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MCCBs - Icw less than Ics - Why?

Nick Parker
Why the MCCBs (with Icw rating) do not have the values same as the Ics value? What is the use of providing the very low Icw value? why does IEC permits the use of Icw less than Ics by manufacturers?


The XH630NE Terasaki mccb has an Icu value = 65kA (rated ultimate short-circuit current) and Ics = 50kA (rated service short-circuit current). However, the rated short-circuit withstand current (Icw) is only 10kA for 0.3s. This tells me it is only good for a threephase throughfault of 10kA for 0.3s. Why so low? Normally the given Ics and calculated Ith values are within 15% of each other.
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    mapj1,  Full marks for your explanation which I agree with 100%, but I think that Nick is calling it low in relation to the potential 65kA fault current. I have had a look at the standard and it has changed from the old, more useful, test for withstand current which used to be at around Ics or Icu for one second to a test for four cycles with at least twelve times the rated current.  As we can have the instant trip held off for twelve times the rated current we can have about 7.5kA for around 8 seconds, an I^2t value of 450 MA^2s, compared to the Icw value of around 20 MA^2s which is around the same value as the instantaneous trip let through energy.

    Nick,  I have always found the guys at Terasaki in Clydebank very helpful and it may be worth having a word with them about the dilemma, though my contact there retired a few years ago so I can't give a current contact name.
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    mapj1,  Full marks for your explanation which I agree with 100%, but I think that Nick is calling it low in relation to the potential 65kA fault current. I have had a look at the standard and it has changed from the old, more useful, test for withstand current which used to be at around Ics or Icu for one second to a test for four cycles with at least twelve times the rated current.  As we can have the instant trip held off for twelve times the rated current we can have about 7.5kA for around 8 seconds, an I^2t value of 450 MA^2s, compared to the Icw value of around 20 MA^2s which is around the same value as the instantaneous trip let through energy.

    Nick,  I have always found the guys at Terasaki in Clydebank very helpful and it may be worth having a word with them about the dilemma, though my contact there retired a few years ago so I can't give a current contact name.
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