Indent (i) was about 3-phase installations that had extremely well balanced loads, so that even if the supply PEN conductor became severed, the consumer's end of it would remain at a safe voltage (pretty impossible to guarantee in practice),  so that one's deletion probably isn't relevant.

The other change, the removal of the single phase condition from intent (iv) however, does open the door to using L-N voltage sensing open-PEN devices on installations with 3-phase supplies.

Just be aware that there are disadvantages to the L-N voltage sensing approach to open-PEN detection - specifically that where a 3-phase distribution system is involved it is possible, under certain conditions, for c.p.c to be at a hazardous voltage but the L-N voltage still to be within normal range. So as a designer you might like to consider whether alternatives might be better choices. 3-phase open-PEN detectors that compare the PE or N voltage with an artificial N point created from the 3 lines, don't suffer that particular blindness for instance. Or where tbe EV and EVSE would be well away from anything on the PME system, TT might be an option.

   - Andy.

AJJewsbury said:

it is possible, under certain conditions, for c.p.c to be at a hazardous voltage but the L-N voltage still to be within normal range.

Whilst what Andy said here is true, it's not a statement  that can easily be demonstrated to be low or high probability of serious electric shock in given circumstances, and is certainly not the whole picture.

This seemingly simple statement ignores the fact that rarely in three-phase systems is the phase balance maintained for long periods. In a real broken neutral situation, the line to neutral voltages on all phases constantly change. So if the broken PEN conductor is initially undetected, that situation will not persist continuously. If you have EV charging loads with the single-phase voltage monitoring open-PEN devices on all 3 phases, it is highly improbable that devices on all 3 phases will be "blinded" at the same time; in fact, the conditions are such that it's highly likely devices on two other phases will operate. This will change the phase balance very quickly, making it extremely likely that the remaining phase, on which OPDDs were originally "blinded" now see an "out of range" voltage in a relatively short time after the event.

In a similar way, the operating characteristic of 30 mA RCDs is not guaranteed to protect children ... that doesn't mean we can't use them and they are useless - they offer more protection than not having the RCD., but they wont' protect 100 % of people 100 % of the time.

Thanks for reply, 

Thanks

the line to neutral voltages on all phases constantly change. So if the broken PEN conductor is initially undetected, that situation will not persist continuously.

That would be more reassuring if these devices latched off once an out-of-bounds voltage had been detected - but as far as I can see there's no requirement for them to do that, indeed the need to recover after a "normal" power cut or brownout would be a temptation not to do so. Maybe the upcoming equipment standard will improve on that, but as far as I understand it, the devices are free to re-close as soon as the L-N voltage is back in bounds, even if the PE-Earth voltage is still hazardous.

In mitigation, some models to include extra features to mitigate the risk further - e.g. c.p.c. current monitoring - but that's not universal.

   - Andy.

AJJewsbury said:

That would be more reassuring if these devices latched off once an out-of-bounds voltage had been detected - but as far as I can see there's no requirement for them to do that, indeed the need to recover after a "normal" power cut or brownout would be a temptation not to do so.

BS 7671 doesn't at the moment is silent on auto-reset for any of the devices described in 722.411.4.1 (iii), (iv), or (v).

Given the fact that in the same way L-N (or L-PE) voltages vary over time in broken neutral situations, so would the "PE to true Earth" voltage ... meaning that even if you were looking for the Neutral over 70 V to true Earth, it may (even in open PEN conditions) return to a voltage of below 70 V ... and auto-reset if that is enabled by the manufacturer.

So, the same argument on auto-reset persists.

One issue is that pretty much all conditions that can be monitored for open-PEN may be caused by something else ... which is why the old VOELCBs are no longer used (nuisance operation).

AJJewsbury said:

In mitigation, some models to include extra features to mitigate the risk further - e.g. c.p.c. current monitoring - but that's not universal.

Agreed.

meaning that even if you were looking for the Neutral over 70 V to true Earth, it may (even in open PEN conditions) return to a voltage of below 70 V ... and auto-reset if that is enabled by the manufacturer.

So, the same argument on auto-reset persists.

But in that case you're directly measuring the quantity you want to control - with the L-N proxy you're not - so with the latter is can re-close even though the PE-Earth is still hazardous at that moment.

One issue is that pretty much all conditions that can be monitored for open-PEN may be caused by something else

Indeed. Although time and patterns might give some clues. If the voltage drops off to near zero, remains flat, and then recovers to normal values (with only a little bounding around at each end) it's more likely to be a "normal" power cut, on the other hand if it continues to bounce around all over the place for many minutes, a broken N/PEN seems more likely. Overvoltages lasting more than a few seconds again might suggest a broken N too as faults on other phases should clear reasonably quickly. I guess there's no reason that self-closing devices can't continue to monitor the supply after they've disconnected the EV - and use that data to better inform the re-closing process.

   - Andy.

AJJewsbury said:

But in that case you're directly measuring the quantity you want to control - with the L-N proxy you're not - so with the latter is can re-close even though the PE-Earth is still hazardous at that moment.

Granted, but that situation might only persist for a short time ... and also similarly if you are measuring 70 V to Earth, then things might look OK, and just after you reclose, the voltage rises again. Neither situation is 100 % ideal, but both provides some additional protection (at least in the from of an indication to the user) regarding a potentially hazardous occurrence that a report from HSL termed a "broadly tolerable" risk (specifically in domestic charging situations).

AJJewsbury said:

Overvoltages lasting more than a few seconds again might suggest a broken N too as faults on other phases should clear reasonably quickly. I guess there's no reason that self-closing devices can't continue to monitor the supply after they've disconnected the EV - and use that data to better inform the re-closing process.

In conceptual terms, this seems plausible. In terms of mandating this course of action, say in standards, what "rule of thumb" would (or could) be used for that?

In conceptual terms, this seems plausible. In terms of mandating this course of action, say in standards, what "rule of thumb" would (or could) be used for that?

Good question. I've been pondering how best to answer. I think it needs a bit of detail (so apologies in advance for the length of this...).

I think it's easiest to look at likely faults and what their characteristics would probably be:

L-N fault on the same line (phase) - point of the fault is pulled to around Uo/2 (say 115V for 230V) - if the EVSE Is directly downstream of the fault it'll see L-N voltage as zero; if the EVSE connection branches off somewhere upstream of the fault, there will still be some voltage difference L-N. So apparent L-N voltage somewhere between 0 and Uo - but should clear within 5s or so - either returning to normal or being disconnected.

L-N fault on a different line (phase) - N will be pulled toward the voltage at the point of the fault (Uo/2) but in phase with the faulty line. In the simpler case of a split-phase supply that would mean a difference from our line of 1.5xUo; for 3-phase I think it'll be in the region of 1.3xUo (sqrt( (Uo+Uo/2*cos(60))^2 + (Uo/2*sin(60))^2 if you want to check my trig). So EVSE should see L-N of between Uo and 1.3xUo (for 3-phase). Again should typically clear within around 5s.

L-L fault between this line and one other (3-phase) - N stays put but both Ls pulled towards a mid-point voltage - (please check my phasor thinking here) - which I think would put it a little below Uo/2. So again depending on the connections, EVSE would see something between a bit below Uo/2 and Uo. Likewise clearance in the region of 5s.

L-L-L fault - all lines pulled towards 0V - so EVSE sees something between 0 and Uo.

Broken PEN (or N) - L-N could be anywhere between 0V and U (U=√3 Uo) - may well fluctuate as loads connect/disconnect (or fry) - and won't clear for some considerable time (although may at times appear to be within normal range).

So observations so far:

If L-N remains within 0 to 1.3x Uo (or 1.5x Uo for a split phase system), and after a short period (a little over 5s say) it either becomes a steady 0V or steady Uo - then chances are it was a conventional fault rather than a broken PEN.

Conversely, if the voltage at any point in time exceeded 1.3x Uo (or 1.5x Uo for split-phase) or the disruption existed for longer than 5s, then chances are it is a broken PEN.

By and large I've presumed that L-PEN faults are similar to L-N ones. Although the DNOs often have longer disconnection times - so maybe extend 5s a bit - as long as the device opened at first hint of any trouble and we're only looking at decision to auto re-close or not, there's little harm in making the period longer (60s maybe? I'm not sure what DNO disconnection times are likely). Likewise L-PE faults, although reduced c.p.c.s and/or parallel paths can influence the numbers to some extent. We can probably debate what sort of number to assume for Uo - eg.. 230V, 253V or even 262.2V (maybe Uo isn't the best symbol).

Power cuts (or local disconnection) will of course appear as 0V L-N at the EVSE for a considerable period (as might a broken PEN on a single-phase system - but in that case it can't have PE-Earth > 70V, so less of an issue).

So my thinking so far beings me to:

1. As soon as the L-N voltage goes out of range, disconnect (within 0.4s say).

2. Continue to monitor the L-N voltage for a short period (>5s <60s say). If the voltage returns either to normal or 0V, and never exceeded 1.3x Uo then allow auto reset once the L-N voltage remains within range for another period (e.g. 1 min).

3. Otherwise presume an open-PEN fault, and either don't auto reclose, or demand an extended period of consistent normal L-N voltage (hours) before doing so.

There is a bit of a challenge in this - being able to monitor voltages (and duration) when the supply may be 0V. I think hat's probably doable though - either with a small battery backup (capacitor perhaps) or maybe more likely by using a battery backed-up real time clock (RTC) and logging (non-zero) voltage readings with a timestamp into non-volatile memory (pretty standard stuff these days) - once the power's back the log will reveal now long the power was off for, Likewise it might be necessary to treat all voltages below some safe threshold (50V say?) as 0V.

There might want to be some additional random delay in the re-close, to prevent all the EVSEs in a neighbourhood switching back on at the same moment when power returns after a power cut.

There are a few assumptions in all this - e.g. the voltage measurement isn't affected by surges (e.g. from switching off inductive loads) - so either such surges are filtered out or the voltage reading is averaged over a suitable length of time (a few cycles perhaps); Also that N/PEN and live conductors have roughly matching impedances.

I might have to think a bit more about brownouts (voltage reductions lasting some period) perhaps due to starting currents from nearby heavy motors.

   - Andy.

AJJewsbury  thank you for contemplating this. It's not an easy subject.

It might need to be the subject of some research to fully address this, as you say - especially voltage variations such as "brownbouts" and other network events.