A circuit breaker works as a switching device as well as a current interrupting device. It does this by performing the following two functions:
1. Switching operation during normal working of operation and maintenance
2. Switching operation during abnormal conditions that may arise, such as overcurrent, short-circuit, etc.
Therefore, the need arises that it must withstand the abnormal current conditions, apart from the normal working current. All the switches discussed above, come equipped with a tripping device that constitutes an elementary load interrupter switch. The difference between a load interrupter switch and a circuit breaker lies in the current interrupting capacity.
A circuit breaker must open the circuit successfully under short-circuit conditions. The current through the contacts may be several orders of magnitude greater than the rated current. As the circuit is opened, the device must withstand the accompanying mechanical forces and the heat of the ensuing arc, until the current is permanently reduced to zero.
When any high-voltage circuit is interrupted, there is a tendency towards an arc formation between the two separating contacts.
If the action takes place in air, the air is ionized and plasma is formed by the passage of current. When ionized, the air becomes an electric conductor. The space between the separating contacts thus has a relatively low impedance and the region close to the surface of the contacts has a relatively high voltage drop. The thermal input to the contact surface is therefore relatively large and can be highly destructive.
Therefore, the major aim in a circuit breaker design is to quench the arc rapidly enough, to keep the contacts in a reusable state by one of the following methods: