Arc Phenomena and Quenching Methods — MCQs – EE

1. An electric arc is produced when:
(A) Current continues to flow across a gap between two conducting electrodes
(B) Voltage becomes zero
(C) Circuit is open and no current flows
(D) Circuit breaker contacts are perfectly closed

2. The arc in a circuit breaker is caused mainly due to:
(A) Ionization of air between contacts
(B) Mechanical failure
(C) Voltage drop in contacts
(D) Magnetic field distortion

3. The medium between the circuit breaker contacts becomes conducting due to:
(A) Ionization of the medium
(B) High resistance
(C) Low current flow
(D) Magnetic repulsion

4. Arc voltage is the voltage:
(A) Across the contacts during arcing
(B) Across open contacts after extinction
(C) Before the fault
(D) Across the load

5. The main aim of arc extinction is to:
(A) Interrupt current as quickly and safely as possible
(B) Maintain steady current
(C) Reduce voltage
(D) Increase resistance

6. The process of de-ionization of the medium between contacts helps in:
(A) Extinction of arc
(B) Formation of arc
(C) Increasing current
(D) Improving voltage regulation

7. The ionized particles in the arc path are responsible for:
(A) Conductivity of the arc
(B) Arc cooling
(C) Voltage reduction
(D) Increasing resistance

8. The two main methods of arc extinction are:
(A) High resistance and low resistance methods
(B) Ionization and cooling methods
(C) Chemical and mechanical methods
(D) Magnetic and thermal methods

9. In the high resistance method, the arc resistance is:
(A) Increased rapidly to reduce current to zero
(B) Decreased gradually
(C) Maintained constant
(D) Made zero

10. The low resistance or current zero method is used in:
(A) AC circuits
(B) DC circuits
(C) Both AC and DC circuits
(D) Low voltage systems only

11. In AC circuit breakers, the arc is extinguished at:
(A) Current zero instant
(B) Maximum current instant
(C) Voltage peak
(D) Power factor angle

12. Cooling of the arc helps in:
(A) De-ionization of the arc path
(B) Increasing ionization
(C) Maintaining conductivity
(D) Increasing current

13. Lengthening the arc results in:
(A) Increased arc resistance
(B) Decreased arc resistance
(C) Constant arc voltage
(D) No effect on arc current

14. Splitting the arc into a number of smaller arcs is done to:
(A) Increase total arc voltage and facilitate extinction
(B) Reduce arc voltage
(C) Maintain steady arc
(D) Increase ionization

15. Blowing out the arc with magnetic field helps in:
(A) Cooling and lengthening the arc
(B) Ionizing the path
(C) Increasing current
(D) Heating contacts

16. The arc voltage in AC circuit breakers is usually:
(A) Higher than the normal supply voltage
(B) Lower than the normal voltage
(C) Zero
(D) Equal to rated voltage

17. The dielectric strength between breaker contacts after current zero should:
(A) Rise rapidly for successful arc extinction
(B) Remain constant
(C) Fall gradually
(D) Be zero

18. The re-striking voltage appears:
(A) Immediately after the current zero
(B) Before the arc is established
(C) During steady-state condition
(D) After full recovery

19. The recovery voltage is the voltage:
(A) Across contacts after arc extinction
(B) During fault
(C) Before contact separation
(D) During normal operation

20. The medium used for arc quenching in air circuit breakers is:
(A) Air at atmospheric pressure
(B) Oil
(C) SF₆ gas
(D) Vacuum

21. Oil circuit breakers use:
(A) Transformer oil for arc quenching and insulation
(B) Air blast
(C) SF₆ gas
(D) Water

22. SF₆ circuit breakers use:
(A) Sulphur hexafluoride gas as an arc quenching medium
(B) Compressed air
(C) Mineral oil
(D) Water vapor

23. The main advantage of SF₆ gas is:
(A) Excellent dielectric and arc quenching properties
(B) High leakage current
(C) Corrosive nature
(D) High cost and maintenance

24. In vacuum circuit breakers, arc extinction occurs due to:
(A) Rapid dielectric recovery in vacuum
(B) Cooling by gas
(C) Magnetic blowout
(D) Oil decomposition

25. The medium in a vacuum circuit breaker is:
(A) High vacuum
(B) Compressed air
(C) Nitrogen
(D) Mineral oil

26. The arc in a vacuum circuit breaker is maintained by:
(A) Metal vapors between the contacts
(B) Air molecules
(C) Ionized oil particles
(D) SF₆ gas

27. Arc quenching in oil circuit breakers takes place due to:
(A) Decomposition of oil into gases which cool and de-ionize the arc
(B) Ionization of air
(C) Heating of oil
(D) Magnetic blowout

28. The arc chute in an air circuit breaker is used to:
(A) Split and cool the arc
(B) Ionize the air
(C) Increase contact resistance
(D) Maintain arc voltage

29. The dielectric strength of SF₆ gas compared to air is about:
(A) 2 to 3 times higher
(B) Equal
(C) Half
(D) One-tenth

30. The key objective of arc quenching methods in circuit breakers is to:
(A) Quickly extinguish the arc and restore insulation between contacts
(B) Maintain current flow
(C) Reduce supply frequency
(D) Increase contact resistance permanently