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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.


Parents
  • 0
    it depends how you twiddle it - unlike a normal MCB with a thermal and a magnetic part, in the modern devices there is a current transformer and some very flexible programmable electronics, so you can make the instant threshold move, or have a variable multiplier. However, as you only use the adjustable because you really do want a funny rating, then it behoves the person making the adjustment to understand the conditions well enough to make an informed choice

    (and maybe to note the settings in a label on the panel and then seal it against tampering.)

    Note that really this is all about faults to earth and shock protection, the fast trip multiplier part is irrelevant for overloads and slow overload is normally what matters for fires, where a 5 second response time is just fine. (hence the preference for either a seperate earth fault relay, or a breaker that includes a programmable RCD function, as then you can separate the cases quite easily.)
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  • 0
    it depends how you twiddle it - unlike a normal MCB with a thermal and a magnetic part, in the modern devices there is a current transformer and some very flexible programmable electronics, so you can make the instant threshold move, or have a variable multiplier. However, as you only use the adjustable because you really do want a funny rating, then it behoves the person making the adjustment to understand the conditions well enough to make an informed choice

    (and maybe to note the settings in a label on the panel and then seal it against tampering.)

    Note that really this is all about faults to earth and shock protection, the fast trip multiplier part is irrelevant for overloads and slow overload is normally what matters for fires, where a 5 second response time is just fine. (hence the preference for either a seperate earth fault relay, or a breaker that includes a programmable RCD function, as then you can separate the cases quite easily.)
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