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 into the atmosphere.
Thus the dielectric strength of the medium is increased, prevents from re-establishing the arc. The arc gets extinguished and flow of current is interrupted.
Axial Blast Air Circuit Breaker
Under Normal Condition
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
Since the air valve connects the air reservoir and the arcing chamber, a high-pressure air enters 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 ionized gases along with it. Consequently, the arc is extinguished and the current flow is interrupted.
- The contact separation required for arc extinction is very small generally (1.75 or so ).
- This small gap may sometimes inadequate clearance for the normal service voltage. Therefore an isolating switch is included as a part of this CB.
- This switch opens immediately after the fault interruption to provide the necessary clearance for insulation.
Cross Blast Circuit Breaker
The figure shows the essential components of a typical cross-blast air circuit breaker.
The high-pressure cross-blast forces the arc into a chute consisting of arc splitters and baffles.
The result is that arc is extinguished and flow of current is interrupted. Since blast pressure is the same for all currents, the inefficiency at low currents is eliminated.
The final gap for interruption is great enough to give normal insulation clearance so that a series isolating switch is not necessary.