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How big does a 3 phase in-balance need to be to mask an open pen fault

Alan B

This is looking at the previous project I was  discussing with two EVCP's on two phases of a 3 phase supply.
Just realised that I may have issues relying on single phase voltage based open pen detection when connecting to a 3 phase supply.


AI says Regulation 722.411.4.1(iv) of BS 7671:2018+A1:2020 prohibits the use of a protection device for open-PEN (Protective Earth and Neutral) detection in single-phase EV charging points supplied by a three-phase installation. This is because this specific type of open-PEN protection is designed for single-phase installations only, and a three-phase supply has different electrical characteristics that are not compatible with this protective measure. 

Looking at amendment 2 I can't see this wording, am I just missing it, or has it been removed?


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    The clue is, "artificial", whose intelligence has got it wrong.

    Aren't all SP circuits (or indeed installations) derived from TP ones?

    My understanding is that TP EVCPs (please forgive the alphabet soup) are inherently safer than SP ones. Thus it is not that open-PEN devices are incompatible with TP installations, but that they are unnecessary.

  • 0

    AI's confused (again).

    There used to be a limitation on using L-N voltage monitoring for open-PEN detection where 3-phase is available, but that's long since gone (and the current version of BS 7671 is 2022, not 2020).

    There was some logic to the limitation - the L-N voltage approach isn't exactly 100% - because of the phase differences you can, in some circumstances, have PEN-true Earth voltages of well over 100V and still not have the open-PEN device notice and open - which is a bit of a flaw.  If 3-phase is available you would, in principle, be better measuring PE against an "artificial N" point derived from the 3 lines instead - so the old limitation tried to encourage better approaches where available. But if we accept the ("flawed") method for single phase installations (derrived from 3-phase LV public distribution), it's hard to then say the same risk is unacceptable just because the installation has a 3-phase supply (which might be a very inconvenient distance from the EV point). Many modern designs incorporate additional methods of protection - e.g. tripping on c.p.c. currents over 10mA to reduce the risk these days too.

       - Andy.

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