الأربعاء، 2 فبراير 2011

Important Terms and Definitions In Circuit Breker

 Important Terms and Definitions In Circuit Breker

Here are described some vital terms, used in the circuit breakers’ analysis, with simple definitions and a short description.


Arc Voltage
Arc voltage is defined as the voltage that appears across the contacts of the circuit breaker during the arcing period (the period in which the arc persists).
As soon as the contacts of the circuit breaker are separated, an arc is formed between them. The voltage that appears across the contacts during this period, until the arc is extinct, is called the arc voltage. The value of this voltage is highest at the zero current point. This peak value of the arc voltage helps maintain the current flow in the form of arc.

Restriking Voltage
Restriking voltage is the transient voltage that appears across the contacts at or near current zero during the arcing period.
The current interruption in the circuit depends upon the high frequency transient voltage, the restriking voltage. If the restriking voltage rises more rapidly than the dielectric strength of the medium between the contacts, the arc will persist for the next half-cycle. On the other hand, if the dielectric strength of the medium builds up more rapidly than the restriking voltage, the arc fails to restrike and the current will be interrupted.

Recovery Voltage
Recovery voltage is defined as the normal (50 Hz) voltage that appears across the contacts of the circuit breaker immediately after the final arc extinction. It is approximately equal to the system voltage.
When the contacts of the circuit breaker are opened, current drops to zero after every half cycle. At some current zero, the contacts are separated adequately apart and dielectric strength of the medium between the contacts attains high value due to the elimination of ionized particles. At such an instant, the medium between the contacts is strong enough to prevent the breakdown by restriking voltage. Consequently, the final
arc extinction takes place and circuit current is interrupted. Immediately after the final current interruption, the voltage that appears across the contacts has a transient part. However, these transient oscillations cave in rapidly due to the damping effect of the system resistance and normal circuit voltage appears across the contacts. The voltage across the contacts is of normal frequency and is identified as recovery voltage.

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