Principles of Arc Extinction in Circuit Breaker
Before discussing the methods of arc extinction, it is necessary to examine the factors responsible for the maintenance of arc between the contacts. These are :
- potential difference between the contacts.
- ionised particles between contacts
Taking these in turn,
Methods of Arc Extinction in Circuit Breaker
- High resistance method
- Low resistance or current zero method.
1. High Resistance Method
- 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 contacts.
- Cooling the arc – Cooling helps in medium between the contacts. This increases the arc may be obtained by a gas resistance. Efficient cooling blast directed along the arc.
- Reducing X-section of the arc – If the area of X-section of the arc is reduced, the voltage necessary to maintain the arc is increased. In other words, the resistance of the arc path is increased. The cross-section of the arc can be reduced by letting the arc pass through a narrow opening or by having smaller area of contacts.
- 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 these arcs experiences the effect of lengthening and cooling. The arc may be split by introducing some conducting plates between the contacts.
2. Low Resistance or Current zero Method
- (a) causing the ionised particles in the space between contacts to recombine into neutral molecules.
- (b) sweeping the ionised particles away and replacing them by unionised particles.
Therefore, the real problem in AC arc interruption is to rapidly deionise the medium between contacts as soon as the current becomes zero so that the rising contact voltage or restriking voltage cannot breakdown the space between contacts. The de-ionisation of the medium can be achieved by :
- (i) lengthening of 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.
- (ii) high pressure. If the pressure in the vicinity of the arc, is increased, the density of the particles constituting the’ discharge also increases. The increased density of particles causes higher rate of de-ionisation and consequently the dielectric strength of the medium between contacts is increased.
- (iii) cooling : Natural combination of ionised particles takes place more rapidly if they are allowed to cool. Therefore, dielectric strength of the medium between the contacts can be increased by cooling the arc
- (iv) blast effect : If the ionised particles between the contacts are swept away and replaced by un-ionised particles, the dielectric strength of the medium can be increased consider-ably. This may be achieved by a gas blast directed along the discharge or by forcing oil into the contact space.