[Power supplies with earth return] Another difference! It is very unusual in the UK to give a domestic customer more than 100A in the main incoming fuse. 400A on each side of your 115-0-115 supply is getting on for 100kW. Do you really use that? I would have thought 10kW would be more likely (my parents use 36kW at the winter peak, mostly heating load. 50A from each of three phases at 240V) Anyway, 400A on the 240V side corresponds to nearly 7A to be returned through the earth at 14kV. Ouch! I wonder what the resistance of the earth connection is. There will be a minimum resistance to prevent undue voltage rise under short circuit conditions, but I don't know how this relates to earth return circuits... The only places where I've seen this sort of arrangement, there are still 2 conductors. One insulated to HV for the line, and one lightly insulated for the (HV) neutral. ISTR that in the UK there is a minimum distance between LV and HV earth electrodes. Earth open circuit? Hmm. The entire HV winding rises to 13kV and the HV neutral flashes over to the transformer tank? Assuming the tank is part of the LV earth system. Otherwise the transformer tank rises too, and flashes over to your house supply... Philip.