400 Most Important MCQ Of Switchgear and Protection with explanation - 2022 Part - 10

91. A circuit breaker is

A. power factor correcting device
B. a device to neutralize the effect of transients
C. a waveform correcting device
D. a current interrupting device

Answer
D. a current interrupting device

Explanation:
A circuit breaker is a switching device that interrupts the abnormal or faults current. 
It is a mechanical device that disturbs the flow of high magnitude (fault) current and in addition, performs the function of a switch. 
The circuit breaker is mainly designed for closing or opening an electrical circuit, thus protecting the electrical system from damage.

92. Low voltage circuit breakers have rated voltage of less than

A. 220 V
B. 400V
C. 1000 V
D. 10,000 V

Answer
C. 1000 V

Explanation
The circuit breaker whose value lies below 1000V is known as the low voltage circuit breaker, and above 1000V it is known as a high voltage circuit breaker.

93. The medium employed for the extinction of arc in air circuit breaker is

A. SF6
B. Oil
C. Air
D. Water

Answer
C. Air

Explanation 
An Air Circuit Breaker (also known as an Air Blast Circuit Breaker or ACB) is an automatically operated electrical switch that uses air to protect an electrical circuit from damage caused by excess current from an overload or short circuit. 
Its primary function is to interrupt current flow after a fault is detected. 
When this happens, an arc will appear between the contacts that have broken the circuit. 
Air circuit breakers use compressed air to blow out the arc, or alternatively, the contacts are rapidly swung into a small sealed chamber, the escaping of the displaced air, thus blowing out the arc.

94. For high voltage, ac circuit breakers, the rated short circuit current is passed for

A. 0.01 sec
B. 0.1 sec
C. 3 sec
D. 30 sec

Answer
C. 3 sec

Explanation 
An electrical circuit breaker is a switching device that can be operated manually and automatically for controlling and protecting an electrical power system. 
As the modern power system deals with huge currents, special attention should be given during the designing of a circuit breaker to ensure it is able to safely interrupt the arc produced during the closing of a circuit breaker. 
This was the basic definition of the circuit breaker.
For high voltage, ac circuit breakers, the rated short circuit current is passed for 3 sec.

95. Interrupting medium in a contactor maybe

A. air
B. oil
C. SF6 gas
D. any of the above

Answer
D. any of the above

Explanation:
A contactor is an electromechanical control device that is used to make or break the connection between the load and power supply. 
The use of a contactor is similar to the relay. But the device used for higher current carrying applications is known as a contactor and the device used for lower current applications is known as Relay.
A contactor has several contacts as per the application and load. 
Generally, these contacts are normally open (NO) contact. And hence the load is shut off when the coil of the contactor is de-energized. 
But the contactor can design for both normally open and normally close applications. 
The most common application of contactor is in the starter that is used to turn ON and OFF the equipment like motor, transformer, etc.
A contactor is an electrically controlled switch used for switching a power circuit, similar to a relay except with higher current ratings. 
A contactor is controlled by a circuit that has a much lower power level than the switched circuit. Contactors are often used for 150 Hp motor.

96. SF6 gas is

A. sulphur fluoride
B. sulphur difluoride
C. sulphur hexafluoride
D. sulphur hexafluoride

Answer
D. sulphur hexafluoride

Explanation
It is colourless, odourless, non-toxic, and non-inflammable gas.
SF6 gas is extremely stable and inert, and its density is five times that of air.
It has high thermal conductivity better than that of air and assists in better cooling current-carrying parts.
SF6 gas is strongly electronegative, which means the free electrons are easily removed from discharge by the formation of negative ions.
It has a unique property of fast recombination after the source energising spark is removed. It is 100 times more effective as compared to arc quenching medium.
Its dielectric strength is 2.5 times that of air and 30% less than that of the dielectric oil. At high pressure the dielectric strength of the gas increases.
Moisture is very harmful to the SF6 circuit breaker. Due to a combination of humidity and SF6 gas, hydrogen fluoride is formed (when the arc is interrupted) which can attack the parts of the circuit breakers.

97. SF6 gas

A. is lighter than hydrogen
B. is lighter than air
C. has density 2-times as compared to that of air
D. has density 5-times as compared to that of air

Answer
C. has density 2-times as compared to that of air

Explanation
 SF6 gas has density 2-times as compared to that of air

98. While selecting a gas for the circuit breaker, the property of gas that should be considered is

A. high dielectric strength
B. non-inflammability
C. non-toxicity
D. all of the above

Answer
D. all of the above

Explanation
High dielectric strength
Take minimum time in installation
The gas is non-inflammable and chemically stable; The decomposition products are non-explosive
It gives noiseless operation it does not make a sound like an air-blast circuit breaker during operation
Minimum maintenance
The same gas is re-circulated into the circuit thereby reducing the requirement of SF6 gas.


99. Which of the following circuit breaker can be installed on a 400 kV line

A. Tank type oil circuit breaker
B. Miniature circuit breaker
C. Vacuum circuit breaker
D. Air blast circuit breaker

Answer
D. Air blast circuit breaker

Explanation
The second ‘air-based circuit breakers are air blast circuit breakers.
In this type, a high-pressure air blast is used as an arc extinguishing medium. In case of a fault, the air blast, controlled by a blast valve, will open the contacts and also cool the arc.
The arc and the arching products are swept into the atmosphere, which rapidly increases the dielectric strength of the medium.
As a result, the restriking of the arc is prevented. The arc is extinguished consequently and the flow of current is completely interrupted.
These are finding their best applications in systems at 132 kV and above (up to 400 kV) with breaking capacity up to 7,500 MVA and above.
The pressure of the air is of the order of 20 kg / cm2 to 30 kg / cm2

100. In modern EHV circuit breakers, the operating time between the instant of receiving trip signal and final contact separation is, of the order of

A. 0.001 sec
B. 0.015 sec
C. 0.003 sec
D. 0.03 sec

Answer
D. 0.03 sec

Explanation
In modern EHV circuit breakers, the operating time between the instant of receiving trip signal and final contact separation is, of the order of 0.03 sec.

101. In Extra voltage lines which of the following Circuit breakers are most suitable?

A. SF6 circuit breakers
B. Air circuit breakers
C. Oil circuit breakers
D. Vacuum circuit breakers

Answer
A. SF6 circuit breakers

Explanation
A circuit breaker in which SF6 under pressure gas is used to extinguish the arc is called SF6 circuit breaker. 
SF6 (sulphur hexafluoride) gas has excellent dielectric, arc quenching, chemical and other physical properties which have proved its superiority over other arc quenching mediums such as oil or air.


102. _______ is not a method of high resistance interruption of arc extinction

A. Blast effect
B. Arc lengthening
C. Arc constraining
D. Arc cooling

Answer
A. Blast effect

Explanation:
Arc is a sparking current in the circuit breaker when the two contacts of it are open. There are two methods of arc extinction in circuit breakers as follows:
1) High resistance method: In the high resistance method, the arc resistance is made to increase gradually with time so that the current force in between the two contacts is insufficient to maintain the arc. Hence the current is interrupted by high resistance the arc is extinguished without creating any damage to the circuit breaker compartment. The different methods to increase the resistance are:
a) Increase the length of the arc
b) Reducing the cross-section area of the arc
c) Arc cooling method
d) Arc splitting method

2) Low resistance method: In this method, the arc resistance is kept low until the current magnitude becomes zero. The arc extinguishes naturally and is prevented by restricting by making the medium dielectric strength high compared to a voltage between contacts. The different methods to achieve are
a) Increase the length of the contact gap
b) Cooling the ac medium
c) High pressure in the arc chamber
d) Blast effect of ions

103. _______ does not have any repairable component

A. Air Blast CB
B. MCB
C. OCB
D. Air Break CB

Answer
B. MCB

Explanation:
MCB (miniature circuit breaker) is an electromechanical device that guards an electrical circuit against an overcurrent that may effect by a short circuit, overload or imperfect design. It doesn’t have any repairable components.
MCB is substituting the rewirable switch-fuse units for low power domestic and industrial applications in a very quick manner. In a wiring system, the MCB is a blend of all three functions such as protection of short circuits, overload and switching. Protection of overload by using a bimetallic strip and short circuit protection by using a solenoid.

104. Resisting capacity of Vaccum circuit breaker is:

A. Zero
B. Moderate
C. Lower as compared to others
D. High

Answer
D. High

Explanation:
A vacuum circuit breaker is such a kind of circuit breaker where the arc quenching takes place in a vacuum. This is suitable for medium voltage applications. The resisting capacity of this type of circuit breaker is very high.

105. Air Blast Circuit breaker is employed among which of the following?

A. Arc furnace duty
B. Traction system
C. Both A & B 
D. None of the above 

Answer
C. Both A & B 

Explanation:
Airblast circuit breakers are used in arc furnace duty, traction systems, and used for railway electrification because they are suitable for repeated duty.




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