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Calculating Earth Fault Loop Impedance in three phase + Neutral circuits

Nick Parker

I would like to calculate the Zs (external impedance). Would it be fine just calculating the impedance of "L1" and "N" in series from the secondary of the transformer till the final load and completing the loop.

Just concerned the "Ze" zone being the three phase circuit and have a thinking that just adding the impedance would yield inaccurate "Zs".

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    Your suggestion is the right way to think about it but the loop is L-E. Note that if you had said 'maximum fault current' for breaker ratings or something, it may not have been - a phase to phase fault is normally more zappy than a phase to neutral or phase to earth one. But to earth it is always one phase, adding more phases does not increase the fault current in the earth path, indeed shorting all three phases leaves almost no amps at all to go down the earth path ;- )

    Usually the 3 phase part is high current and chunky and the 1 phase part  is thin and so the latter provides most of the ZS, but not always.
    Mike.

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    Your suggestion is the right way to think about it but the loop is L-E. Note that if you had said 'maximum fault current' for breaker ratings or something, it may not have been - a phase to phase fault is normally more zappy than a phase to neutral or phase to earth one. But to earth it is always one phase, adding more phases does not increase the fault current in the earth path, indeed shorting all three phases leaves almost no amps at all to go down the earth path ;- )

    Usually the 3 phase part is high current and chunky and the 1 phase part  is thin and so the latter provides most of the ZS, but not always.
    Mike.

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