Electrical Dost

Circuit Breaker

Circuit Breaker, Electrical Dost
Circuit Breaker, Electrical Dost
Image by Jürgen Diermaier from Pixabay

Definition:
“A circuit breaker is an equipment which can be used to Break a circuit automatically under fault conditions Can make a circuit manually or by remote control system under fault condition.”

Working principle:
A circuit breaker usually consists of fixed or moving contact which is called electrodes. Under operating or normal conditions, these contacts are closed and will not open until a fault occurs. When a fault occurs, the trip coils are energized and thus moving contacts are pulled off by opening a circuit breaker.

When the contacts of the circuit breaker are opened during fault conditions, an arc is produced between them, this arc is removed by some medium. The main problem in the circuit breaker is the arc quenching phenomenon.

Arc Quenching Phenomenon:

When there is a fault in the power system, heavy current flows through the contacts of circuit breaker and a large amount of heat is produced due to heavy flow of current and this arc can be removed by using any medium. During the arc period, the current between the contacts of a circuit breaker depends upon the arc resistance which depends upon the following factors

Degree of ionization: Arc resistance is increased by a decrease in the number of ionization particles between contacts of the circuit breaker.
Length of Arc: If the length of the arc increases then arc resistance also increases.
Cross-section of Arc: By decreasing the cross-sectional area, the arc resistance increases

Types of Circuit breakers:
The medium used for arc quenching is different for different rating, mostly oil, air, SF6 or vacuum is used.

Following are the types of circuit breakers:

1) Airblast or Air break Circuit breakers (ACB)
2) Oil Circuit breakers (OCB)
3) SF6 Circuit breakers
4) Vacuum circuit breakers

1) Airblast or Air break circuit breaker:

Such circuit breakers can use an air blast as an arc quenching medium. The contacts of C.B are opened inflow of air blast by opening a valve. The air first cools down the arc and then sweeps away the arcing to the atmosphere which usually increases the dielectric strength of a medium between contacts

Advantages of Air Blast Circuit breakers:

1) It has very less arc energy, ACB is used where frequent operation is required.
2) The dielectric growth is very rapid which can quench the arc fast.
3) The arc is completely removed by the blast.

Disadvantages of Air Blast Circuit breakers:

1) The ACB is very sensitive in the rate of re-striking voltage
2) It needs high maintenance.
3)It has very inferior arc quenching properties

2) Oil Circuit Breakers:

Such circuit breakers used some insulating oil (e.g transformer oil) for arc extinguishing medium. The contacts under oil and arc are struck with them.

This oil circuit breaker has further types:

  1. Bulk Oil circuit breaker: It uses a large quantity of the oil that has two purposes, the one is it extinguishes the arc during the opening of the circuit breaker and the second is too insulating current-conducting parts from one another.
  2. Low oil Circuit breakers: It uses the minimum amount of oil for the arc quenching of arc, the current conducting parts are insulated by air or porcelain.

Advantages of Oil Circuit breakers:

1) It absorbs the arc energy to decompose oil into gases.
2) It is an insulator and used to cool down the arc very easily

Disadvantages of Oil circuit breakers:

1) There is a risk of fire because it is inflammable.
2) It may form an explosive mixture with air.


3) SF6 Circuit breakers:

Such circuit breakers use SF6 gas is used for arc quenching medium. The SF6 C.B is an electro-negative gas and has a strong tendency to absorb electrons. The contacts of the breakers are opened in a high pressure of SF6 gas and arc is struck between them.

Advantages of SF6 Circuit breaker:

1) Due to good properties, SF6 C.B has very short arcing time.
2) The Sf6 is a soundless operation due to its closed gas circuit and no exhaust to atmosphere.
3) There is no risk of fire because Sf6 is non-inflammable.

Disadvantages of SF6 Circuit breaker:

1) SF6 circuit breakers are costly.
2) SF6 gas has to be reconditioned after every operation of breaker, so for this purpose additional equipment is required.


4) Vacuum Circuit breaker:

It uses the vacuum as a medium for arc quenching. A vacuum has the highest insulating strength. When a circuit breaker contacts are opened in a vacuum, the dielectric strength is greater and the arc is easily quenched.

When the Circuit breaker operates, the Moving contacts separate from the fixed contact and arc is between the contacts, this arc is quickly quenched by vacuum medium since the vacuum has an in no time rate of recovery of dielectric strength.

Advantages of Vacuum Circuit breaker:

1) They are no risk of fire in VCB.
2) There is no generation of gas in VCB.
3) VCB usually require less maintenance.
4) VCB has low arc energy.

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