Arc is a sparking current in the circuit breaker when the two contacts of it are open. There are two methods of arc extinction in circuit breaker as follows:
- High Resistance method
- Low resistance method.
High Resistance method:
In high resistance method, the arc resistance is made to increase gradually with the time so that the current force in between the two contacts is insufficient to maintain the arc. Hence the current is interrupted by with high resistance the arc is extinguished without creating any damage to the circuit breaker compartment. The major work is to increase the resistance for the arc current. The following are the different ways to increase the resistance of the arc medium.
Increase the length of the arc:
The resistance of the wire or any conductor can be increased by increasing the length of it. Here also the air (gap between conductors) acts as conductor so that by increasing the gap between the contacts then the resistance of the arc can be increased. I.e. the resistance of the arc is directly proportional to the length of the arc.
Reducing the cross section area of arc:
Reducing the cross section area of the arc directly implies up on the voltage to be maintained for the arc persisting. I.e. if the area has been reduced, the resistance of the arc path increases so that the voltage of the arc should be high enough to maintain the arc. The cross section area of the arc can be reduced by letting the arc passed through the smaller area compared to wide area. This is done by routing the area with smaller gap of contacts in the circuit breaker compartment.
Arc Cooling method:
Cooling arc medium helps in deionization of the medium which carries the arc current. This intern increases the arc resistance which prevents the arc to persist. Effective cooling may be the blasting type which forces the cooling element along the arc.
Arc splitting method:
The resistance of the arc can be increases by splitting the arc in to smaller pieces which are in series. I.e. each one of this arc is experiences the methods of lengthening and cooling then the total arc resistance in series is high which prevents the arc persistence. The arc may be split by introducing some conducting plates between the contacts of circuit breaker.
The major disadvantage in this method is that a high energy is dissipated across the arc medium where it offers high resistance. So this is method of arc extinction is generally used in DC circuit breakers and low capacity AC circuit breakers.
Low Resistance method (Current zero method):
This method is used in AC power supply only where the voltage and current signals are alternating in their amplitudes (Max, zero, Min). In this method the arc resistance is kept low until the current magnitude becomes zero where the arc extinguish naturally and is prevented by restricting by making the medium dielectric strength high compared to voltage between contacts.
In an AC system the current or voltages change their polarity for every half cycle which crosses zero magnitude point. At this point of current zero the arc extinguishes naturally for a moment. And then when the voltage rises to restriking voltage, it easily breaks the dielectric strength offered by the fewer ions in the contact medium. This helps in arc persisting for another half cycle. In another case if the dielectric strength of the arc medium can be increased to maximum after the current zero point, then the arc fails to restrike and current will be interrupted. This rapid increase of dielectric strength of contact medium at zero current point is achieved by:
- By recombining the ionised particles in the contact medium in to neutral molecules (SF6).
- Forcing the old ionised particles away from the medium and replacing them with the new ones (blast type)
Therefore it is clear by the above discussion the rapid de-ionization of the contact medium immediately after the current zero point so that the raising voltage between contacts or restriking voltage can break the space medium between the contacts. The rapid de-ionization can be achieved by the following methods.
Increase the length of the contact gap:
The dielectric strength of the medium is directly proportional to the length of the contact gap. Therefore if the two contacts are opened rapidly with in less time higher dielectric strength of the medium can be obtained.
Cooling the Arc medium:
If the ions are cooled rapidly they will recombine naturally to form the neutral molecules, so in this way the dielectric strength of the arc medium can be increased.
High pressure in arc chamber
If the pressure in the arc chamber of circuit breaker compartment is increased then density of the density ionized particles will be high. This increases the de-ionization rate in the chamber which increases the dielectric strength between the contacts.
Blast effect of ions
This is one of the best ways. In this method the ionized particles in the arc chamber are swept way at zero current point and replaces with the new ones, so these new particles offers high dielectric strength in between the circuit breaker contacts.
Due to advantages of efficient arc extension without any high energy loss all the modern AC circuit breakers uses this method of arc extinction.