I think it's a great discussion and clearly quite important as far as the htg guys are concerned.
My gut feeling says it's normal practice and acceptable.
It is a good discussion and as I mentioned earlier I would have argued the toss with you too had I seen this thread earlier
I have always been of the mind set that you shouldn't use the earth in a multicore in case someone breaks into it upstream and taps into the earth
Its common sense to me and so I don't do it.
but if there is nothing prohibiting you from doing that then that's not for me to argue, I still think its good practice not to though for the reason I've just given
I'm just glad that most of the heating controls I do are ELV
Matt