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Short circuit fault current calculations

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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  • Former Community Member
    0 Former Community Member
    Yes there is a risk of someone meddling, but in general we have that risk anyway - cartridge fuses may be replaced by one of a higher rating (not all values are physically different sizes), an incorrect size of fuse wire might be selected for a rewireable fuse - even an MCB might be replaced by one of a different rating or type or even manufacturer.


    Ultimately there would be little point in manufacturing adjustable devices if lower settings couldn't be relied upon - you might as well just install a fixed device of the maximum rating.


       - Andy.


    There are many other reasons for changing the previously calculated and tested SC-currents. For example, it can be:
    -replacing a transformer at a substation (although this often goes in the direction of increasing power),
    -changing the distribution network configuration during repairs and operational switching (various temporary schemes can be in operation for a sufficiently long time), etc.
    There are always risks.


Reply
  • Former Community Member
    0 Former Community Member
    Yes there is a risk of someone meddling, but in general we have that risk anyway - cartridge fuses may be replaced by one of a higher rating (not all values are physically different sizes), an incorrect size of fuse wire might be selected for a rewireable fuse - even an MCB might be replaced by one of a different rating or type or even manufacturer.


    Ultimately there would be little point in manufacturing adjustable devices if lower settings couldn't be relied upon - you might as well just install a fixed device of the maximum rating.


       - Andy.


    There are many other reasons for changing the previously calculated and tested SC-currents. For example, it can be:
    -replacing a transformer at a substation (although this often goes in the direction of increasing power),
    -changing the distribution network configuration during repairs and operational switching (various temporary schemes can be in operation for a sufficiently long time), etc.
    There are always risks.


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