Arc Generation in Circuit Breaker

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what is the ARC in Circuit Breaker

The Arc is an “electrical spark” between fixed electrode and movable electrode when they are separated during a fault. The electrical spark occurs due to the flow of free electrons which are produced by ionizing the medium between to electrons.

Generation of arc in circuit breaker

When a short circuit current of usually 10 times greater than the normal circuit flows through the circuit breaker under fault condition. During this if the circuit breaker opens its contacts then the contact area reduces which increases current density (electron density) further it increases the temperature near the contacts. This temperature ionizes the medium (oil, air, gas) between the contacts which forces the medium to acts a conductor for electron flow. This instant electron layer with high temperature is known as arc of the circuit breaker.

So during this short time the circuit remains uninterrupted to short circuit current flow as long as the arc persists. This high current persists through the circuit longer than the intended creates high temperature and fire in the electrical circuit. To avoid this arc of the circuit breaker should be quenched as soon of the possible. The arc existence depends on the resistance offered for the circuit by the contact medium and it depends on the following factors:

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Dielectric strength of the medium: The arc resistance increases with increase in dielectric strength of the medium as it offers an opposite force to the electron flow.

Degree of ionization: With the decrease in the number of particles to be ionized the arc resistance will increases.

Length of the arc: The arc resistance increases with the length of the arc i.e. separation of contactors. If the contacts separation is high then arc extinction will be less.

Cross section of the arc: The arc resistance increases with decrease in area of cross section of the arc.

Separation time: with increase in separation time of contacts the arc existence will be less.

ARC persistence factors in circuit breaker contacts:

1)    Potential difference between the contacts after separation of the contacts. i.e. the two contacts would be separated as far as possible to reduce the pd between the contacts which helps the arc to persist. But in high voltages this is not suitable because the contacts have to be separated for some meters.

2)    Reduce the ionized particles: Theses ionized particles creates a conductive path for electron flow which helps the arc to persist. So to extinct the arc the ionized particles should be deionized by perfectly cooling the arc area with proper cooling methods.

ARC Extinction methods

There are two ARC Extinction methods based on above said arc persistence factors;

• How resistance method
• Low resistance or current zero method.