Arcing in Circuit Breaker

Electric Arc

For interrupting power supply to a part of a network or circuit, or to clear a fault, circuit breakers are employed. On receipt of trip signal the circuit breaker operating mechanism operates to separate moving contact from the fixed contact (Fig-A). As the moving contact starts moving away from the fixed contact, the contact area of the tips of  both moving contact and fixed contact reduces. But the same current now passes through this reduced contact area. The current density of the contact area increases very much. This situation makes the areas of the tips in contact very hot, may be several thousand degrees celsius. Now as the contacts just separate these hot spots becomes source of electron emission. High energy electrons are emitted from the separated contact tips. This is called thermionic emission.

The other main cause of electron emission is field emission. As the movable contact moves away from the fixed contact voltage difference between the two electrodes (fixed and moving contacts) exist (Fig-B). Which gives rise to an electric field between the electrodes.

Electric Field = V / d

Where  V is the potential difference between the electrodes and d is the separation between the two.

So from the above formula it is clear that just after separation when the movable contact has not moved much away, then d is small so the electric field strength is very high (order of several kV per millimetre). High electric field gives rise to emission of high energy electrons from the contact surface. The high speed electrons emitted bombard the molecule or atoms of the medium and dislodge electrons from the atoms. This is secondary emission.

The high energy electrons so emitted ionise the gas or oil used as medium. Arc plasma is formed between the fixed and moving contacts. The current continues to flow through the arc plasma between the contacts (see Fig-A).

It is clear that just separating the contacts does not automatically break the circuit and flow of current does not stop. The arc is required to be extinguished. More importantly the dielectric strength of the medium between the fixed and moving contacts should be restored quickly, otherwise arc may re-strike between the contacts. This is the job of the arc interruption chamber of the circuit breaker.

Several techniques are employed for the arc extinction. The arc can be extinguished by increasing the resistance of the arc path. This is achieved by lengthening the arc or splitting the arc using one of the techniques and by reducing the diameter of the arc by cooling. By increasing the resistance the voltage drop across the arc increases. It achieves such a value that the supply voltage cannot sustain the voltage drop across the arc and the arc is extinguished.
Also in AC circuit the current varies sinusoidally so the arc is extinguished at next current zero. Although the arc is extinguished but the medium is having enough ionised particles. Hence to stop the re-striking of arc due to Transient Restriking Voltage the space between the separated contacts should rapidly regain dielectric strength. This can be done by blowing gas or air at high speed to the region between the contacts in Gas or Air Blast breaker. In case of oil circuit breaker rapid flow of oil to the contact region helps regain high dielectric strength. Regaining dielectric strength is the result of recombination of ions and electrons.

The working principles of some breakers will be discussed later.    

1 comment:

Anonymous said...

good and simple explaination....

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