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Air Circuit Breaker definition
It is a circuit breaker with compressed air as arc quenching medium to exterminate the arc produced when the contacts are separated. So this is called Air Circuit Breaker. The air circuit breaker is used for the voltages less than 3 KV.
Construction of Air circuit Breaker
The air circuit breaker consists of three types of contacts in a compressed air chamber. The three contacts are
- The main contacts which carries the load current
- The arching current which carries arching current
- The arching horns.
Main contacts of Air Circuit breaker
The main contacts are made from a good conducting material, usually made up of silver, copper. These offers low resistance to the load current flowing through the circuit. This also avoids over heating of the contacts while in service.
Arching contacts of air circuit breaker
The arching contacts are made of harder material with high resistance for example tungsten. Because the aching contacts should with stand very high arching current. Once the main contacts opens the arching current flows through the arching current with high resistance path.
Arching horns and chutes
the arching horns are also made up of hard material(tungsten or copper) to carry the arching current. The arc is transferred from arching contacts at last before extinction. The arc chutes made up of the fire proof insulating material because these are the last contacts of arc. The cooling metal plates are attached to arc chutes to convect the hot gasses way from the gases and to cool the contacts arc the extinction of arc. The metal plates further stretches the arc and helps in reducing the heating of contacts.
The above parts are fitted in circuit breaker chamber with compressed air which acts as dielectric material for the arching current to flow.
Operating principle of Air circuit Breaker
The operating mechanisms of air circuit breaker in two modes i.e. opening mode and closing mode are explained as follows.
Breaker opening mode
Under normal operation with the breaker close the load current flows through the main contacts because of low resistance path compared to arching contacts.
Under fault condition or normal opening condition, as the breaker opens, the main contacts which are carrying the load current opens first by transferring the current to the arching contacts. As the arching contacts opens then arc produces due to ionization of the medium at their faces. the arching contact faces are made up of hard materials the damage is less for these arching current.
As the arching contacts still opens the arc widens and increases due to convection of surrounding gases. The stretched arc when enters to arc chute area is rapidly cooled by the cooling metallic plates. The cooled gases offers high dielectric force to the electron conduction and hence the arc extincts.
Breaker Closing Mode
Closing mode is the reverse process of breaker opening mode where the arching contacts closes when the closing command is initiated to the circuit breaker. The main contacts closes about a half a cycle after the arching contacts. Then the main contacts carries the load current.
Advantages of Air Circuit Breaker
- The Operation of Air circuit Breaker is easy and simple.
- The whole construction is simple and inexpensive
- The arc quenching medium is easily available in atmosphere
- Simple maintenance requirements only monitoring the air pressure in the cabinet.
- Fire hazards will not happen in case of insulation damage of the cabinet as in the case of oil circuit breaker.
Disadvantages of Air Circuit Breaker
- The operating voltage is limited to <10 KV because of less dielectric strength compared to SF6 and air blast.
- The interrupting current capacity is limited to 10000 A.
Applications of air circuit breaker
- The air circuit breakers are used in power plants for medium voltage i.e. <3KV for safe operation.
- These are widely used as out door circuit breakers due to its simplicity.