When a fault occurs in a power system, an abnormal value of current flows through the contacts of the circuit breakers before they are opened the protective system. During the operation of circuit breaker, the medium (air or oil) around the contacts becomes ionized and fault current start flowing through the air and hence rise in temperature. The ionized medium (air or oil) acts as a conductor and an arc is struck between the contacts. The potential difference is sufficient to maintain the arc. This arc provides a low resistance path to the flow of fault current. Consequently, the fault current in the circuit remains uninterrupted so long as the arc persists.
When a fault occurs on a power system, the relay gives a signal to the circuit breaker. The contacts of the circuit breaker begin separate, at this instant arc is formed around the contacts. For isolation the healthy section of the system from the faulty section without causing any harm to the power system, the arc will interrupt as soon as possible after occurring of fault. The current interruption results in arc extinction. There are two methods of arc extinction in circuit breakers, namely, High Resistance Methods and Low Resistance or Current Zero Interruption.
High Resistance Method
In this method, the arc resistance is increased in time to such a high value so that the current is reduced to such a value insufficient to maintain the arc. As a result, the current is interrupted or the arc is extinguished. The rate at which the effective resistance is increased or current is decreased is not abnormal, so as to cause harmful induced voltages in the system. The main drawbacks of this method, is that enormous heat is generated in the arc. This method is employed only in dc circuit breakers and Air circuit breakers.
The arc resistance can be increased by
- Lengthening the arc
- Cooling the arc
- Splitting the arc
- Constraining the arc
Low Resistance or Zero Current Method
This method is used only in an AC circuit for the extinction of arc. We Know that, In case of AC 50 Hz supply there are 100 zeros per second. This property of AC current used for interruption purposes. In this method, when current reaches to zero is not allowed to rise again. In present day, all modern AC circuit breakers employ this method for arc extinction.
This is achieved by
- Blast Effect
- Gap lengthning