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End of line fault

Nick Parker
Why calculate end of line fault current? 

Is it for verifying that thermal stress could be withstood by the cable before the breaker gets tripped?

Is it to verify that breaker to be tripped in a certain time (1s or 5s) for the fault current magnitude at the end of line?


I have a colleague who says, only phase to phase faults to be calculated at the end of line.  I ask why not the phase to earth or Phase to neutral for which his reply is, Generic MCCB or MCBs would not work for earth faults.


The Generic MCCBs would work for earth fault provided the fault magnitude is greater than the magnetic trip threshold (Im). Now what if the Phase to earth fault's magnitude is lesser than the Magnetic Pick up. Should RCD be provided?


I think it is the job of the designer to find the minimum fault current at the end of the line (whether it is P-P or P-N or P-E) and select the breaker which guarantees trip the minimum fault current
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    Yes your colleague is mistaken, in the general case there may be no RCD, so the MCB/fuse is handling all cases, L-N L-E and L1-L2. in such a case the far end Zs is important, as it sets the breaking time  - in an extreme case if the cable is long and thin enough, nothing may trip at all, you just get some very hot cable....


    If, and only if, there is an RCD or other earth fault detection, then you may argue that the MCB is only handling the faults of the L-N and L1-L2 kind. It is not the general case, and always with one protective mechanism or another, all credible cases should be considered.

    Mike.
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  • 0
    Yes your colleague is mistaken, in the general case there may be no RCD, so the MCB/fuse is handling all cases, L-N L-E and L1-L2. in such a case the far end Zs is important, as it sets the breaking time  - in an extreme case if the cable is long and thin enough, nothing may trip at all, you just get some very hot cable....


    If, and only if, there is an RCD or other earth fault detection, then you may argue that the MCB is only handling the faults of the L-N and L1-L2 kind. It is not the general case, and always with one protective mechanism or another, all credible cases should be considered.

    Mike.
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