Circuit Breakers (Air, Oil, SF₆, Vacuum) — MCQs – EE

1. The main function of a circuit breaker is to:
(A) Interrupt fault current and isolate faulty sections
(B) Step up voltage
(C) Measure current
(D) Control power factor

2. A circuit breaker must be capable of:
(A) Making and breaking normal and fault currents
(B) Regulating voltage
(C) Measuring frequency
(D) Changing phase sequence

3. The arc in a circuit breaker is produced when:
(A) Contacts separate carrying current
(B) Voltage drops to zero
(C) Frequency increases
(D) Resistance decreases

4. The breaking capacity of a circuit breaker is expressed in:
(A) MVA
(B) kWh
(C) Ohms
(D) Volts

5. The process of arc extinction in circuit breakers depends mainly on:
(A) Current and voltage at the time of contact separation
(B) Power factor only
(C) Frequency
(D) Load connected

6. Air circuit breakers are generally used for:
(A) Low and medium voltage applications
(B) Very high voltage systems
(C) DC traction systems only
(D) Battery protection

7. In air blast circuit breakers, arc extinction occurs by:
(A) High-pressure air blowing across the arc
(B) Cooling oil
(C) Magnetic field
(D) Vacuum chamber

8. The air blast in an air circuit breaker helps in:
(A) Cooling and deionizing the arc path
(B) Increasing resistance of the contacts
(C) Maintaining arc temperature
(D) Boosting voltage

9. A disadvantage of air circuit breakers is:
(A) Noisy operation and large size
(B) Slow operation
(C) High maintenance of oil
(D) Poor insulation

10. Oil circuit breakers use oil as:
(A) Both an arc-quenching and insulating medium
(B) A conductor
(C) A coolant only
(D) A magnetic material

11. The main disadvantage of oil circuit breakers is:
(A) Fire hazard and frequent maintenance
(B) High cost of oil
(C) Slow speed of operation
(D) Requirement of vacuum

12. A minimum oil circuit breaker uses:
(A) Less oil than bulk oil circuit breakers
(B) More oil than bulk oil circuit breakers
(C) No oil
(D) Water as dielectric medium

13. The main function of oil in a circuit breaker is to:
(A) Quench the arc and insulate contacts
(B) Lubricate moving parts
(C) Increase current flow
(D) Reduce mechanical friction

14. In an oil circuit breaker, the arc energy decomposes oil into gases such as:
(A) Hydrogen and acetylene
(B) Carbon dioxide and oxygen
(C) Nitrogen and helium
(D) Methane and ammonia

15. SF₆ gas used in circuit breakers is chemically:
(A) Inert, non-toxic, and non-flammable
(B) Magnetic
(C) Highly reactive
(D) Conductive

16. SF₆ circuit breakers are preferred for:
(A) Extra high voltage (EHV) and high reliability applications
(B) Only low voltage
(C) DC systems only
(D) Marine applications

17. The main advantage of SF₆ circuit breakers is:
(A) Excellent arc-quenching and insulation properties
(B) High weight
(C) Low dielectric strength
(D) High noise level

18. The main disadvantage of SF₆ circuit breakers is:
(A) High gas pressure maintenance and cost
(B) Poor dielectric strength
(C) Slow operation
(D) Poor insulation

19. SF₆ gas circuit breakers are mostly used for voltages above:
(A) 220 kV
(B) 11 kV
(C) 33 kV
(D) 66 kV

20. The dielectric strength of SF₆ gas is approximately:
(A) 2 to 3 times that of air
(B) Same as air
(C) Half of air
(D) Lower than oil

21. In vacuum circuit breakers, arc extinction occurs because:
(A) There are no ionized particles to sustain the arc
(B) The air is compressed
(C) Oil absorbs the arc
(D) Magnetic field rotates the arc

22. The dielectric strength in a vacuum circuit breaker is:
(A) Very high compared to air and oil
(B) Very low
(C) Equal to air
(D) Same as SF₆

23. Vacuum circuit breakers are most suitable for:
(A) Medium voltage applications (up to 33 kV)
(B) Low voltage applications only
(C) Very high voltage systems
(D) DC circuits

24. The main advantage of a vacuum circuit breaker is:
(A) Long life and minimal maintenance
(B) Noisy operation
(C) Large size
(D) Use of oil as dielectric

25. The main limitation of a vacuum circuit breaker is:
(A) Limited current interrupting capacity
(B) High maintenance
(C) Fire hazard
(D) Large oil requirement

26. The arc in a vacuum circuit breaker is initiated by:
(A) Ionization of metal vapors from contacts
(B) Compressed air
(C) SF₆ gas
(D) Oil decomposition

27. The contact material in vacuum circuit breakers is usually:
(A) Copper-chromium alloy
(B) Pure copper
(C) Aluminum
(D) Steel

28. The operating mechanism of modern circuit breakers is generally:
(A) Spring, pneumatic, or hydraulic type
(B) Manual only
(C) Magnetic only
(D) Electrical motor only

29. The most eco-friendly and maintenance-free circuit breaker among the following is:
(A) Vacuum circuit breaker
(B) Oil circuit breaker
(C) Air circuit breaker
(D) SF₆ circuit breaker

30. The selection of a circuit breaker for a power system depends primarily on:
(A) System voltage, fault level, and type of application
(B) Size of the substation
(C) Manufacturer’s country
(D) Cable length