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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.


  • 0
    This raises an issue that I have always thought is dangerous, that of relying on an instantaneous trip to clear faults within the defined time. 

    When you rely on a thermal trip, then a small error in the calculation of the fault current will result in a small change in time to trip, but, really, is anyone bothered if it is 5 seconds or 5.1 seconds? 

    With an instantaneous trip, a small error can make the difference of trip or no trip, or at least no trip for a very long time. That really is a safety issue.
  • 0
    I agree, and further to that, in some situations there is the possibility of faults of non zero impedance in some cases, involving people or leaking fluids as part of the fault current path, so there is a lot to be said for earth current monitoring in such cases, even while not set as sensitive as a an safety of life RCD, there is still an advantage to a prompt disconnetion for earth faults that may be precursors to a  dangerous situation.
  • 0
    Another problem being that some manufacturers recommend their devices are rebuilt after a given number of short circuit conditions. The strain to the devices must be such that they can't guarantee continued successful activation. The problem being that such events are never recorded and we put our faith in the ability of the device to work as intended with no knowledge at all of the condition of the internal workings. I wonder how many industrial MCCBs would not perform satisfactorily under fault conditions, a reasonably high proportion is my guess. Especially when most of these devices are left on continuously for years in some cases.
  • 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.
  • 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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