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Air Blast Circuit Breakers - Classification and Working

Air blast circuit breakers employ a high pressure air blast as an arc quenching medium. 


Under normal condition the contacts are closed. When a fault occurs contacts are opened and an arc is struck between the them. The opening of contacts are done by a flow of air blast established by the opening of blast valve (located between air reservoir and arcing chamber ). The air blast cools the arc and sweeps away the arching products in to the atmosphere. Thus the dielectric strength of the medium is increased, prevents from re-establishing the arc. The arc get extinguished and flow of current is interrupted.

Types of Air Blast Circuit Breakers


The circuit breakers are classified on the basis the direction of air blast to the arc. They are classified into :


1. Axial Blast Type - air blast is directed along the arc path.

2. Cross Blast Type - air blast is directed at right angles to the arc path.

3. Radial Blast Type - air blast is directed radially.



Axial Blast Air Circuit Breaker


Under Normal Condition
The fixed and moving contacts are held in closed position with the help of spring pressure.
There is an air reservoir connected to the arcing chamber through an air valve. The air valve
control the flow of air into the arcing chamber. The valve is closed under normal conditions.

Under Faulty Condition
When a fault occurs a tripping impulse is produced which causes opening of the air valve. Since the air valve connects the air reservoir and the arcing chamber, a high pressure air enters to the arcing chamber. This air pushes away the moving contact against the spring pressure. The moving contact is separated and an arc is struck. At the same time high pressure air blast flows along the arc and takes away the ionised gases along with it. Consequently the arc is extinguished and current flow is interrupted.

Note: 
  • The contact seperation required for arc extinction is very small generally (1.75 or so ).
  • This small gap may sometimes inadequate clearence for the normal sevice voltage. Therefore an isolating switch is included as a part of this CB.
  • This switch opens immediately after the fault interruption to provide necessary clearence for insulation.


Cross Blast Circuit Breaker


In this type of breaker, an air-blast is directed at right angles to the arc. The cross blast lengthens and forces the arc into a suitable chute for arc extinction. Figure shows the

essential components of a typical cross-blast air circuit breaker. 

When the moving contact is withdrawn, an arc is struck between the fixed and moving contacts. The high pressure cross-blast forces the arc into a chute consisting of arc splitters and baffles.


The splitters serve to increase the length of the arc and baffles give improved cooling. The result is that arc is extinguished and flow of current is interrupted. Since blast pressure is same for all currents, the inefficiency at low currents is eliminated. The final gap for interruption is great enough to give normal iusulation clearance so that a series isolating switch is not necessary.