What is the difference between Momentary Duty, and Interrupting Duty for fault currents?

rbulsara (Electrical)25 Jun 08 16:48

Momentarty duty is the maximum current (usually Asymmetric peak in first half cycle) it can withstand without interrupting the current.

Interruptinng duty is the current it can safely interrupt. Interrution may occur several cycles after the fault to allow for downstream device to open. Thus innterrutping current's rms value would typcially be less than momentary duty.

For more look up IEEE Red book and Buff Book.

rbulsara (Electrical)25 Jun 08 17:06

I believe momentary duty also indicates ability to close in on (make the circuit) a faulty circuit without getting damaged. This is besides ability to withstant without interrupting.

There also some differences as to how fault currents and duty's are caculated between ANSI and IEC standards. But that is details!.

Gianoli (Electrical)25 Jun 08 23:54

Momentary Duty:

Is the Maximum RMS asymmetrical current that the circuit breaker will withstand including short-circuits from all sources and motors and the DC component. This rating is independent of the operating voltage. It defines the ability of the circuit breaker to close against and withstand the mechanical stresses produced by the very large offset first cycle of the short-circuit current.

Interrupting Duty:

This is the maximum total RMS amperes that the circuit breaker will interrupt regardless of how low the voltage is.