Paul,
What's on the end of that Submain feed cable?.
Is there a possibility that someone could have driven an Earth stake at the other end?.
I'm just wondering if there isn't some sort of a circulating earth current here, between the Main earthing of the house and the barn.
How many cores does the SWA have?.
Are there any Electric fence systems near the house?.
It wouldn't be the first time a farmer has driven a fence earth through a pipe and not realised. [Linked Image]