I cs is the maximum perspective fault current which the circuit breaker can clear and still remain serviceable. Tests are carried out at the specified fault current. Time interval (short as possible, but minimum of 3 minutes) Making operation followed by breaking operation The breaker is then subject to both dielectric withstand and temperature rise tests. I cs is subject to an O-t-CO-t-CO sequence. The breaker is then certified safe by a simple dielectric test. When tested against the standard, circuit breakers under go the following tests: This point is particularly important in circuit breakers when the I cs is lower than the I cu.
After clearing a fault the circuit breaker does not have to remain serviceable and could be dangerous to operate. If these conditions change then it may be necessary to derate the circuit breaker. This is verified by testing in accordance with the standard and is applicable at a specific set of electrical and environmental conditions. I cu is really the maximum perspective fault which a circuit breaker can clear (with the fault current being expressed as rms for ac). While the definitions are pretty straight forward, perhaps a little discussion is worthwhile. I cm - short-circuit breaking (or making) capacityĪ breaking (or making) capacity for which the prescribed conditions include a short circuit What does it all mean? The rated short-time withstand current of a circuit-breaker is the value of short-time withstand I cw - rated short-time withstand current I cs - service short-circuit breaking capacityĪ breaking capacity for which the prescribed conditions according to a specified test sequence include the capability of the circuit-breaker to carry its rated current continuously I cu - ultimate short-circuit breaking capacityĪ breaking capacity for which the prescribed conditions according to a specified test sequence do not include the capability of the circuit-breaker to carry its rated current continuously On this, it's probably easiest to start of with the IEC definition of the relevant ratings:
Just to be clear, we are only talking about fault ratings (not rated current). Hopefully we should be able to clear some of the misunderstanding. While the IEC low voltage circuit breaker Standard has been around for many years now, it's surprising how often fault ratings are misunderstood. NT MasterPact Circuit BreakerI think this post is going to be helpful to several of our readers.
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