Function, Working and Construction of Air Circuit Breaker (ACB)

Air circuit breaker is designed to overcome the defects and safeguard the machine before it breakdowns. The main function of air circuit breaker is to:

1. Open and close a 3 phase circuit, manually or automatically
2. Open the circuit automatically when a fault occurs. Faults can be of various types – under or over voltage, under or over frequency, short circuit, reverse power, earth fault etc.

The main feature of ACB is that it dampens or quenches the arcing during overloading.

Air Circuit Breaker (ACB) Construction & Working

1. 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.
2. The main contact normally carries most of the load current. All the contacts are made of cadmium-silver alloy, which has good resistance to damage by arcing.
3. 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 and then the main contact follows.
4. 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.
5. The main contact closing pressure is kept high so that the rise in the temperature in the contacts while carrying current remains within limits.
6. 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 arc is achieved by:

1. Using arcing contacts made of resistance alloy and silver tips for the main contacts. Arcing contacts close earlier and open later than the main contacts.
2. When opening contacts have travelled at high speed to stretch the resultant arc, which is transferred to the arcing contact.
3. Cooling and splitting of the arc is done by arc chutes, which draw the arc through splitters by magnetic action and quickly cools and splits the arc until it snaps. The circuit breaker opens when the arc is quenched.