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Open PEN detection for 722.411.4.1 (iv)

Following on from the recent discussion about the adequacy of open-PEN detection devices that disconnect when the L-N voltage goes outside of the 207-253V range and how, when the single phase installation is fed from a 3-phase supply system, some open-PEN situations might go unnoticed even though the PEN/PE voltage can be significantly higher than 70V from true Earth.

Can people check that I'm understanding this correctly please?

As I see it, once the PEN becomes disconnected, it's free to float from Earth and can therefore be dragged towards the various L1/L2/L3 voltages depending on any impedances left between the severed N and the line conductors (usually any connected loads).

So thinking about it in phasor diagram terms, you have the usual triangle formed by L1/L2/L3 and N can then be dragged to any position within that triangle.

The detection device is looking at L-N voltages - or to put it another way, it's measuring the voltage on the N by using L as a reference. It's only seeing one phase, so can't work out anything about phase angles so it measures the magnitude of the voltage difference only - the phase angle of the measured L-N voltage could be anything. So I'm thinking that any given voltage different like that would show up on a phasor diagram as an arc (or circle) centred on the L position (L1 say) - the radius represent the magnitude of the voltage difference from L and the angle could be anything (within limits) - hence an arc rather than a single point.

So drawing arcs for the 207V and 253V from L, I get a region in between where the open-PEN device will think all is well.

So I've got this:

so the coloured regions are were the device thinks all is well - green where the N/PE voltage is actually ≤50V from true Earth, yellow between 50V and 70V (usually deemed acceptable for EV situations) and red where the device thinks all is well but the N/PE voltage is actually in excess of 70V from true Earth (anything up to 128V in this particular example). For the remaining unshaded areas of the triangle, the device will see L-N out of range and disconnect. Does that all make sense?

I've had to make an assumption about the actual line voltage present - since the N/PEN voltage is measured against that - I picked 240V as being typical for a UK installation, but of course it could vary considerably - which I suspect will mean that there could be an even wider range of PEN voltages that under particular circumstances the device would regard as being acceptable.

   - Andy.

Parents
  • Thank you to Andy for such a thoughtful posting.

    Of course, the question with any "voltage" is really the "potential difference", which begs the question, "between what and what?" I remain to be convinced by these open PEN devices. I shall stick with 3 phases 'cos at least you can compare the actual and theoretical values.

  • I shall stick with 3 phases 'cos at least you can compare the actual and theoretical values.

    Until you have a prosumer's installation that generates on all phases when it switches to island mode (but remains connected to PME earthing arrangement) ...

    Of course, the question with any "voltage" is really the "potential difference", which begs the question, "between what and what?"

    Between Line and neutral at the charging point in the case of 722.411.4.1 (iv)

    Between PE and Earth at the charging point according to 722.411.4.1 (iii), although neutral to Earth would also achieve the same in a TN-C-S system.

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  • I shall stick with 3 phases 'cos at least you can compare the actual and theoretical values.

    Until you have a prosumer's installation that generates on all phases when it switches to island mode (but remains connected to PME earthing arrangement) ...

    Of course, the question with any "voltage" is really the "potential difference", which begs the question, "between what and what?"

    Between Line and neutral at the charging point in the case of 722.411.4.1 (iv)

    Between PE and Earth at the charging point according to 722.411.4.1 (iii), although neutral to Earth would also achieve the same in a TN-C-S system.

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