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Details of BS7671:2018 Amendment 1 are here.

Alan Capon
Details of Amendment 1 of BS7671:2018 is available here: https://electrical.theiet.org/bs-7671/updates/


Regards,


Alan.
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    AJJewsbury:




    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.

     




    Interesting discussion.


    The 70 V rms is derived, as discussed in earlier post, from IEC TR 60479-5, considering two hands to feet in wet conditions.


    In terms of the effects of TT systems, we don't need to consider simultaneous contact with exposed-conductive-parts of other earthing systems because:

    (a) The TT should have adequate separation below ground from the PME earthing system, and


    (b) Regulation 411.3.1.1 requires simultaneously accessible exposed-conductive-parts to be connnected to the same earthing system.


    In any event, disconnection times in TN systems can be up to 5 s to comply with Chapter 41 (Reg 411.3.2.3), and the touch voltage there may well be over 70 V! Consider further that this could be the disconnection time required at larger charging equipment. Only the charging point (i.e. vehicle connector or socket-outlet) requires RCD protection unless it's required for other reasons in the General Rules (Parts 4 & 5) ! So, is 70 V really a concern?

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  • 0

    AJJewsbury:




    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.

     




    Interesting discussion.


    The 70 V rms is derived, as discussed in earlier post, from IEC TR 60479-5, considering two hands to feet in wet conditions.


    In terms of the effects of TT systems, we don't need to consider simultaneous contact with exposed-conductive-parts of other earthing systems because:

    (a) The TT should have adequate separation below ground from the PME earthing system, and


    (b) Regulation 411.3.1.1 requires simultaneously accessible exposed-conductive-parts to be connnected to the same earthing system.


    In any event, disconnection times in TN systems can be up to 5 s to comply with Chapter 41 (Reg 411.3.2.3), and the touch voltage there may well be over 70 V! Consider further that this could be the disconnection time required at larger charging equipment. Only the charging point (i.e. vehicle connector or socket-outlet) requires RCD protection unless it's required for other reasons in the General Rules (Parts 4 & 5) ! So, is 70 V really a concern?

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