Switchgear and Protection MCQs With Explanation Part-01

1. The main function of a fuse is to 
A. protect the line
B. open the circuit
C. protect the appliance
D. Prevent excessive currents
E. none of the above

Answer: D. Prevent excessive currents

  • A fuse is an electrical safety device that operates to provide overcurrent protection of an electrical circuit.
  • Its essential component is a metal wire or strip that melts when too much current flows through it, thereby stopping or interrupting the current.
  • It is a sacrificial device; once a fuse has operated it is an open circuit and must be replaced or rewired, depending on its type.

Fuses can be divided into two types:
DC Type: 
  • In a DC system, when the metallic wire melts because of the heat generated by the overcurrent, then Arc is produced and it is very difficult to extinguish this arc because of the DC constant value.
  • So in order to minimize the fuse arcing, DC fuses are a little bigger than an AC fuse which increases the distance between the electrodes to reduce the arc in the Fuse.
AC Type:
  • In the AC system, voltage with 60Hz or 50Hz frequency changes its amplitude from zero to 60 times every second, so arc can be extinct easily as compared to DC.
  • Therefore, AC fuses are a little bit small in size as compared to DC fuses.

2. The arcing contacts in a circuit breaker are made of
A. Copper tungsten alloy
B. porcelain
C. electrolytic copper
D. aluminum alloy

Answer: A. Copper tungsten alloy

  • In a circuit breaker, the main contacts are usually made up of copper and conduct current in closed positions. 
  • Circuit breakers have low contact resistance and they are silver plated. 
  • The arcing contacts are solid, resistant to heat and are made up of copper tungsten alloy.

3. On which of the following routine tests are conducted ?
A. Oil circuit breakers
B. Air blast circuit breakers
C. Minimum oil circuit breakers
D. All of the above

Answer: D. All of the above

Routine Test of Circuit Breaker:
  • Routine tests are performed as per recommendations of the Indian Standards.
  • These tests are performed on the manufacturers’ premises.
  • Routine tests confirm the proper functioning of the circuit breaker.
  • The routine tests confirm the proper functioning of the circuit breaker.
Various routine tests are performed for ensuring the quality, performance of a circuit breaker and these are:
  • Power frequency overvoltage withstand test for 1 minute
  • Dielectric test on auxiliary circuit and control circuit
  • Measurement of resistance of main circuit or contact resistance test
  • Tightness test or SF6 gas leakage test
  • Design and visual checks
  • Mechanical operation tests / operational tests.
Type tests can be broadly classified as:
  • Mechanical tests.
  • Thermal tests.
  • Dielectric tests.
  • Short-circuit tests:
  • Making capacity test
  • Breaking capacity tests
  • Duty cycle tests
  • Short time current tests.

4. Which of the following medium is employed for extinction of arc in air circuit breaker ?
A. Water
B. Oil
C. Air
D. SF6

Answer: C. Air


Type of circuit breaker


Operating voltages 

 Air Circuit Breaker

Air at atmospheric pressure

Low voltage up to l000 V

Tank type oil circuit breaker


Up to 33 kV

Minimum oil circuit breaker


36 kV, 1500 MVA

132 kV, 3000 MVA


Compressed air

132 kV, 220 kV

400 kV, 760 kV


SF6 gas

132 k V, 220 kV

400 kV, 760 kV

Vacuum C.B.


11 kV. 33 kV

High voltage direct current C.B.

Vacuum or SF6 gas

± 500 kV DC

Important Notes:

Circuit breaker

Arc Quenching medium



SF6 circuit breaker

SF6 gas

Up to 760 kV

Used in a complete range of medium voltage and high voltage electrical power system

Oil circuit breaker

Insulating Oil

Up to 132 kV

It is mainly used for interrupting low current at high voltage

Vacuum circuit breaker


Up to 66 kV

It is mainly used for high current at low voltage

Used for remote and rural area

Air circuit breaker


up to 15 kV

Used for railway traction purpose


5. With which of the following, a circuit breaker must be equipped for remote operation ?
A. Inverse time trip
B. Time-delay trip
C. Shunt trip
D. None of the above
E. All of the above

Answer: C. Shunt trip

  • Inverse time tripping is a characteristic of circuit breakers in which the breaker trips in more time with lower over current and in less time with higher over current.
  • Long time delay trip action causes the breaker to wait for a certain amount of time to allow temporary inrush currents, such as those encountered when starting a motor, to pass without tripping.
  • Short time pickup is used for selective tripping. The short-time pickup function determines the amount of current that will be carried by breaker for a short period of time. Short-time delay is used in conjunction with short-time pickup, controls the time involved in postponing a short-time pickup trip.
  • A shunt trip device is an optional accessory in a circuit breaker that mechanically trips the breaker when power is applied to the shunt trip terminals. The power for the shunt trip does not come from within the breaker, so it must be supplied from an external source. It is used for remote operations.
6. SF6 gas
A. is yellow in colour
B. is lighter than air
C. is nontoxic
D. has pungent small
E. none of the above

Answer: C. is nontoxic

  • In SF6 circuit breaker, Sulphur hexafluoride gas is used as the arc quenching medium.
  • Due to its low gaseous viscosity and high molecular weight, the SF6 gas can efficiently transfer heat by convection.
  • It has high insulating properties and highly electronegative properties.
  • Pure Sulphur hexafluoride gas is inert and thermally stable.
  • SF6 has a very high dielectric strength (breakdown strength). It has 100 times more effective than air and oil as interrupting mediums.
  • Its dielectric strength is 2.5 times that of air and 30% less than that of the dielectric oil.
7. Fault diverters are basically
A. fuses
B. relays
C. fast switches 
D. circuit breakers 

Answer: C. fast switches 

  • Fault diverters are basically fast acting switches
  • These are very effective protection against transient-induced power arcs within accelerator oscillator tubes
  • This device short-circuits the oscillator-plate power supply in the event of an over-current, thus removing the power flow from the fault within a few microseconds
8. A thermal protection switch can protect against
A. short-circuit 
B. temperature
C. overload
D. over voltage 

Answer: D. over voltage 

  • A thermal protection switch is used to protect against overload conditions
  • Thermal circuit protectors utilize a bimetallic strip electrically in series with the circuit
  • The heat generated by the current during an overload deforms the bimetallic strip and trips the breaker
  • Thermal protectors have a significant advantage over fuses in that they can be reset after tripping
9. Arc in a circuit behaves as
A. a capacitive reactance
B. an inductive reactance
C. resistance increasing with voltage rise across the arc
D. resistance decreasing with volt-age rise across the arc

Answer: D. resistance decreasing with volt-age rise across the arc

Arc phenomenon in circuit breaker:
  • Arc initiated at the instant of contact separation due to high field gradient or field ionization.
  • The arc is maintained not because of the field gradient.
  • It is maintained because of thermal ionization due to the high amount of heat produced during the arcing process.
  • So, the arc is just a column of ionized gases.

Properties of the Arc:
  • The conductance of the arc is directly proportional to the number of free electrons generated during ionization.
  • The conductance of the arc is directly proportional to the cross-section area of the arc.
  • Conduction of the arc is inversely proportional to the length of the arc.
  • The path of the arc is purely resistive and it is having a negative temperature coefficient (means if the temperature increases arc resistance decreases and vice versa).
  • The resistance of the arc is highly non-linear. So that it behaves as a resistance decreasing with voltage rise across the arc.

Important Points
As the path of the arc is purely resistive, we know that for purely resistive elements the voltage across the element is always in phase with current flowing through the element. So, the arc voltage produced in the circuit breaker is always in phase with the current.

10. Thermal circuit breaker has 
A. delayed trip action
B. instantaneous trip action
C. both of the above
D. none of the above

Answer: A. delayed trip action

  • This type depends upon the effect of heat on a bimetal strip for its operation. Such a strip bends on heating and, in the thermal circuit breaker, bending of the strip causes contact to open, which breaks the circuit. The bimetal strip is usually in series connection in the electrical circuit, its temperature being governed by the current flowing through it. As the current increases, the temperature of the bimetal strip rises, bending more the hotter it becomes. 
  • The load current is passed through a small heater coil wrapped around a bi-metal strip inside the MCB housing; the heat created depends on the current it carries. This heater is designed to warm the bi-metal strip either directly (the current passes through the bi-metal strip which in effect is part of the electrical circuit) or indirectly (where a coil of current-carrying conductor is wound around the bi-metal strip) and the excess current warms the bi-metal strip.
  • The bi-metal strip is made of two different metals, normally brass and steel (brass expanding more than steel). These two dissimilar metals are securely riveted or welded together along their length. The rate of expansion of the two metals is different so that when the strip is warmed, it will bend and will trip the latch, due to its expansion coefficient, its action is time delayed.
  • The bi-metal strips are arranged so that normal currents will not heat the strip to the tripping point. However, if current increases beyond the rated value, the heater increases in temperature, and thus the bi-metal strip is raised in temperature, trips the latch, and breaks the circuit.
  • The thermal relay is not suitable for operation on short-circuit as it will burns the elements sufficiently before the strip may deflect so as to close the contacts.

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