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Electrical Engineering

What is Service short circuit breaking capacity?


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June 01, 2012 11:37AM

Icu is really the maximum perspective fault which a circuit breaker can clear (with the fault current being expressed as rms for ac). This is verified by testing in accordance with the standard and is applicable at a specific set of electrical and environmental conditions. If these conditions change then it may be necessary to derate the circuit breaker. After clearing a fault the circuit breaker does not have to remain serviceable and could be dangerous to operate. This point is particularly important in circuit breakers when the Ics is lower than the Icu.

Ics is the maximum perspective fault current which the circuit breaker can clear and still remain serviceable. The standard does allow some minor welding of the contacts to take place, so after a large fault it would still be necessary to inspect the breaker. When specified as a percentage of Ics, the standard proposes ranges of 25%, 50%, 75% and 100%.

Icw is the perspective fault withstand rating (rms for ac). Circuit breakers may be subject to through fault which they are not intended to clear. While not clearing these faults, the breaker will still need to withstand the thermal and mechanical stress imposed by the fault current. The longer a fault is present the more the effects build up and Icw always has a time element associated with it (i.e. 50 kA for 1 second). The standard specified preferred time ranges of 0.05, 0.1, 0.25, 0.5 and 1 second (although 3 seconds is also often used in practice).

Icm is the peak current which the circuit breaker can safely break or make. It is expressed as the maximum perspective peak current at a rated voltage, frequency and power factor and is always greater than Icu. From a safety aspect this is particularly important as it will be the primary mechanism to protect the operator if the circuit breaker is closed on to a fault.

All ratings are derived under specific electrical and environmental conditions and are verified with the circuit breaker in free air. As soon as the breaker is enclosed in in any kind of panel or cabinet the ratings change and need to be re-assessed as part of the assembly testing.