Rather specialised. Much depends on the fence. In the Falklands we used New Zealand fences which could run for 5 miles. In UK that would be very rare. Unlike the earth for normal uses if the earth fails it fails safe. So even 1mm cable is OK if it works. Even with 5 miles of twin wire fence the earth was only a 2 meter rod with a short length of 2.5mm cable. If it does not work then you add extra. However I will admit I never got hold of the fence to test it! Seem to remember there was a tester which came with fence to see if all was well or not.
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