Open PEN detection for 722.411.4.1 (iv) - Part 2

Following on from from the discussion  Open PEN detection for 722.411.4.1 (iv) -  and the various discussions about uncleared TT earth faults and similar causing the substation earth reference to wander about, I've been thinking (bad idea I know, but...) and there's something that's been troubling me. Maybe someone can reassure me.

These open-pen devices (for the (iv) option) work by measuring the L-N voltage, and using that to approximate the voltage difference between N/PE/PEN and true earth. Previously we discussed how the nature of the 3-phase connected loads (either 3-phase loads or combinations of single phase loads) could cause the severed N to wander about the place - and not all the combinations where it was more than 70V away from true earth would be detected by the L-N voltage monitoring.  We had this (phasor) diagram:

(Green was the bit where the device remains connected with touch voltages <50V, yellow where it remains connected despite touch voltages >50V and red >70V)

So, I think, in effect the monitor is effectively using the substation's earth electrode as its reference for true Earth, offset by the line voltage, but still ultimately referenced to the substation's electrode. In the above diagram the substation's Earth is the centre point.

So I now add in the other thought - say we had some current returning to the substation's star point via the general mass of the earth. Probably quite likely in an Open-PEN situation, especially where there are still a few true extraneous-conductive-parts about, or someone's up with the latest version of BS 7671 and installed an additional electrode on their PME installation. We'd then create a p.d. across the soil surrounding the substation's electrode(s). Exact values would vary tremendously, as substation electrode resistance values could be anything from well below 1Ω to nearly 20Ω. For the sake of illustration say we had a substation electrode say 2Ω and 20A of return current - V=IR so our substation star point would be offset by 40V from true Earth. The phase angle of the return current would depend on the mix of loads across the 3 phases - but for the sake of argument (and because it makes the diagrams a bit simpler to draw) lets say it's in phase with the line our charger is on. So "0v" at the substation is really 40V and L1's "240V" is really 280V (similarly the other two phases will be offset, but by odd angles which makes calculating the exact values tiresome). We still have the normal 230/400V nominal relationship between conductors, but they're all offset from Earth by that 40V.

In a simple single phase system the additional resistance on the broken N but now through the general mass of the earth would severely limit the current that could flow through the loads, so the voltage across the loads would collapse, almost certainly pushing the open-PEN device to trip. But with a 3-phase supply I'm thinking that the bulk of the load currents is supplied between lines - from one line, via "artificial star point" of the far end of the broken PEN to the other lines, so isn't limited by the ground resistance, and so voltages across loads is likely to hold up fairly well - the ground resistance only affecting the N current, resulting from the imbalance of the loads, which may be small in comparison to the load currents, so only have a small effect on the voltage on the load end of the severed PEN conductor.

So now the true earth point is 40V to the left of the centre of our 3-phase triangle, which I think means the open-PEN device is even more likely to mos-interpret unsafe touch voltages as acceptable.

... so have I gone wrong somewhere?

- Andy.

• Well, yes ... the "true earth" point is what the star point of the transformer is connected to.

The line conductors are referenced to that point via the transformer Neutral.

In the open-PEN event, it's the local Neutral which shifts based on the balance of the loads and the current flowing in the Earth.

Also, you've moved the 70 V circle to the left with the local Neutral - and, whilst I can see the point of the thinking here:

(a) within a building the main protective bonding connected to the Neutral will move up and down with the Neutral - so regardless of whether you've moved your feet with the Neutral, what you're touching (extraneous-conductive-parts and exposed-conductive-parts) are also at the same voltage.

(b) outdoors, if you are close to buried metalwork connected to the MET, again your feet are almost at the same potential as the parts you are touching. As you move away from the influence of anything buried connected to the MET, the voltage you can touch increases gradually, until you get to about 8 to 10 m away. At that point, your feet are at the same Earth potential as the transformer star point. What this means, is that  in cases where there is some ground potential rise due to broken PEN, the voltage transferred to you becomes less the further away from the influence of MET you are (so the actual voltage rise in a PEN event could be higher than 70 V theoretically, for that position).

And then, there's also the influence on the materials on which you are standing ...

• Well, yes ... the "true earth" point is what the star point of the transformer is connected to.

Iff there is no current in the star point electrode Otherwise, as Andy notes, the Tx star is some volts off the local terra-firma (TF) - and the L-L and L-N voltages may not indicate a problem. This is not the same as the Open PEN fault, but is really  another fault case, and another one that is fortunately quite rare but also scary.

A better definition is that the true earth  potential is that of an electrode driven into the ground ( and here by 'ground' I mean TF, not connected to the CPC) far enough removed to avoid the  near fields of any other electrodes either intentional or adventitious, that is itself neither sourcing nor sinking current. Sort of thing you do for 3 or 4 probe electrode tests.

Mike.

• you've moved the 70 V circle to the left

Good point. Keeping the 70V circle in the same point (centred about true earth) and moving the triangle (and the arcs centred from the end of L1) to the right would probably have been a better description. The point being that the transformer's star point is offset from true Earth. (I've also realized I got the scaling wrong when I amended the diagram - it's actually showing the results of an 80V offset rather than 40V - so doesn't match my (arbitrary) example numbers - I'll fix that when I get the chance).

This is not the same as the Open PEN fault, but is really  another fault case, and another one that is fortunately quite rare but also scary.

Another good point. An open PEN supplying our installation is certainly one way to cause this sort of thing. I think an open PEN elsewhere on the same distribution system could do so as well. Maybe even an uncleared earth fault on someone else's TT installation on the same distribution system. In many ways it's similar to the problem of open-PEN detectors on power islanded prosumer installations - the root cause that the supply's earth reference is compromised by having current flowing through the reference electrode.

- Andy.

• Also occurs to me that the 3-phase option (iii) devices that measure for >70V between PE and an artificial N will also be vulnerable in this situation.

- Andy.

• Something else to consider - your diagram is showing a single electrode connected to the star point of the transformer - it would be very unusual for there to be only the single electrode connected to the star point/PEN - perhaps a reason why PNB is limited to 4 customers? in PME installations, there would likely be a number of LV PEN conductors connected to the star point, so on a break in one PEN conductor, there are lots of return path earth electrodes.

• Some other thoughts Andy:

• In the worst-case of a single installation downstream of broken PEN, the combined resistance N-Earth upstream of the PEN could be argued to reduce the touch-voltage someone experiences at the affected installation, because the total volt-drop between transformer neutral and the MET is split between the combined electrode resistances of the installation extraneous-conductive-parts (and any consumer earth electrode) and the combined earth resistances upstream of the break ... in series.
• There is no direct relationship between the operation of the open-PEN detection device that measures L-PE (or L-N) voltage at a charging point, and an actual touch-voltage. It purely detects a symptom of, effectively, a broken neutral to a single-phase installation in a single-phase or three-phase and neutral network.
• In a single-phase network, the device would always detect a neutral break
• In a three-phase network, when the neutral breaks, the neutral moves around constantly as the balance of loads changes.
• Something else to consider - your diagram is showing a single electrode connected to the star point of the transformer - it would be very unusual for there to be only the single electrode connected to the star point/PEN - perhaps a reason why PNB is limited to 4 customers? in PME installations, there would likely be a number of LV PEN conductors connected to the star point, so on a break in one PEN conductor, there are lots of return path earth electrodes.

Yes, although I did indicate a lower resistance than is likely from a single rod. If you consider a single phase situation the transformer winding is kicking out something in the region 250V and in a broken PEN situation that will be divided between the connected load, connections to earth on the far side of the PEN break and connections to earth on the supply side of the break (ignoring v.d. along the line and PEN conductors as a first approximation). The offset of the star point at the substation depends more on the balance between those three impedances rather than any absolute value. If on the load side of the break, additional PME electrodes would be more of a hindrance than a help in terms of keeping the star point close to 0V. With loads across 3-phases downstream of the break it gets more complicated to analyse - the current through the ground will be the normal N current (resulting from the imbalance of the loads, rather than controlled directly by the loads themselves) and limited by the impedance of connections to ground on the load side of the break, but still the substation's star point is going to be offset by the product of the impedance of the earth connections on supply side of the break and the N current - and my gut feel is that even quite reasonable numbers quite rapidly give worrying results. The chances might be small, but as it's the very circumstances that we're relying on open-PEN devices to operate correctly in.

I suspect the in quite a few (non-urban) situations the source electrodes might have a relatively high impedance - I'm thinking of  a typical village setup - decent sized 3-phase transformer on poles behind the pub, separate LV and HV earthing systems, and a mix of ABC overheads and underground distribution and maybe a steel framed commercial/industrial building or two as well. All we can say for sure is that the transformer's LV electrode should be <20Ω and any additional electrodes might be significantly higher than that (as there's no specific limit specified) - overall we're still likely to be looking at several Ohms.

- Andy.