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Short circuit fault current calculations

Simon Allen
Hi,


When using an adjustable circuit breaker e.g. current rating adjustable between 15 and 36Amps. What value do you use for rated current in the prospective short circuit fault current calculation, I.e. do you use the maximum current setting (36A) or the actual current setting used in operation, (e.g. 20A).

In the example given above this can make a large difference to the maximum length of cabling that can be used and still ensure instantaneous tripping of the CB.

Also if the actual current setting is used in the calculation, where does responsibility lie if someone adjusts the setting to the maximum, which could mean that the prospective short circuit fault current is no longer enough to guarantee instantaneous tripping, which could in turn lead to a fire risk? Is that foreseeable misuse?


any help or suggestions appreciated.


thanks.


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  • 0
    I can't remember any details but I do remember hearing that identical devices can have very different fault ratings in different countries because some countries define the rating as the fault current that they can break without suffering any change to their ability to operate correctly next time and other countries define the rating as the current that they can successfully interrupt, even if they destroy themselves in the process.

    It should only be an issue where the fault level is near the limit and the fault is near the device which is, hopefully, fairly rare but it is a concern as there really is no way of knowing other than the experience of whoever repairs the fault.

    The only way round this would be some sort of indicator , or cut out, built into the MCCB or MCB which is triggered by the fault current. In Power Stations, where the fault levels are often very high, they often have fuses upstream of MCBs for this reason.
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  • 0
    I can't remember any details but I do remember hearing that identical devices can have very different fault ratings in different countries because some countries define the rating as the fault current that they can break without suffering any change to their ability to operate correctly next time and other countries define the rating as the current that they can successfully interrupt, even if they destroy themselves in the process.

    It should only be an issue where the fault level is near the limit and the fault is near the device which is, hopefully, fairly rare but it is a concern as there really is no way of knowing other than the experience of whoever repairs the fault.

    The only way round this would be some sort of indicator , or cut out, built into the MCCB or MCB which is triggered by the fault current. In Power Stations, where the fault levels are often very high, they often have fuses upstream of MCBs for this reason.
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