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Senin, 18 April 2011

Types of Air-Blast Circuit Breakers


Depending upon the direction of air blast in relation to the arc;air blast circuit breakers are classified into:
(i)Axial-blast type in which air-blast is directed along the arc path as shown in figure below.

(ii) Cross-blast type in which air blast is directed at right angles to the arc path as shown in figure below

(iii) Radial-blast type in which the air blast is directed radially as shown in figure below

(i) Axial-blast air circuit breaker
The figure below shows the essential components of a typical axial blast circuit breaker.The fixed and moving contacts are held in closed position by spring pressure under normal conditions.The air reservoir is connected to the arcing chamber through an air valve.This valve remains closed under normal conditions but opens automatically by tripping impulse when a fault occurs on the system.

When a fault occurs,the tripping impulse causes the opening of the air valve which connects the circuit breaker reservoir to the arcing chamber.The high pressure air entering the arcing chamber pushes away the moving contact against 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.
It may be noted that in such circuit breakers,the contact separation required for interruption is generally small about 1.75 cm. Such a small gap may constitute inadequate clearance for the normal service voltage.Therefore,an isolating switch is incorporated as part of this type of circuit breaker.This switch opens immediately after fault interruption to provide necessary clearance for insulation.
(ii) Cross Blast air breaker
In this type of circuit 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 below shows the parts of a typical cross-blast air circuit breaker
When the moving contact is withdrawn,am arc is struck between the fixed and moving contacts.the high pressure cross-blast forces into a chute consisting of an arc splitters and baffles.The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 the 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 insulation clearance so that series isolating switch is not necessary.

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