Circuit protection for distribution circuit

Can 3 individual mcbs(instead of a 3 or 4 pole mcb) be used for circuit protection of a 3 phase distribution circuit, where a 4 or 5 core swa is used? Is there a requirement that all phases must be disconnected in the event of a fault?

  • So long as by the time the distribution circuit splits up in one place to supply the individual single phase loads , and from then on each of them has a separate neutral, you should be OK - so for example a 3 phase fed to one 3 phase board with lots of single phase outbound circuits should be   fine - as the board  itslef is a 3 phase item, and might just as well be fed by 3 fuses. However, if the load and of the cable somehow splits to supply 3 separate singe phase boards, and did so in such a way that the outbound neutrals are shared, then that is not right.

    It gets more complex with things like 3 phases directly supplying a bank of light switches, where really that is the final circuit, and you should be careful to avoid the shared neutral , as an example then, someone may well pull the fuse to kill the lights on one room and then open the neutral for a different zone and get a belt off it from the other phase still energised.

    But the question was not originally about neutrals - yes it needs watching, but the ganged MCB at the origin is not the thing that makes a circuit a 3 phase circuit  one or 3 separated single phase ones - it is rather what is at the far end.


  • and might just as well be fed by 3 fuses

    I am not sure that I agree. It is much easier to open an MCB (just like a light switch) than to remove a fuse. The arrangement would provide fault protection and overload protection perfectly well, but that is not quite the point.

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  • but on a distribution circuit quite often we do not really want folk casually flicking things  off - if they want to work on the final circuits then they ought to be turning off at the last DB - in this example the one where things split out from 3 phases to lots of singles but they could just as easily be a mix of 1p and 3P loads. 

    Also given how badly MCBs co-ordinate in the face of faults  of varying currents,  there is a lot to be said for fuses further back  and MCBs nearer the load - and with high PSSC they can act as an energy limiter, reducing the size and expense of the downstream switchgear.
    I agree that at a pop festival or similar plug and play you will have MCBs and ganged ones, all the way back to the genset, but in a factory setting that freedom may be more of a hindrance than a help.

    Not saying always, just that it can be . And, in cases where we accept 3 fuses, and the failure of one is not a big deal, then we ought to be OK with non-ganged breakers, though that would be more unusual.