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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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  • 0

    Speak to the Open-PEN protection manufacturers and the EVSE manufacturers, the measurement methods vary between single and three-phase Open-PEN equipment and a reference earth rod may be required to take the measurements in a three-phase installation. 

    You will probably need a three-phase Open-PEN protection upfront of the single phase EVSE, because it is a three-phase installation, allowing the use of single-phase EVSE without built in Open-PEN protection. 

    You need to take guidance from equipment manufacturers, not AI.

  • 0 in reply to Sparkingchip

    and a reference earth rod may be required to take the measurements in a three-phase installation. 

    The reference electrode approach (iii) can be used with either 3-phase or single phase - and in theory is the ideal approach if the electrode accurately represents the ground surface potential around the vehicle. The trouble is making sure that the electrode isn't unduly influenced by other voltages (e.g. buried PME cables or bonded metallic pipework) and not hitting any services when banging the thing in. If you do go to the effort of installing a good electrode, then in many (although not all) cases you could just TT the EV using it, and not bother with the expensive open-PEN device at all.

    The difference with 3-phase is that it gives the option of inferring a 0V reference by averaging together the three line voltages (like an artificial N point) - it's like having a perfectly balanced star connected load - the star point naturally sits at 0V even without needing a N conductor. As long as the three phases are all present and correct (e.g. all have 400-ish volts between them) the implied N point should be reliable reflection of the substation's electrode, even during open PEN events - so that can make a good reference for the open-PEN device.

    Single phase open-PEN devices without an electrode can only monitor L-N (or L-PE) voltages ... which has its flaws ...  Open PEN detection for 722.411.4.1 (iv) 

       - Andy.

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  • 0 in reply to Sparkingchip

    and a reference earth rod may be required to take the measurements in a three-phase installation. 

    The reference electrode approach (iii) can be used with either 3-phase or single phase - and in theory is the ideal approach if the electrode accurately represents the ground surface potential around the vehicle. The trouble is making sure that the electrode isn't unduly influenced by other voltages (e.g. buried PME cables or bonded metallic pipework) and not hitting any services when banging the thing in. If you do go to the effort of installing a good electrode, then in many (although not all) cases you could just TT the EV using it, and not bother with the expensive open-PEN device at all.

    The difference with 3-phase is that it gives the option of inferring a 0V reference by averaging together the three line voltages (like an artificial N point) - it's like having a perfectly balanced star connected load - the star point naturally sits at 0V even without needing a N conductor. As long as the three phases are all present and correct (e.g. all have 400-ish volts between them) the implied N point should be reliable reflection of the substation's electrode, even during open PEN events - so that can make a good reference for the open-PEN device.

    Single phase open-PEN devices without an electrode can only monitor L-N (or L-PE) voltages ... which has its flaws ...  Open PEN detection for 722.411.4.1 (iv) 

       - Andy.

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