Arc Interruption Method
· High resistance method – In this method arc resistance is made to increase with time so thatcurrent becomes insufficient to maintain the arc. Disadvantage of this method is enormous energy is dissipated in the arc. Hence it can be used only in dc circuit breakers and low capacity ac circuit breakers.
Arc resistance can be increased by:
1. Lengthening the arc – Arc resistance is directly proportional to length of arc so to increase resistance separation between the contacts are increased
2. Cooling the arc – Cooling helps in deionisation of medium thus increasing arc resistance
3. Reducing cross section of the arc – When area of arc reduced, voltage necessary to maintain arc increased i.e. resistance is increased. Allowing the arc to pass through narrow opening can reduce cross section area.
4. Splitting the arc – The resistance can be increased by splitting the arc into number of smaller arcs in series. Each arcs experiences the effect of lengthening and cooling. Arc may be split by introducing some conducting plates between the contacts.
· Low resistance method – This method is employed for arc extinction in ac circuits only. In this method the arc resistance is maintained low till current zero during which arc extinguishes naturally and is prevented from restriking inspite of rising voltage across the contacts.
In ac system current drops to zero after every half cycle, during which the arc extinguishes for a brief moment. The medium still contains ions and electrons so has small dielectric strength which can be easily broken down by the rising voltage between the contacts known as restriking voltage. So if break down occur arc will persist for another half cycle. If at current zero the dielectric strength is built up more rapidly than the voltage across the contacts the arc will fail to restrike and current will be interrupted.
Dielectric strength can be increased by:
- Recombination of ionized particles into neutral molecules
- Replacing ionised particles by unionised particles
Deionisation can be achieved by:
1. Lengthening of the gap – Dielectric strength is directly proportional to length of gap between contacts. So by opening contacts rapidly dielectric strength can be achieved.
2. High pressure – When pressure increases, density of particles increases, which causes high rate of deionisation and hence increases dielectric strength of medium.
3. Cooling – Natural combination of ions occur rapidly when they are cooled. Therefore cooling the arc can increase dielectric strength.
4. Blast effect – If ionized particles are swept away and replaced by unionized particles dielectric strength can be increased. It can be achieved by gas blast directed along the discharge or by forcing oil into the contact space.