Arc Interruption Theory
- It should provide sufficient insulation between the contacts when circuit breaker opens.
- It should extinguish the arc occurring between the contacts when circuit breaker opens.
Methods of Arc InterruptionThere are two methods by which interruption is done.
- High resistance method,
- Low resistance method or current zero interruption method.
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Limitations of high resistance method: Arc discharge has a resistive nature due to this most of the energy is received by circuit breaker itself hence proper care should be taken during the manufacturing of circuit breaker like mechanical strength etc. Therefore this method is applied in DC power circuit breaker, low and medium AC power circuit breaker. Low resistance method is applicable only for ac circuit and it is possible there because of presence of natural zero of current. The arc gets extinguished at the natural zero of the AC wave and is prevented from restricting again by rapid building of dielectric strength of the contact space.
There are two theories which explain the phenomenon of arc extinction:
- Energy balance theory,
- Voltage race theory.
- Restriking Voltage: It may be defined as the voltages that appears across the breaking contact at the instant of arc extinction.
- Recovery Voltage: It may be defined as the voltage that appears across the breaker contact after the complete removal of transient oscillations and final extinction of arc has resulted in all the poles.
- Active Recovery Voltage: It may be defined as the instantaneous recovery voltage at the instant of arc extinction.
- Arc Voltage: It may be defined as the voltages that appears across the contact during the arcing period, when the current flow is maintained in the form of an arc. It assumes low value except for the point at which the voltage rise rapidly to a peak value and current reaches to zero.
- Energy Balance Theory: When the contact of circuit breaker are about to open, restriking voltage is zero, hence generated heat would be zero and when the contacts are fully open there is infinite resistance this again make no production of heat. We can conclude from this that the maximum generated heat is lying between these two cases and can be approximated, now this theory is based on the fact that the rate of generation of heat between the the contacts of circuit breaker is lower than the rate at which heat between the contact is dissipated. Thus if it is possible to remove the generated heat by cooling, lengthening and splitting the arc at a high rate the generation, arc can be extinguished.
- Voltage Race Theory: The arc is due to the ionization of the gap between the contact of the circuit breaker. Thus the resistance at the initial stage is very small i.e. when the contact are closed and as the contact separates the resistance starts increasing. If we remove ions at the initial stage either by recombining them into neutral molecules or inserting insulation at a rate faster than the rate of ionization, the arc can be interrupted. The ionization at zero current depends on the voltages known as restriking voltage.