Current Zero Interruption
Current Zero Interruption is toxic to switch Gear protection. In a 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. If at current zero the dielectric strength is built up more rapidly than the voltage across the contacts the arc I'll fail to restrike and the current will be interrupted.
Dielectric Strength can be increased by:
(a) Recombination of ionized particles into neutral molecules.
(b) Replacing ionized particles by unionized particles.
Deionization can be Achieved by:
(a) Lengthening of the gap:
Arc resistance is directly proportional to the length of the gap between contacts. So by opening contacts rapidly dielectric strength can be achieved.
(b) High pressure:
When pressure increases, the density of particles increases, which causes a high rate of deionization and hence increase the dielectric strength of the medium
The natural combination of ions occurs rapidly when they are cooled. Therefore the arc cooling can increase dielectric strength.
(d) Blast effect:
If ionized particles are swept away and replaced by unionized particles dielectric strength can be increased.