LuxPower - emergency backup enabling

[Oldgreybeard]

TN-C-S/PME poses an interesting question for those with an eye for the fine detail. Because the regs now allow an additional intermediate to be installed in parallel with the incoming protective earth (or more accurately the PEN) then it isn't really TN-S when in EPS mode, simply because there may well be multiple earth fault current paths back through all the other PME earth electrodes, right back to the grid transformer.

Difficult to see how fault current could flow back to the transformer earth, as although there may be an earth path there cannot be a live or neutral conductor going thereto as the relay in both of them will be open.

But I do agree with you that converting the whole installation to TT (on grid) is much the best way forward. Fortunately I never got round to converting mine to TN-C-S in the first place as proposed by WPD some years ago.

My real concern is the PEN fault problem. I gather there are still lots of these every year and in the event of a power failure on the distribution side I would guess that the risk of there being a PEN fault as a part of that may be quite high. If this results in the potential of the incoming PEN rising (as it often seems to) then by leaving the PEN connected to the local earth electrode there is a risk that this could still rise to an unsafe touch voltage due to conditions outside the consumer's control. What's more, if this happens the incoming RCD on the grid supply won't see it, so that offers no protection either. Doesn't seem to me to make a lot of sense in retaining the PEN as the protective earth, in parallel with a local earth electrode, as it offers zero benefit, given that we no longer need a very low impedance earth fault path with RCDs, as we did when all we had were fuses/MCBs.

[Swwils]

They will sell you a £800 magic box that will fix it. Just ignore that 32A heat pump outside the house that's exempt.
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[Oldgreybeard]

They will sell you a £800 magic box that will fix it. Just ignore that 32A heat pump outside the house that's exempt.
.

Never understood why heat pumps were exempted from the normal requirement to not have outdoor conductive parts not earthed via an incoming PEN. All other outdoor bits of kit that aren't double insulated are supposed to use a local TT earthing arrangement, AFAIK.

A ~22kW single phase heat pump is probably a fairly rare beast, too. Our 6kW heat pump draws a maximum of a bit over 8A, and I thought that if you wanted a heat pump that was over about 15kW output you're really into three phase territory. Not sure why, as the current shouldn't be an issue, perhaps it's just that three phase makes more sense for larger compressor motors (although I have a feeling these are all run from VFDs, anyway, even on the single phase machines).

[marshman]

When in EPS mode would the measured/recorded earth loop impedance of the local earth rod still be applicable? Under normal conditions the measurement is back through the local transformer, some distance away. When the grid is disconnected and you are running from the inverter (EPS mode) the local earth rod impedance as measured when the grid was present can no longer be valid as the circuit whose impedance was measured is no longer there. The earth loop impedance really ought to be checked again when in EPS mode as the path is then through the local earth stake and back to the inverter. In this instance I would expect, (will have to measure it to check), the loop impedance of the local earth stake to be lower than any external PME earth provided by the DNO as we are measuring to the local generator (inverter) rather than the more distant local transformer.

[Oldgreybeard]

When in EPS mode would the measured/recorded earth loop impedance of the local earth rod still be applicable? Under normal conditions the measurement is back through the local transformer, some distance away. When the grid is disconnected and you are running from the inverter (EPS mode) the local earth rod impedance as measured when the grid was present can no longer be valid as the circuit whose impedance was measured is no longer there. The earth loop impedance really ought to be checked again when in EPS mode as the path is then through the local earth stake and back to the inverter. In this instance I would expect, (will have to measure it to check), the loop impedance of the local earth stake to be lower than any external PME earth provided by the DNO as we are measuring to the local generator (inverter) rather than the more distant local transformer.

As long as Zs is low enough to allow 30mA of earth fault current to flow without raising the voltage at any exposed and earthed conductive part above the safe touch voltage of 50V then it's fine, as the RCD will trip before there is an unsafe condition, There is a deeper issue, potentially, with some inverters that measure Zs as a part of their pre-connection checks, though. I assume that these will all be OK with a measured Zs of 200 ohms for Ra plus the minute impedance of the wiring loop, but this may not be the case. Our EV checks the earth loop impedance before it starts charging and will refuse to charge if this is over 150 ohms. Annoying, as occasionally there are some public charge points that have an earth loop impedance greater than this. I once resorted to asking for a bucket of water from a hotel we were staying at to pour around their earth electrode so we could get the car to charge overnight.

[openspaceman]

When in EPS mode would the measured/recorded earth loop impedance of the local earth rod still be applicable? Under normal conditions the measurement is back through the local transformer, some distance away. When the grid is disconnected and you are running from the inverter (EPS mode) the local earth rod impedance as measured when the grid was present can no longer be valid as the circuit whose impedance was measured is no longer there. The earth loop impedance really ought to be checked again when in EPS mode as the path is then through the local earth stake and back to the inverter. In this instance I would expect, (will have to measure it to check), the loop impedance of the local earth stake to be lower than any external PME earth provided by the DNO as we are measuring to the local generator (inverter) rather than the more distant local transformer.

'Scuse my ignorance but how can you check the impedance of your local earth rod when the supply from the DNO is TN-S and you have a changeover switch that has swapped L and N but the earth via the DNO supply cabling is still present (though not to be relied upon)?

[marshman]

When in EPS mode would the measured/recorded earth loop impedance of the local earth rod still be applicable? Under normal conditions the measurement is back through the local transformer, some distance away. When the grid is disconnected and you are running from the inverter (EPS mode) the local earth rod impedance as measured when the grid was present can no longer be valid as the circuit whose impedance was measured is no longer there. The earth loop impedance really ought to be checked again when in EPS mode as the path is then through the local earth stake and back to the inverter. In this instance I would expect, (will have to measure it to check), the loop impedance of the local earth stake to be lower than any external PME earth provided by the DNO as we are measuring to the local generator (inverter) rather than the more distant local transformer.

'Scuse my ignorance but how can you check the impedance of your local earth rod when the supply from the DNO is TN-S and you have a changeover switch that has swapped L and N but the earth via the DNO supply cabling is still present (though not to be relied upon)?

That's kind of my point ! The "generator" has changed, therefore the earth path has changed, so the effective earth loop impedance has changed.

[openspaceman]

That's what I was thinking when the electrician tested the earthing when he fitted my changeover switch (I'll have another question about that later) until the DNO cuts the power and the supplied neutral my TT earth runs parallel with it.

Also the N-E bonding switch, is this only for the EPS use or for any emergency generator?

I was just going to ground the generator with a spike, do I assume the neutral and earth are bonded within the generator?

[marshman]

__

[Oldgreybeard]

Sorry can't answer that. You certainly can't assume anything. My big diesel generator did not have an earth neutral link, nothing in the documentation, I had to find out by inspection!.

My small, and pretty ancient, generator doesn't have a neutral link either. When I checked it the earth terminal was connected to what seems to be a centre point, so it's really a split phase generator, 120V either side of earth. I never bothered to earth it, just used it as IT, as almost all small leisure type generators are used in practice. Nothing wrong with IT, works just fine and some countries still use IT earthing (Norway, for example).

I don't think there was any sort of standardisation of earthing with generators for years. I got mine second hand a long time ago and it had been repaired and modified a bit before I got it, so whether the way it's wired now is the way it left the factory I don't know. I also have a small 12V inverter, rated at a joke 1000W (it's good for around 300W in reality) and that has no earth to neutral link, doesn't even have a connection from the outlet earth socket to anything, not even the case. Probably not uncommon for cheaper stuff.