15-10-2015, 03:25 PM
That document predates current wiring regs but still looks pretty much OK. One point I overlooked is that if you export a PME earth to an outbuilding with extraneous metal parts (water pipes etc) then they must be bonded back to the main earthing terminal with a 10mm^2 cable. The cable armour doesn't do this, nor does the 4mm core used as earth. If you export a TNS earth to an outbuilding I'm not sure how extraneous metal parts should be bonded. May still need to be 10mm but could be less.
All in all, if there are water pipes etc I'd almost certainly TT an outbuilding. If it's a caravan or boat then you MUST TT it. If it's on a farm there are additional requirements which I haven't read up. One particular problem on farms is that livestock are mor vulnerable than humans. Partly because they don't wear wellies (no rude jokes please) but mainly because their feet are further apart. So a voltage gradient in the earth due to a fault that would be harmless to a biped could be harmful to a quadruped. I think this means that PME is rarely used on farms, or if used it's only for the farmhouse. Everything else would be TT. You don't get much TNS in rural areas.
All in all, if there are water pipes etc I'd almost certainly TT an outbuilding. If it's a caravan or boat then you MUST TT it. If it's on a farm there are additional requirements which I haven't read up. One particular problem on farms is that livestock are mor vulnerable than humans. Partly because they don't wear wellies (no rude jokes please) but mainly because their feet are further apart. So a voltage gradient in the earth due to a fault that would be harmless to a biped could be harmful to a quadruped. I think this means that PME is rarely used on farms, or if used it's only for the farmhouse. Everything else would be TT. You don't get much TNS in rural areas.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
