Air Blast Circuit Breaker

                 What is Air Circuit Breaker?

These breaker employ a high pressure air blast and air quenching medium. The contact are opened in a flow of air blast established by the opening of blast valve. The air blast cools the arc and sweep away the arcing product  to the atmosphere. This rapidly increase the dielectric strength of the medium between contacts and prevents from reestablishing the arc. Consequently the arc is extinguished and flow of current is interrupted.

   Working Principle of Air Circuit Breaker

The working principle of this breaker is rather different from those in any other types of circuit breakers. The main aim of all kind of circuit breaker is to prevent the reestablishment of arcing after current zero by creating a situation where in the contact gap will withstand the system recovery voltage.

The air circuit breaker does the same but in different manner. For interrupting arc it creates an arc voltage in excess of the supply voltage. Arc voltage is defined as the minimum voltage required maintaining the arc. This circuit breaker increases the arc voltage by mainly three different ways,

1. It may increase the arc voltage by cooling the arc plasma. As the temperature of arc plasma is decreased, the mobility of the particle in arc plasma is reduced; hence more voltage gradient is required to maintain the arc.
2. It may increase the arc voltage by lengthening the arc path. As the length of arc path is increased, the résistance of the path is increased, and hence to maintain the same arc current more voltage is required to be applied across the arc path. That means arc voltage is increased.

Advantages:-

1)The risk of fire is eliminated.

2) The products completely removed by the blast.

3) The arcing time is very small due to the rapid buildup of dielectric strength between contacts.

4) Due to lesser arc energy, air blast circuit breaker are very suitable for condition where frequent operation is required.

Disadvantages:-

1) the air has relatively inferior arc extinguishing properties.

2) The air blast circuit breaker are very sensitive  to the variation in the rate of raise of re-striking voltage.

Application:-

The air blast circuit breaker are finding wide application in high voltage installation. Majority of Circuit breaker for the voltages beyond the 110KV are of this type.

               Types Of Air Blast Circuit Breaker

        Axial blast Air Circuit Breaker

In the air blast circuit breaker, the flow of air is longitudinal along the arc.  Air blast circuit breaker may be a single blast or double blast. Breaking employing double blast arrangement are sometimes called radial blast circuit breakers as the air blast flows radially into the nozzle or space between the contacts.

The essential feature of air blast circuit breaker is shown above. The fixed and moving contacts are kept in a closed position by spring pressure under normal operating conditions. The air reservoir tank is connected to the arc chamber through an air valve, which is opened by a triple impulse.

          Cross Blast Air Circuit Breaker

In such breaker,  an arc blast is directed at right angles to the arc. The schematic representation of the cross principle of cross blast air circuit breaker is given in the figure below.  A moving contact arm is operated in close spaces to draw an arc which is forced by a transverse blast of air into the splitter plates, thereby lightening it to the point when it cannot restrike after zero current.

Resistance switching is not normally required as the lightening of arc automatically introduces some resistance to control the re-striking voltage transient but if extra resistance is thought desirable. It is possible to introduce it by connecting it in the section across the arc splitter.