Selectivity categories and rated short-time withstand current (Icw)

IEC 60947-2 defines two types of circuit breaker defined by their “selectivity category:

• Selectivity category B comprises circuit breakers providing selectivity by having a short-time withstand current rating and an associated short-time delay. With an intentional short-time delay (which may be adjustable) provided for selectivity under short-circuit conditions. Such circuit-breakers have a short-time withstand current rating (Icw), that can be withstood for a specified time. This is generally applied to power open type or “Air” circuit breakers and to certain heavy-duty molded-case types. Icw is the maximum current that the B category circuit breaker can withstand, thermally and electrodynamically, without sustaining damage, for a period of time given by the manufacturer.

Selectivity category A comprises all other circuit breakers. This category of circuit breaker has no deliberate delay in the operation of the “instantaneous” short-circuit magnetic tripping device. Usually molded-case type circuit breakers or modular circuit breaker are category A These circuit breakers may provide selectivity under short-circuit conditions by other means. But manufacturer will not provide Icw value.

Category A : Without an intentional short-time delay for selectivity under short-circuit condition and therefore without a short-time withstand current rating.        

References:

1. IEC 60947-1 Low-voltage switchgear and controlgear – Part 1: General rules
2. IEC 60947-2 Low-voltage switchgear and controlgear – Part 2: Circuit-breakers
3. Electrical Installation Guide (Schneider Electric)