Seems to me the obvious solution is to use 3-core & earth lighting cable and use the third core as the linking signal. Is this the way it's normally done?
Although I have seen quite a few SD linked via TE jobs with the earth being used. When I mentioned it to the guy his logic was that the Earthing point on the SD was a dead stop, so the unit didn't need an earth.
I gave up, wasn't my place to offer up good practice and CPC sense
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