Where metallic fences are continuous for long distances, or the length of a metallic fence is unknown, it is assumed the fence continues to "remote earth" (0 V) and so during an earth fault at the substation will transfer a voltage of magnitude equal to the soil voltage at the fence.  In other words the fence will transfer zero volts, or remote earth, to a point nearest to the faulted earthing system.

If the transferred voltage is lower than the allowable touch voltage limit then touch voltages on the metallic fence due to a fault at the substation are safe and no mitigation is required.  The transferred voltage is the surface potential value at the nearest point of the fence to the earth grid (also the highest).


Where the length of the continuous metallic fence is known, the worst case touch voltage is equal to the difference between the surface voltage at the furthest point on the fence from the earthing system to that of the nearest point.  See Figure below.


References:

[1]  Earthing Design Instruction, Distribution earthing design, Document No: EDI 100 Amendment No: 1, Endeavour Energy.