How the arc extinction done on circuit breaker?

 There are two methods of extinguishing the arc.

1. High resistance method.
2. Low resistance method. 

High resistance method for arc extinction. 

In this method, the arc resistance is made to increase with time so that the current is reduced to a value insufficient to maintain the arc. Thus, the current is interrupted or the arc is extinguished. The principal disadvantage is that huge loss occurs in the arc. For this reason, it is used only in DC circuit breakers and low capacity AC circuit breakers.

The arc resistance increasing procedures.

1. Lengthening the arc: The resistance of the arc is directly proportional to its length. The length of the arc can be increased by increasing the gap between the contacts.

2. Cooling the arc: Cooling helps in the deionization of the medium between the contacts which increases the arc resistance. Efficient cooling could be obtained by a gas blast directed along the arc.

3. Reducing cross-section of the arc: If the area of X-section of arc is reduced, The voltage necessary to maintain the arc is increased. In other words, the resistance of the arc is increased. The cross section of the arc can be reducer by letting the arc to pass through a narrow opening or by having smaller area of contacts.

4. Splitting the arc: The resistance of the arc can be increased by splitting the arc into a number of smaller arcs in series. Each one of this arcs experiences the effect of lengthening and cooling. The arc splitting may be done by introducing some conducting plates between the contacts.

Low Resistance or Current zero method for arc extinction.

This method is employed for arc extinction in AC circuits only. In this method, the arc resistance is kept low until current is zero where the arc extinguishes naturally and is prevented from re-striking inspite of rising voltage across the contacts. All modern high power a.c. circuit breakers employ this method for arc extinction.

In an a.c. system, current drops to zero after every half-cycle. At every current zero, the arc extinguishes for a brief moment. During this time the medium between the contacts contains ions and electrons so the it has small dielectric strength and it can be easily breakdown by the rising contact voltage. 

If such a breakdown occurs, the arc will persist for another half cycle. If immediately after current zero, the dielectric strength of the medium between contacts is built up more rapidly than the voltage across the contacts, the arc fails to restrike and current will be interrupted. 

The rapid increase of dielectric strength of the medium near current zero can be achieved by...

=>> Causing the ionised particles in the space between contacts to recombine into neutral molecules.

=>> Sweeping the ionised particles away and replacing them by un-ionised particles.

 

The de-ionization of the medium can be achieved by:

Lengthening the gap. The dielectric strength of the medium is proportional to the length of the gap between contacts. Therefore, by opening the contacts rapidly, higher dielectric strength of the medium can be achieved.

High pressure. If the pressure in the vicinity of the arc is increased the density of the particles increases which causes higher rate of de-ionization and consequently the dielectric strength of the medium between contacts is increased.

Cooling. Neutral combination of ionized particles takes place more rapidly it the are allowed to cool. Therefore, dielectric strength of the medium between the contacts can be increased by cooling the arc.

Blast effect. If the ionized particles between the contacts are swept away and replaced by un-ionised particles, the dielectric strength of the medium can be increased considerably. This may be achieved by a gas blast directed along the discharge or by forcing oil into the contact space. 

Read more:

1. What are the different types of fuse?

2. What are the difference between Fuse and Circuit breaker?