Construction & Working [Video Lesson]

ACB has two sets of contacts i.e. main and auxiliary contacts. Each set of contact consists of a fixed contact and a moving contact. The main contact normally carries most of the load current. All the contacts are made of cadmium-silver alloy which has very good resistance to damage by arcing.

 

When the ACB is closed, the powerful spring is energized and the ACB is then latched shut against spring pressure. The auxiliary contact makes first & breaks last i.e. when ACB is closed, the auxiliary contact closes first then the main contact follows.

 

When the ACB is open, the main contact open firsts, and then the auxiliary contact opens. Thus the auxiliary contacts are subjected to arcing during the opening of ACB and can easily be replaced. The main contact closing pressure is kept high so that the temperature rises in the contacts while the carrying current remains within the limit. A closing coil operating on D.C voltage from a rectifier is provided to close the circuit breaker by operating a push button.

 

How Arc Quenching is Achieved?

Quenching of the arc is achieved by:

1. Using arcing contacts made of resistance alloy and silver tips for the main contacts. Arcing contacts close earlier and opens later than the main contacts.

2. When opening contacts have long travelled at high speed to stretch the resultant arc, which is transferred to the arcing contact.

3. Cooling and splitting of the arc are done by arc chutes which draw the arc through splitters by magnetic action and quickly cool and split the arc until it snaps. The circuit breaker opens when the arc is quenched.

 

Video Lesson:

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