Classification of Circuit Breakers MCQ Quiz in मराठी - Objective Question with Answer for Classification of Circuit Breakers - मोफत PDF डाउनलोड करा

Air Blast Circuit Breaker:

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.

Applications:

• The air blast circuit breakers are preferred for arc furnace duty and traction systems because they are suitable for repeated duty.
• These type of circuit breakers are finding their best application in systems operating in the range of 132 kV to 400 kV with breaking capacities up to 7000 MVA.

Advantages:

• No fire hazards are possible with this type of circuit breaker
• The high-speed operation is achieved.
• The time for which arc persists is short. Thus the arc gets extinguished early.
• As arc duration is short and consistent, the amount of heat released is less and the contact points are burnt to a less extent. So the life of a circuit breaker is increased.
• The extinguishing medium in this type of circuit breaker is compressed air which is supplied fresh at each operation. The arc energy at each operation is less than that compared with the oil circuit breaker. So air blast circuit breaker is most suitable where frequent operation is required.
• This type of circuit breaker is almost maintenance-free.
• It provides a facility for high-speed reclosure.
• The stability of the system can be well maintained.

Disadvantages:

• If an air blast circuit breaker is to be used for frequent operation it is necessary to have a compressor with sufficient capacity for high-pressure air.
• The maintenance of the compressor and other related equipment is required.
• There is the possibility of air leakages at the pipe fittings.
• It is very sensitive to restriking voltage. Thus current chopping may occur which may be avoided by employing resistance switching.

• 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.

Additional Information

Circuit breaker:

A circuit breaker is a switching device which can be operated manually and automatically for controlling and protecting an electrical power system.

There are four types of circuit breaker

• Oil circuit breaker
• Air circuit breaker
• SF6 circuit breaker
• Vacuum circuit breaker

The correct answer is (option 4) i.e. Moulded.

Explanation:

MCCB stands for Moulded Case Circuit Breaker.

So here 'M' stands for Moulded.

• It is a type of electrical protection device that can be used for a wide range of voltages, and frequencies of both 50 Hz and 60 Hz.
• The wide range of current ratings available from MCCB allows them to be used in a wide variety of applications.
• MCCBs are available with current ratings that range from low values such as 15 amperes to industrial ratings such as 2500 amperes.
• This allows them to be used in both low-power and high-power applications.

Oil Circuit Breakers:

An Oil circuit breaker aka OCB is a type of circuit breaker that uses insulating oil as a dielectric medium to quench the arc and break the circuit safely.

The oil used is insulating oil used usually transformer oil that has better dielectric strength than air.

Types of Circuit Breakers: 

1. Bulk oil Circuit Breaker
   (i) Plain oil circuit breaker
   (ii) Arc control oil circuit breaker.
2. Minimum or low oil circuit Breaker

Bulk oil Circuit Breaker:

• The oil has to serve two purposes.
• Firstly, it extinguishes the arc during the opening of contacts and secondly, it insulates the current-conducting parts from one another and from the earthed tank.
• In the case of a plain-break oil circuit breaker, there is very little artificial control over the arc, and the quantity of oil requirement is also more than arc control CB.
• Therefore, a comparatively long arc length is essential in order that turbulence in the oil caused by the gas may assist in quenching it.
• However, it is necessary and desirable that final arc extinction should occur while the contact gap is still short. For this purpose, some arc control is incorporated and the breakers are then called arc control circuit breakers.
   

Minimum or Low oil circuit breaker: 

• A low oil circuit breaker employs solid materials for insulation purposes and uses a small quantity of oil which is just sufficient for arc extinction.
• As regards quenching the arc, the oil behaves identically in bulk as well as low oil circuit breaker.
• By using suitable arc control devices, the arc extinction can be further facilitated in a  minimum oil circuit breaker.

Circuit Breakers: A circuit breaker is a piece of equipment that can,

• Make or break a circuit either manually or by remote control under normal conditions
• Break a circuit automatically under fault conditions
• Make a circuit either manually or by remote control under fault conditions
• It can open or close a circuit under no-load, full load, and fault conditions
   

Thus a circuit breaker incorporates manual (or remote control) as well as automatic control for switching functions.

Operating principle:

• A circuit breaker essentially consists of fixed and moving contacts, called electrodes.
• Under normal operating conditions, these contacts remain closed and will not open automatically until and unless the system becomes faulty. Of course, the contacts can be opened manually or by remote control whenever desired.
• When a fault occurs on any part of the system, the trip coils of the circuit breaker get energized and the moving contacts are pulled apart by some mechanism, thus opening the circuit.
• When the contacts of a circuit breaker are separated under fault conditions, an arc is struck between them.
• The current is thus able to continue until the discharge ceases.
• The production of arc not only delays the current interruption process but it also generates enormous heat which may cause damage to the system or to the circuit breaker itself.
• Therefore, the main problem in a circuit breaker is to extinguish the arc within the shortest possible time so that heat generated by it may not reach a dangerous value.

Sulphur Hexaflouride (SF₆) Circuit Breakers:

• In these circuit breakers, sulphur hexaflouride (SF₆) gas is used as the arc quenching medium.
• The SF₆ is an electro-negative gas and has a strong tendency to absorb free electrons.
• The contacts of the breaker are opened in a high-pressure flow of SF₆ gas and an arc is struck between them.
• The conducting free electrons in the arc are rapidly captured by the gas to form relatively immobile negative ions.
• This loss of conducting electrons in the arc quickly builds up enough insulation strength to extinguish the arc.
• The SF6 circuit breakers have been found to be very effective for high power and high voltage service.
   

Advantages:

• Due to the superior arc quenching property of SF₆, such circuit breakers have a very short arcing time.
• Since the dielectric strength of SF₆ gas is 2 to 3 times that of air, such breakers can interrupt much larger currents.
• The SF6 circuit breaker gives noiseless operation due to its closed gas circuit and no exhaust to the atmosphere, unlike the air blast circuit breaker.
• The closed gas enclosure keeps the interior dry so that there is no moisture problem.
• There is no risk of fire in such breakers because SF₆ gas is non-inflammable.
• The size of SF6 breaker is smaller than a conventional circuit breaker of the same rating.
• There are no carbon deposits so that tracking and insulation problems are eliminated.
• The SF6 breakers have low maintenance costs, light foundation requirements, and minimum auxiliary equipment.
• Problems connected with current chopping are minimum.
• Since SF6 breakers are totally enclosed and sealed from the atmosphere, they are particularly suitable where explosion hazard exists e.g., coal mines.
   

Disadvantages:

• SF₆ breakers are costly due to the high cost of SF₆.
• Since SF6 gas has to be reconditioned after every operation of the breaker, additional equipment is required for this purpose.

The correct answer is 'option 3'

Solution:

The voltage levels of different circuit breakers are given below:

Additional Information

Advantages of SF₆ circuit breaker: ​

• The SF₆ gas is  non-flammable and chemically stable.
• Their decomposition products are non-explosive and hence there is no risk of fire or explosion.
• Electric clearance is very much reduced because of the high dielectric strength of SF₆.
• It gives noiseless operation.
• There is no over-voltage problem because the arc extinguishes at natural current zero.
• There is no reduction in dielectric strength because no carbon particles are formed during arcing.
• It requires less maintenance and no costly compressed air system is required.
• SF6 performs various duties like clearing short-line faults, switching, opening unloaded transmission lines, transformer reactor, etc. without any problem.

Oil switches

• The oil pressure switch is installed in the oil circuit of the engine.
• It monitors the oil pressure and turns off or on a warning light or controls an oil pressure indicator.
• These switches are used for circuits of high voltage and large current carrying capacities.
• The oil switch has its live parts immersed in oil to reduce arcing.
• The oil pressure switch is normally closed. When the ignition is switched on, the oil pressure warning light illuminates.

Concept:

Current chopping in circuit breaker is defined as a phenomenon in which current is forcibly interrupted before the natural current zero.

the voltage across the capacitor during current chopping

\(V=\frac{1}{\sqrt{L/C}}\)

Current Chopping is mainly observed in the air blast circuit breaker. There is no such phenomenon in oil circuit breaker.

SF6 circuit breaker:

• SF6  circuit breaker is a type of circuit breaker in which sulfur hexafluoride gas or SF6 gas is used as the arc quenching medium.
• SF6  circuit breaker may suffer from gas leakage problem. Because of that, the pressure of the gas inside the extinguisher chamber decreases.
• The arc quenching capability of the breaker may be weaker if the pressure of the gas decreases. to avoid this, regular checking/maintenance of an SF6 CB. 
• The current chopping phenomenon is less in SF6 circuit breaker and so high reliability is obtained.

Additional Information

​Circuit breaker:

A circuit breaker is a switching device that can be operated manually and automatically for controlling and protecting an electrical power system.

There are four types of circuit breaker

• Oil circuit breaker
• Air circuit breaker
• SF6 circuit breaker
• Vacuum circuit breaker