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.

Er, for starters - your Earth fault loop should consist of the L plus c.p.c. impedances, not N.

If it's a 'large' system (e.g. conductors over 25mm²) for accuracy, you might need to add the resistive and reactive components separately rather than just adding the overall impedances.

    - Andy.

AJJewsbury said:

If it's a 'large' system (e.g. conductors over 25mm²) for accuracy, you might need to add the resistive and reactive components separately rather than just adding the overall impedances.

If armoured cable is used, even relatively small csa, the armour csa is usually > 16 sq mm, and therefore the relevant method in PD CLC/TR 50480 should be used due to the reactance of the armour, or the armour/external cpc combination. Note that often - but especially if the cable is buried - the armour (perhaps in parallel with a copper conductor) has to be capable of acting as a cpc, whether you use it downstream or not, because a fault to it is possible, and it's often connected to exposed-conductive-parts.

The 5th Edition (2022) of the IET Electrical Installation Design Guide, and the 9th Edition of Guidance Note 6 Protection Against Overcurrent, discuss the relevant calculation methods according to PD CLC/TR 50480.

Just doing these on a 2396 class today. The two currents are not in phase so do not add arithmetically