In the magnetic blow out type, the arc is extinguished between auxiliary contacts confined by a chute in which the arc is rapidly blown out due to a powerful magnetic field from one or more electromagnets. This type may be used in air or watertight boxes and is peculiarly adapted for service where the arc must be confined.
Fig. 2,247.—Thermal overload circuit breaker. In construction two contact blocks are fixed rigidly to, but insulated from, the switch arm. They are connected electrically by two parallel strips of suitable metal, each fitted with a steel catch piece. When the switch is closed the strips are sprung apart over a fixed catch, and the full rated current does not release the catch. Overload causes the strips to move apart, and the circuit breaker flies off under the action of a spring.
In a carbon break type, the arc is finally ruptured between carbon break contacts. The breaking of the circuit is accomplished progressively, that is to say, it is done in three stages, by several sets of contact, known respectively as
In operation, as the circuit breaker acts to break the circuit, first the main contacts, separate, then the intermediate contacts, and finally the carbon contacts between which the arc is ruptured.
Ques. What is the object of the intermediate contacts?
Ans. To prevent the forming of an arc on the main contacts.
Fig. 2,248.—Carbon break discs of Condit circuit breaker. The two pairs of similar discs which slide past each other are so arranged that these surfaces coincide at the instant the intermediate contacts separate after which, as the contact arm opens further, they gradually disengage.