Calculating current in a supplementary earth conductor.

When cable armour is insufficient to carry expected earth fault current for the necessary period of time, a supplementary earth cable can be provided.

The size and location of this cable needs to be chosen so that the desired proportion of earth fault current flows in it.
The sharing of current depends on the impedances of the cable armour and the earth cable and the mutual impedance between them. Having determined these the calculation of the current sharing is straightforward though tedious.
A worksheet is attached to this paper and can be downloaded. The principal parts of the theory are described here.


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