PE advice by Napit (hot tubs)

AJJewsbury: 
 On a TT system earth leakage currents (either from filters etc or just damp getting into things) will raise the voltage on the Earthing system (Ohm's Law - leakage current x Ra) - say 50mA and 200 Ohms would produce 10V - and for indefinite lengths of time.  BS 7671 only require we keep it below 50V.

   - Andy.

Does BS 7671 really require we keep the touch voltage below 50 V? I accept that, in general, for “leakage” currents, it is likely to be the case.

But I'm sure BS 7671 doesn't even specifically require supplementary local equipotential bonding to achieve that under all conditions - although it's likely in a number of cases to be achieved, it's not guaranteed. The fault current causing operating of a device, Ia, may well be far below the prospective fault current, especially where RCDs are used … if OCPDs are used then it's the current causing the device to operate in 5 s …

Worth remembering that for ADS we do calculations using worst-case (lowest) prospective fault current (i.e. that to the measured earth electrode resistance), but when we connect extraneous-conductive-parts, particularly if the TT is derived from a PME system, and we share extraneous-conductive-parts with a PME installation, it's entirely possible the prospective fault current and loop impedances could effectively pretty much revert back to PME values … 

So, how's this relevant?

Well, using Reg 415.2.2 if I have a 30 mA RCD (in either a TT or TN system), I can “get away with” bonding resistances in excess of 1667 ohms - perhaps more than average dry body impedance !!!!!

Now, I'm not saying BS 7671 is in any way wrong, or that there's an issue here, but “50 V” isn't the requirement …

AJJewsbury: 
 On a TT system earth leakage currents (either from filters etc or just damp getting into things) will raise the voltage on the Earthing system (Ohm's Law - leakage current x Ra) - say 50mA and 200 Ohms would produce 10V - and for indefinite lengths of time.  BS 7671 only require we keep it below 50V.

   - Andy.

Does BS 7671 really require we keep the touch voltage below 50 V? I accept that, in general, for “leakage” currents, it is likely to be the case.

But I'm sure BS 7671 doesn't even specifically require supplementary local equipotential bonding to achieve that under all conditions - although it's likely in a number of cases to be achieved, it's not guaranteed. The fault current causing operating of a device, Ia, may well be far below the prospective fault current, especially where RCDs are used … if OCPDs are used then it's the current causing the device to operate in 5 s …

Worth remembering that for ADS we do calculations using worst-case (lowest) prospective fault current (i.e. that to the measured earth electrode resistance), but when we connect extraneous-conductive-parts, particularly if the TT is derived from a PME system, and we share extraneous-conductive-parts with a PME installation, it's entirely possible the prospective fault current and loop impedances could effectively pretty much revert back to PME values … 

So, how's this relevant?

Well, using Reg 415.2.2 if I have a 30 mA RCD (in either a TT or TN system), I can “get away with” bonding resistances in excess of 1667 ohms - perhaps more than average dry body impedance !!!!!

Now, I'm not saying BS 7671 is in any way wrong, or that there's an issue here, but “50 V” isn't the requirement …