When the current carrying contacts of the circuit breaker are moved apart, an arc is formed, which insist for a short period after the separation of contacts. This arc is dangerous on account of the energy generated in it in the form of heat which may result in explosive force.
The circuit breaker should be capable of extinguishing the arc without causing any disturbances to the equipment or danger to the personnel. The arc plays a vital role in the behaviour of the circuit breaker. The interruption of DC arc is relatively more difficult than AC arcs. In an AC arc, as the current becomes zero during the regular wave, the arc vanishes and it is interrupted from restriking.
The conductances of the arc are proportional to the number of electrons per cubic centimetre produced by ionisation, the square of the diameter of the arc and the reciprocal of the length. For arc extinction, it is, therefore, necessary to reduce the density of free electrons, i.e., reduces the ionisation and decrease the diameter of the arc.
Methods of Arc Extinction
There are two methods of arc extinction in circuit breakers.These methods are
High Resistance method
In this method, the arc is controlled in such a way that its effective resistance increase with the time, so that the current is reduced to such a value that heat formed by it is not sufficient to maintain the arc or thus arc is extinguished.
Because of the resistive nature of the arc discharge, most of the energy in the system will be dissipated within the circuit breaker. This is the main drawback of this method of arc extinction. The following are the reasons which can increase the resistance of the arc.
- Cooling of arc
- Increasing the length of the arc
- Reducing the cross section of the arc
- Splitting of arc
Low resistance or Zero Current Interruption method
This method is applicable only in AC circuit interruption because there are natural zero of current, 100 times in a second for 50Hz three-phase supply system. This is one of the most significant advantages of AC circuit for arc interruption purpose because the current is not allowed to rise again.
In this method, the arc resistance is kept low until the current is zero where the arc extinguishes naturally, and it’s prevented from restriking after it has gone out at a particular