You don't calculate it, you read it off the nameplate. The relevant IEEE or IEC standard may then provide guidance for adjusting that rating (downward) when the prospective fault current X/R exceeds the test value.
I’ll see your silver lining and raise you two black clouds. - Protection Operations
Mehak kar (Mechanical)(OP)2 Dec 23 18:55 " Kindly through some light on how to calculate the breaking current capacity of a MCCB?"
The learned advice by Mr. davidbeach is short and sharp. A great post.
I would like to add the following for clarity, for your information.
1. In the IEC world, a) rated ultimate short-circuit breaking capacity (I[sub]cu[/sub],..is the maximum short-circuit current value which the CB can break twice (O-t-CO) at the corresponding rated operational voltage. After the opening and closing sequence, the CB is NOT required to carry the rated current.
b) Rated service short-circuit breaking capacity (I[sub]cs[/sub]), ..._is the maximum short-circuit current value which the CB can break three times in accordance with a sequence of opening and closing operation (O-t-CO-t-CO) at a defined voltage of (I[sub]e[/sub]) and at a defined power factor. After the sequence, the CB is required to carry its rated current.
2. Note: a) Testing procedure and judgment differ between IEC (in Europe) and IEEE/NEMA (in US). The same CB rated X kA per IEC is very likely to be rated Y kA per IEEE/NEMA.
b) This result to test certification or EC mark in Europe is NOT automatically as being listed/recognized by UL in the US.
3. IEC in (Europe) and IEEE/NEMA (in US), each publishes/issues their own "standard" without any co-ordination or harmonisation.
4. The IEC and IEEE/NEMA do NOT carryout any testing or policing that their "standard" is correctly implemented/interpreted.
Che Kuan Yau (Singapore)
