b) less than that of used in OCB
(c) more than that of used in OCB
(d) more at lower pressure and low at higher pressure.

5. Which of the following is demerit of SF6 circuit breaker?

(a) sealing problem of gas
(b) in flux of moisture in the gas system is dangerous
(c) deterioration of quality of circuit breaker affects reliability of circuit breaker
(d) all of the above.

6. Sphere gaps are used for

(a) measurement of high DC voltages
(b) measurement of high AC voltages
(c) measurement of impulse voltages
(d) all of the above.

7. Which of the following is not valid in case of aluminum as compared to copper? Aluminum has higher

(a) resistivity
(b) coefficient of linear expansion
(c) tensile strength
(d) joint resistance.

8. The isolator is interlocked with circuit breaker and earthing switch. While opening the circuit, ……. opens first, then the …… and only after this …… can close.

(a) isolator, circuit breaker, earthing switch
(b) earthing switch, isolator, circuit breaker
(c) circuit breaker, earthing switch, isolator
(d) circuit breaker, isolator, earthing switch

9. Which of the following are the voltage waves of magnitude higher than the desirable value?

(a) over-voltages
(b) surges
(c) transients
(d) all of the above.

10. Over voltage transients may occur due to

(a) lightening
(b) switching
(c) arcing grounds
(d) any of the above.

11. Surge impedance of overhead transmission line is of the order of

(a) 20 to 30 ohms
(b) 300 to 500 ohms
(c) 3000 to 5000 ohms
(d) 30 KΩ to 300 KΩ

12. The surge impedance of underground cable is of the order of

(a) 20 to 60 ohms
(b) 200 to 600 ohms
(c) 2000 to 3000 ohms
(d) 30 KΩ to 300 KΩ

13. The surge impedance of a transmission line is given by

(a) √(LC)
(b) √(L/C)
(c) √(C/L)
(d) √(L+C)

14. Surge modifiers are used to

(a) reduce the current of wave front
(b) reduce the voltage of wave front
(c) reduce the steepness of wave front
(d) modify the shape of wave front.

15. The steepness of the wave front can be reduced by

(a) connecting a capacitance between line and earth
(b) connecting an inductor in series with the line
(c) connecting an capacitor between line and earth or connecting an inductor in series with the line
(d) connecting an inductor between line and earth and connecting a capacitor in series with the line.

16. The disadvantage offered by ungrounded system is

(a) frequent arcing grounds
(b) difficult earth fault relying
(c) voltage oscillations
(d) all of the above.

17. Solid grounding is used for voltages

(a) above 220 kV
(b) above 11 kV
(c) below 660 V
(d) below 115 V

18. Resistance grounding is used for voltages

(a) below 220 V
(b) upto 660 V
(c) between 3.3 kV to 11 kV
(d) above 66 kV

19. Switchover voltages are more hazardous than lightening surges in case of

(a) low voltage systems
(b) 11 kV systems
(c) unbalanced systems
(d) EHV and UHV systems.

20. Current limiting reactors may be

(a) air cooled, air cored
(b) oil immersed magnetically shielded
(c) oil immersed non-magnetically shielded
(d) any of the above.

21. Series reactors are installed at strategic locations of power system to

(a) bring down the fault current level within the capacity of switchgear
(b) directly pass the fault surges to the ground
(c) pass neutralizing surges of opposite nature
(d) discharge the capacitors.

22. Fault diverters

(a) divert the current to the earth in the event of short-circuits
(b) neutralize the surges by resistors
(c) modify the surge wave shapes
(d) none of the above.

23. In star connected system without neutral grounding, zero sequence currents are

(a) same as the peak value of phase currents
(b) same as the RMS value of phase currents
(c) vector sum of the phase currents
(d) zero.

24. In which portion of the transmission system faults occur most frequently?

(a) transformers
(b) overhead lines
(c) alternators
(d) underground cables.

25. Which portion of the transmission system is least prone to faults?

(a) switchgear
(b) CT, PT
(c) alternators
(d) feeders.

26. A 3 phase, 5000 kVA, 6.6 kV generator having 12% sub-transient reactance. A 3-phase short-circuit occurs at its terminals. Fault MVA is

(a) 21.5
(b) 41.66
(c) 53.33
(d) 75.75

27. A 3 phase, 5000 kVA, 6.6 kV generator having 12% sub-transient reactance. A 3-phase short-circuit occurs at its terminals. Fault current is

(a) 3640 A
(b) 2460 A
(c) 1680 A
(c) 880 A

28. The ohimic value of impedance to be connected in the neutral to ground circuit of a 2000 kVA transformer with earth fault relay set to 40% with respect to 400 V side will be

(a) 0.2 ohm
(b) 2.0 ohm
(c) 20 ohm
(d) 200 ohms

1. MCQ Questions on Electrical Cable
2. MCQ on Switchgear and Protection
3. Circuit Breaker MCQ Question Answer
4. Circuit Breaker Multiple Choice Questions

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