What is meant by Symmetrical and Asymmetrical breaking currents?

issue:
Customer wants to know the value of the symmetrical and asymmetrical breaking current of the circuit breaker  and what do they mean.

Environment:
Circuit Breaker

Resolution:
- Symmetrical breaking current is the RMS value of AC component of short circuit current at the instant of separation of the contacts. It does not take into account the DC offset.
- Asymmetrical breaking current is is the RMS value of the combined sum of the DC and AC components of the short circuit current at the instant of separation of the breaker contacts. This DC offset is of decaying nature which is dictated by the time constant of the system.



As shown in the above picture the DC offset decays in a few cycles and is left only with the symmetrical component. Hence if a breaker has to close before the DC component decays, then the breaker’s breaking capacity is to be calculated for the asymmetrical current at the time of breaking.

Released for:Schneider Electric South Africa