OPERATING PRINCIPLE OF A CIRCUIT BREAKER (CB)

A circuit breaker consists of two contacts namely “fixed” and “moving contacts” which touches each other under normal condition i.e. when circuit breaker is closed. Whenever a fault occurs, the trip coils get energized, the moving contacts get pulled by some mechanism and therefore, the circuit breaker is opened and the circuit is broken.

Initiation of Arc

When the moving contacts of a C.B. begins to part, the heavy current passes through the gap causing a small voltage drop. A small separation of contacts does not cause immediate interruption of current because as the contact separate, the resistance between them increases and a lot of heat is produced due to the ohmic loss.
This heat is sufficient to ionize the air or vaporize and ionize the oil between the contacts. The ionized air or vapour acts as a conductor and thus the current remains uninterrupted across the low resistance arc produced. The potential difference between the contacts is quite small and is just sufficient to maintain the arc.

Note:

• When the flow of charge takes place in a dielectric medium then, it is called “arc”.
• Arc in a circuit breaker is initiated due to “field emission process” and is maintained due to “thermionic emission (thermal ionisation)”.
• Because of the random nature of ionisation process, the voltage has high frequency components which also contribute radio frequency noise.
• The heat of the arc causes the oil in circuit breaker to decompose and liberate gases which comprises of almost 70% hydrogen, 20-25%% acetylene and 5-10% others (methane and other hydrocarbons)

Comparison between Circuit Breaker and Isolator/Disconnecting Switch: