Details of BS7671:2018 Amendment 1 are here.

I note and concur with the 25V maximum exposed voltage instead of 50 instead of 70

The thinking behind the 25V was that that earth leakage currents can raise a TT earthing system significantly above true Earth potential - so in theory an ordinary TT system could be hovering at anything up to 50V from Earth without the RCD tripping. An adjacent charge point might possibly also be at 50V - but there's no guarantee that the leakage currents are in phase - so you could have 50V x √3 between two adjacent TT systems on different phases - or even 50V x 2 = 100V if they're supplied from different sides of a split-phase supply. Ensuring the RCD trips above 25V was just intended to ensure the touch voltage between any two adjacent point couldn't exceed 50V rather than looking for a lower limit as such.


Of course with a typical 30mA RCD and a 200 Ohms electrode we'd be looking at about 6V per system or 12V max between adjacent points - so in practice it's a lot safer even than that.


  - Andy.

I note and concur with the 25V maximum exposed voltage instead of 50 instead of 70

The thinking behind the 25V was that that earth leakage currents can raise a TT earthing system significantly above true Earth potential - so in theory an ordinary TT system could be hovering at anything up to 50V from Earth without the RCD tripping. An adjacent charge point might possibly also be at 50V - but there's no guarantee that the leakage currents are in phase - so you could have 50V x √3 between two adjacent TT systems on different phases - or even 50V x 2 = 100V if they're supplied from different sides of a split-phase supply. Ensuring the RCD trips above 25V was just intended to ensure the touch voltage between any two adjacent point couldn't exceed 50V rather than looking for a lower limit as such.


Of course with a typical 30mA RCD and a 200 Ohms electrode we'd be looking at about 6V per system or 12V max between adjacent points - so in practice it's a lot safer even than that.


  - Andy.