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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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    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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  • 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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