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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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    Nick Parker:
    AJJewsbury:

    That's generally only true for TT systems - in TN it's usual for overcurrent devices (MCBs, MCCBs, fuses) to provide earth fault protection too.

       - Andy.


    Do you meant to say that in TN systems, the earth fault current is sufficient to operate the generic breakers?




    It certainly can be, especially in TN-C-S situations. Just draw the circuit and see the earth fault loop path. In B.S. 7671 the earth fault current is defined as resulting from a fault of negligible impedance. The cables to and from the earth fault point will in practice have an impedance of course.



    411.4 onwards.

    Z.


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  • 0
    Nick Parker:
    AJJewsbury:

    That's generally only true for TT systems - in TN it's usual for overcurrent devices (MCBs, MCCBs, fuses) to provide earth fault protection too.

       - Andy.


    Do you meant to say that in TN systems, the earth fault current is sufficient to operate the generic breakers?




    It certainly can be, especially in TN-C-S situations. Just draw the circuit and see the earth fault loop path. In B.S. 7671 the earth fault current is defined as resulting from a fault of negligible impedance. The cables to and from the earth fault point will in practice have an impedance of course.



    411.4 onwards.

    Z.


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