breaking capacity of the breaker is defined as
symmetrical breaking current in kA
The breaking duty root-mean-square duty is usually specified to take place after a number of cycles of fault current have passed.
This is usually taken to be the time given by the manufacturer for the circuit breaker to open and clear the fault. This is typically 5 to 8 cycles of the fundamental current.
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If you are in an IEC country, the breaking capacities are given as :
1. Ultimate breaking capacity Icu
2. Service breaking capacity Ics. Usually expressed as a percentage of Icu.
The test circuits, parameters etc are given in IEC 60947 parts 1 and 2.
ACBs are meant to stay closed to give time for the downstream MCCBs to clear the fault usually. in a properly designed distribution scheme with discrimination or cascading.
MCCBs are usually current-limiting and clear the fault well before the prospective peak is reached. Less than half-a-cycle clearing time.
There are a number of discussions in CR4 on this subject, you can use the search window on the right.
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Circuit-Breakers
Re: circuit-breakers
