I will add that I have seen this happen, only once, mind: called to a house owned by a farming estate and let to workers, immersion not working and shocks off pipework, did initially wonder about a connection, but alas no, the immersion was a red herring, the high level safety cutout on the stat had overheated, burnt and gone O/C. Onto the shocks, well TNCS install, Zdb at about 0.23ohm, so got a decent earth, switch off and do a real Ze, thats fine as well, but did I imagine the arcing as I pulled the main earth out the bar... flash it on the bar, nope, definatly arcing. Strange. Metre long drill bit in the garden outside the door and a bit of singles shows that the earth/neutral/PEN is about 190v above true earth, and theres no voltage to the house earth.

Time to call the DNO I think, details taken by 'first line support', get a call back almost immediatly from someone above, did I really mean 190v off true earth, yes, erm, well we will send an enginner. I leave to visit the wholesaler for anew immersion stat, an hour later I am am back (rural lincolnshire, you don't get around it very quickly) and the DNO chap is already there scratching his head and we are waiting for bucket truck to turn up to look at the Tx (which it couldn't get to anyway, but it did have fibreglass extenison ladders on board as well) ANyway more DNo chaps on site now, they go upto the TX and remove the tape on the joint to the conductor running to the local electrode and measure between that and the line and says 'All looks fine here' and getting the feeling that it wasn't as simple as that I go and fetch the temporary electrode and the cable and put it a few metres away from pole and ask him to humour me and sees funny voltages there as well, hes now a bit confused, and makes a call to someone in his office and then talks about getting a digging crew to exacavte a joint further down, becuase I think they did disocnnect the output from the transformer tap and note that voltages became normal at that.

Original DNO chap goes down road to the next house and aaks about issues and to see the mains, comes back and tells me that it seems to be reverse polarity, but does notice that its TT so hasn't got the same issues as the first house, this is odd, as it definatly wasn't RP when I did the EICR a few months back, asks me to come back and have a look, it seems both L and N have volts to the (TT) earth, and he has assumed RP as neutral tail lit up the voltstick, I was kind of wondering if there was an issue here as I had picked up on the EICR that there were some circuits not on RCD so no fault protection, but I seem to remember one of us switched main switch and issue remained, so not that.

Back out to the road with the DNO chap, and the overheads continue down the road, I said, where do they go, and he says some farm buildings, seeing where this is going I suggest we go and have a look, we end up at this old metal framed barb  thats seen better days and the roof is all covered with plastic sheeting, go in, not much in there, couple of lights in some well rusted conduit, probably not been used in years, then we notice the digital meter has the LED flashing away, odd, theres nothing on in here, follow the tails from meter to old VOELCB, then to BILL rewireable board, then conduit across the ceiling for the lights (how the VOELCB was meant to do anything when the conduit was in contact with the frame of the building that was providing the earth, I don't know....) but then the DNO chap saw it, the conduit was happily flashing away to the frame of the building. He ran to truck to get his visor and gloves, I took a quick video to prove what it turned out to be https://www.youtube.com/watch?v=cYj55s0ENdY ,service fuse was removed, senior engineer finally arrived from DNO after being called before and volatages all returned to normal.

I did enquire about if the rod at the tx was the only one, but I was told there was a second one at the end of the line as well, but clealry not an amazing reading compred to a metal framed barn that had conduit with perishing VIR inside fixed to it and a VOELCB that was probably seized and unlikely to work how it was installed anyway.

I think when I left site, most of the DNO chaps were still a bit fuzzy on what had actually gone on, they thanked me for my assistance, but I got the feeling, that only the office based engineer had actually understood why the fault had caused the symtoms it had.

I think I've remembered most of it correctly, so I hope it makes sense, I've just realised it was three and a half years ago and I don't know where the time has gone...

Nice video, and very good to know the voltages returned to normal when the man who understands arrives,  - there is a related effect where intermittent faults only come back while demonstrating to customers they had been fixed..

Sorry, I should not tease. It is as you say fortunately rare, but as folk will soon be encouraged to add earth electrodes etc to foundations of buildings, especially if that involves piles or rebar, then there may be many more properties as well grounded as that barn, and far more scope to pull the 3 phase triangle down by one corner instead of only at the centre. A low Zs, especially on a PME feed, does not guarantee a low earth resistance to terra-firma, just a low fault path via the neutral.

And then how to recognize this fault class will need to be properly taught, as the 'earth is always at 0V'  is pretty much embedded thinking, and of course quite dangerously wrong in that case - the could have been scope for a really nasty shock or dead livestock though earth currents actually.

Mike.

Just thought I'd share an update here:

Humm, maybe time to resurrect the "Campaign for Real Earthing"?

(I'm still rather taken with the modified TN-S approach where neutral earthing is through a resistance (I think it was proposed in one of the Cahier technique papers ages ago) - while you need RCDs (TT style) for ADS it pretty much guarantees that PE stays reasonably close to true earth - even during faults).

    - Andy.

There are some conditions that the neutral earthing resistor  does not guard against - and the grounding to terra firma of any one phase  like that barn in the incident described above, is one of them.It also means that the topology of mains filters needs to be changed a bit to reduce standing CPC currents.

The other approach commonly taken in parts of Africa, and south America, while beyond the pale here, is  of not using an earth connection at all, and if you really need to earth something  it has to be a TT variant. In places where the risk of network side failure is much higher than it is here that actually becomes the safest sensible option.

Consider if you were downstream of this transformer or one like it - would you want PME ?

Note the method of installing and testing (7 mins in ) for an earth is also not one we would recognize

Power of opportunity supplies  in war zones and disaster relief might well be similar quality.

Mike

It strikes me that a functioning VOELCB could be the answer, but only if:-

• All incoming services are plastic, or have plastic inserts.
• The VOELCB breaks all incoming wires, including the earth (so 3-pole or 5-pole).
• The VOELCB has a properly independent earth rod as its reference.

The moment that the incoming PEN drifts too far from real earth, the power would be completely shut off.

 Simon Barker you've made some great suggestions, and believe me, I've looked at this in great detail, but there are still some issues.

It's not just incoming services that would have to be plastic ... there are a number of structures with substantial amounts of steel, which are extraneous-conductive-parts too.

And this leads into the second issue of a VOELCB, which is where to put a suitably segregated earth electrode so it's looking at "true earth".

There actually are some modern VOELCBs that have a measurement earth electrode out there on the market, it's just looking at how we use them, which has been the difficulty with their use for many EV charging installations.

There are some conditions that the neutral earthing resistor  does not guard against - and the grounding to terra firma of any one phase  like that barn in the incident described above

Indeed - nothing's perfect but the vast majority of faults I suspect are L-PE rather than L-true earth, so still an improvement. (I suspect the barn incident would have been L-PE via the steel conduit - its was the fact it was TT that introduced the resistance to true Earth. Had it been supplied from a NER version of TN-S with the conduit connected to the supply PE (and/or the barn frame bonded) then the barn frame, and everyone else's PE would have remained close to 0V - the voltage difference be created between Earth and N (and thus Earth an Ls) rather than between anything people should ordinarily be touching, even if the RCD failed to trip. The low earth fault currents also make it far easier to protect wiring against faults to true earth - as simple braiding or foil screens make more than adequate c.p.c.s.

is one of them.It also means that the topology of mains filters needs to be changed a bit to reduce standing CPC currents.

Yes EMI issues (and SPDs) was something I had in the back of my mind. If the NER was chosen to give an earth fault current of say 1A - i.e. about 230Ω - it might see to be similar to a TT installation in terms of Zs - and in normal circumstances N would still be close to Earth potential (at least comparable to a TN-S or TT system). Certainly still some thinking to do on the subject, but maybe has some potential still.

Nice videos by the way At least there was an attempt at testing, which is more than can be said for some even over here. I can't help thinking that the procedure might have been devised with a filament lamp in mind - the much lower currents through what looked to be an LED one surely undermine things a bit. I don't give much for the longevity of galv steel with all that sodium chloride though.

   - Andy.

There are some conditions that the neutral earthing resistor  does not guard against - and the grounding to terra firma of any one phase  like that barn in the incident described above

Thinking about it, I reckon it could help even then - provided the chosen resistance was much larger than the source's electrode resistance - which shouldn't be too hard - given that DNOs work to max 20Ω anyway (often <1Ω).

e.g. with a resistance >200Ω  and Rb < 20Ω you shouldn't see PE dragged more than 1/10th of Uo above true Earth, no matter how low the resistance of the fault to Earth.

   - Andy.

I must admit I find the faults described a bit confusing  I can understand that if the neutral is broken then the current goes back via customers earth we had that at a house on the estate I lived on  everything metallic  was at a decidedly painful voltage above earth the DNO came out and sorted a damaged neutral in the meter box  the weird thing was the estate was only converted to PME maybe a year before. What's needed now I think is everytime an EV charger is installed is for the DNO to install a long length of copper pipe in a trench in the street bonded to that customers CNE terminal in the house they can afford it  what do you think?