Describe arc extinction process in brief.

Describe arc extinction process in brief.
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1 Answer

Answer :

Arc Extinction Process: During fault condition, when the tripping mechanism operates the circuit breaker to open its contacts, arc is produced between the contacts. There are two methods of arc extinction: i) High resistance extinction method ii) Low resistance or Zero current extinction method

High resistance extinction method: In this method, the arc is so controlled that its effective resistance increases with time so that the current reduces to a value insufficient to maintain the arc. The currents tends to be in phase with the voltage so that at zero current instant, the restriking voltage appearing across the contacts is relatively low and arc can not struck again. Arc path resistance is increased to reduce the current to low values while interrupting the arc. Arc resistance = varc/iarc. The arc resistance mainly increased by: i) Lengthening of the arc by arc runners ii) Splitting the arc by arc splitters: An appreciable voltage is absorbed at the contact surface so that if the arc can be split into a number of small arcs in series, the voltage available for the actual arc column is reduced. iii) Arc cooling: The voltage required to maintain ionization increases with decrease of temperature of arc, so that cooling effectively increases the resistance. iv) Constraining the arc: If the arc can be constrained into a very narrow channel, the voltage necessary to maintain it is increased.

Current zero or Low Resistance Method: This method is employed in a.c. circuit breakers since the ac passes through zero 100 times /second in 50 cycle current wave. When current wave passes through every zero, the arc vanishes for a brief moment. However the arc restrikes again with the rising current waves. In this method, at current zero instant, fresh unionized medium is introduced between the space in between the contacts. Due to this medium deionization effect takes place. The dielectric strength of the contact space increases to such an extent that the arc does not continue after current zero.

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