Electric Arc in

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Arc in Circuitbreaker

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Arc Phenomenon :

Arc Phenomenon

Arc Phenomenon :

When a short-circuit occurs, a heavy current flows through the contacts of the circuit breaker before they are opened by the protective system. At the instant when the contacts begin to separate, the contact area decreases rapidly and large fault current causes increased current density and hence rise in temperature. The heat produced in the medium between contacts (usually the medium is oil or air) is sufficient to ionise the air or vaporise and ionise the oil. The ionised air or vapour acts as conductor and an arc is struck between the contacts. Arc Phenomenon

Arc Resistance:

The arc resistance depends upon the following factors: Degree of ionisation Length of the arc Cross-section of arc Arc Resistance

Methods of Arc Extinction :

There are two methods of extinguishing the arc in circuit breakers: 1. High resistance method. 2. Low resistance or current zero method Methods of Arc Extinction

1. High resistance method:

In this method, arc resistance is made to increase with time so that current is reduced to a value insufficient to maintain the arc. Consequently, the current is interrupted or the arc is extinguished. The resistance of the arc may be increased by: Lengthening the arc Cooling the arc Reducing X-section of the arc Splitting the arc 1. High resistance method Arc Lengthening

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Arc Splitting Lengthening the arc

2. Low resistance or Current zero method:

1.This method is employed for arc extinction in a.c. circuits only. In this method, arc resistance is kept low until current is zero where the arc extinguishes naturally and is prevented from restriking in spite of the rising voltage across the contacts. 2. 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 the current will be interrupted. 2. Low resistance or Current zero method

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The rapid increase of dielectric strength of the medium near current zero can be achieved by: ( 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 un-ionised particles The de-ionisation of the medium can be achieved by: Lengthening of the gap High pressure Cooling Blast effect

Some important terms:

1. Arc Voltage. It is the voltage that appears across the contacts of the circuit breaker during the arcing period. 2. Restriking voltage. It is the transient voltage that appears across the contacts at or near current zero during arcing period. 3. Recovery voltage. It is the normal frequency (50 Hz) r.m.s. voltage that appears across the contacts of the circuit breaker after final arc extinction. Some important terms

Consequences of online Isolator switching.:

Consequences of online Isolator switching.

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