Interesting - I've never heard of such a thing! Where does the DNO 'earth' conductor come from - is there such a conductor run overhead all the way back to the DNO's tranny or is it perhaps derived from rod(s) at local pole(s)?Is that overhead TN-S? ... Staffordshire was the first place I ever saw overhead TN-S and on the same day, subterranean TT supplies.
That was probably true in the past, but things have changed. Many of the rural overhead supplies, like that in my village, which were traditionally TT have now been PMEd, so that they can be used as TN-C-S.When you learn about supply types at college, you are generally told TN are underground and TT overhead. Then you go out into the world and find all sorts of variations!
Kind Regards, John