# Circuit Breaker MCQ Question Answer

### 1. The contact resistance is least affected by

(a) the mechanical force applied
(b) the shape of the contact faces
(c) the amount of surface contamination
(d) the ambient temperature.

### 2. As the force on contact is increased, the contact resistance will

(a) increase linearly
(b) increase exponentially
(c) remain unchanged
(d) decrease.

### 3. Minimum arcing voltage for platinum is 16 V. It can therefore concluded that when the voltage is below 16 V

(a) it will not possible to interrupt the circuit
(b) it will not possible to pass the current
(c) it will be possible to interrupt any value of current without arcing
(d) it will be possible to interrupt any value of current without bringing contact closer to each other.

### 4. The arc voltage produced in the circuit breaker is always

(a) in phase with arc current
(b) leading the arcing current by 90 degrees
(c) lagging the arcing current by 90 degrees

### 4. Sparking between contacts can be reduced by

(a) inserting a resistance in the line
(b) inserting a capacitor in series with the contacts
(c) inserting a capacitor in parallel with the contacts

### 5. For magnetic blow out of the arc, magnetic field is produced

(b) parallel to the axis of the arc
(c) at right angles to the axis of the arc.

### 6. Sparking occurs when a load is switched off because the circuit has

(a) high inductance
(b) high capacitance
(c) high resistance

### 7. The power factor of the arc in circuit breaker is

(a) always zero
(b) always unity
(c) always lagging

### 8. Air blast circuit breaker is usually used for

(a) instantaneous duty
(b) permanent break
(c) intermittent duty
(d) repeated duty

### 9. Flame proof switch gears are usually preferred

(a) on transmission lines of low voltage
(b) substations
(c) in mines
(d) in high MVA capacity circuits

### 10. Pressure of air in air blast circuit breakers is usually

(a) 1 to 5 kg/cm2
(b) 5 to 10 kg/cm2
(c) 10 to 30 kg/cm2
(d) 35 to 100 kg/cm2

# Circuit Breaker MCQ Question Answer

### 11. Air used in air blast circuit breaker

(a) must have least carbon dioxide
(b) must have ionized
(c) must have oil mist
(d) must be free from moisture.

### 12. In a circuit breaker the time duration from the instant of fault to the instant of energizing of the trip coil is known as

(a) lag time
(c) protection time
(d) operation time

### 13. In a circuit breaker the time duration from the instant of fault to the instant of extinction of arc is known as

(a) lag time
(c) total clearing time
(d) operation time

### 14. In a circuit breaker the time duration from the instant of fault to the instant of closing of contact is known as

(a) recycle time
(b) total time
(c) gross time
(d) reclosing time.

### 15. For a high speed circuit breaker the total clearing time is nearly

(a) 1 to 2 cycles
(b) 5 to 10 cycles
(c) 10 to 15 cycles
(d) less than 50 cycles

(a) minimum
(b) 0.5
(c) 0.707
(d) maximum

### 17. Which of the following circuit breaker will produce the least arc energy

(a) Minimum oil circuit breaker
(b) Air blast circuit breaker
(c) Plain oil circuit breaker
(d) All will produce same energy.

### 18. For a circuit breaker ‘break time’ is

(a) same as opening time
(b) opening time + arc duration
(c) opening time + arc duration + resistor current duration

### 19. The breaking capacity of a circuit breaker in MVA (3 phase) is given by

(a) rated service voltage x rated symmetrical current
(b) 1.1 x rated service voltage x rated symmetrical current
(c) √2 x rated service voltage x rated symmetrical current
(d) √3 x rated service voltage x rated symmetrical current

### 20. Which relay is used for feeders?

(a) MHO relay
(b) Translay relay
(c) Merz Price Protection
(d) Buchholz relay

### 21. MHO relay is used for

(a) rectifiers
(b) circuit breakers
(c) transmission lines
(d) feeders

### 22. Merz Price protection is used on

(a) substations
(b) capacitor banks
(c) induction motors
(d) generators

### 23. The advantage of neutral earthing is

(a) simplified design of earth fault protection
(b) over-voltage due to lightening can be discharged to the earth
(c) freedom from persistent arcing ground
(d) all of the above.

### 24. The protection against over-voltage due to lightening is provided by

(a) use of surge diverters
(b) low tower footing resistance
(c) use of over head ground wires
(d) any of the above.

### 25. Which of the following is a conducting medium for electric current?

low temperature gas
(a) high temperature gas
(b) dissociated gas
(c) plasma.

### 26. In circuit breakers the contact space is ionized by

(a) thermal ionization of gas
(b) thermal emission from the surface of contacts
(c) field emission from the surface of contacts
(d) any of the above.

## Circuit Breaker MCQ Question Answer

1. 