After opening the Circuit Breaker under load or fault condition, a transit voltage appears across the breaker contacts which is called Restriking Voltage. Frequency of Restriking voltage goes in kH.
During arcing period, transient voltage appears across the contacts at or near current zero. If dielectric medium between the CB contacts is greater than the rise of restriking voltage then the arc will not restrike.
Recovery transient voltage is the final arc extinction between the contacts of circuit breaker when voltage came to power frequency that is 50Hz or 60 Hz.
Effects of restriking voltage
Circuit Breaker is the most important equipment in electrical substation. Switching of any elements can be done through circuit breaker only. During switching at full load or fault condition, arc will generate across the contact of circuit breaker due go high current flow in the circuit.
The circuit breaker is designed in a way that can easily eliminate the arc by using of high dielectric medium inside the circuit breaker chamber.
However due to high restriking voltage, dielectric medium of CB could not quench arc and it will failed the circuit breaker. The arching contacts or main contacts of the CB also failed due to high restriking voltage across the contacts. It is very much important to minimize the voltage of the circuit breaker.
Methods of Arc Extinction or Interruption
Followings methods are used for extinguishing arc due to restriking voltage in Circuit Breaker.
- High resistance method.
Under this method, arc resistance is made to increase with time so that current can be reduced and it will maintain the arc. Accordingly, it will interrupted the current.
Due to this using of high resistance enormous energy is dissipated in the Arc. This high resistance method generally used in DC circuit breaker or in low capacity AC circuit breaker.
- Current zero method or Low resistance
This low resistance or current zero method is used only in AC circuit. It is commonly used for interruption of arc in high voltage AC circuit breaker. Under this method arc resistance made very low till the current value is zero.
In AC current, current gets zero in every half cycle. At the zero current or low current condition, the CB has been opened and under this it will create very low restriking voltage is generated.