Switchgear and Protection MCQs Part-4

Switchgear and Protection

1. 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.

2. 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.

3. 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.

4. 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.

5. 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°

6. 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.

6. 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

7. 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

8. 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.

9. 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.

10. Up to what voltage can the liquid type HRC fuses be used?

a. 33 kV

b. 132 kV

c. 66 kV

d. 220 kV

11. 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.

12. Which part of the circuit breaker is helpful in breaking the current?

a. Trip coil.

b. Contacts.

c. Medium

d. Handle.

13. 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.

14. Upto what voltage a cartridge type of fuse can be used?

a. 400 V

b. 11 kV

c. 20 kV

d. 33 kV

15. On what basis is the selection of fuse done?

a. Steady load.

b. Fluctuating load.

c. Both (a) and (b)

d. None of these.

16. 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.

17. 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

18. What is the advantage of HRC fuses over Rewirable fuses?

a. High speed operation

b. High rupturing capacity

c. No ageing effect.

d. All of the above.

19. What is the typical value of the pre arcing time?

a. 0.1

b. 0.01

c. 0.001

d. 0.0001

20. A fuse wire of circular cross-section has a radius of 0.8 mm. 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

21. 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.