I've just seen this article in E&T highlighting the increased incidence of reported broken PEN's from 57 in 2003 to 474 in 2021. It seems they are becoming less of a rare event.
David
I've just seen this article in E&T highlighting the increased incidence of reported broken PEN's from 57 in 2003 to 474 in 2021. It seems they are becoming less of a rare event.
David
However, in the case of a TN-S system with no extraneous-conductive-parts, in the event of a PE break, a consumer's earth electrode connected to MET will help divert at least 83 % of the available touch-current (assuming body resistance of 1 kΩ, and earth electrode resistance at the 200 Ω maximum recommended for stability).. So, let's say we had a potentially lethal touch-current of 30 mA, this is reduced to a much less problematic 5 mA (at least in dry conditions).
Agreed - however proper TN-S systems are rapidly going the way of the Dodo - practically every domestic installation these days if it has a DNO supplied Earth will be under PME conditions - so the current available under a PEN break will likely be several amps - and 17% of serval amps is hardly any less lethal than 100% of those same several amps.
- Andy.
however proper TN-S systems are rapidly going the way of the Dodo - practically every domestic installation these days if it has a DNO supplied Earth will be under PME conditions - so the current available under a PEN break will likely be several amps - and 17% of serval amps is hardly any less lethal than 100% of those same several amps.
Andy, you can't compare apples and oranges.
My reply regards controlling touch-currents in certain cases of TN-S systems with broken PE in the distribution network, is in no way relevant, applicable, or comparable, to the case of trying to control touch-voltage PME with broken PEN conductors!
I think the assumed value of 200 Ω in the example calculation gave that away!
As above, to control PME touch voltage, you really need low values of resistance - typically sub 1Ω, which is seen as not realistic in a number of areas in the country (unless you have a large amount of steelwork from a large steel-frame building to use as your electrode).
However, it's also the case that 5 no. premises with sub 5 Ω resistance (that might be achievable with foundation or perimeter-ring style electrodes) could well help ... as discussed above.
however proper TN-S systems are rapidly going the way of the Dodo - practically every domestic installation these days if it has a DNO supplied Earth will be under PME conditions - so the current available under a PEN break will likely be several amps - and 17% of serval amps is hardly any less lethal than 100% of those same several amps.
Andy, you can't compare apples and oranges.
My reply regards controlling touch-currents in certain cases of TN-S systems with broken PE in the distribution network, is in no way relevant, applicable, or comparable, to the case of trying to control touch-voltage PME with broken PEN conductors!
I think the assumed value of 200 Ω in the example calculation gave that away!
As above, to control PME touch voltage, you really need low values of resistance - typically sub 1Ω, which is seen as not realistic in a number of areas in the country (unless you have a large amount of steelwork from a large steel-frame building to use as your electrode).
However, it's also the case that 5 no. premises with sub 5 Ω resistance (that might be achievable with foundation or perimeter-ring style electrodes) could well help ... as discussed above.
5 no. premises with sub 5 Ω resistance (that might be achievable with foundation or perimeter-ring style electrodes)
We have to be careful when the local connection to terra-firma on the fault side gets too good, as now it starts to matter about the "electrode" at the substation side as well.
The traditional assumption is that the transformer star point is well earthed, and the fault is not. If the reverse is true, then it is all the folk on the non- fault side that get shocked as the whole transformer is in effect lifted off true earth by a voltage set by the total current coming back via terra-firma, and the combined electrode resistances on that side.
In effect the planet is at the mid point of two resistors straddling the fault.
I can see this being OK in a street of terraced houses and lots of plumbing and so on, I am less sure about a country lane with overhead singles zigzagging over the road from pole to pole, where one modest pole-pig transformer feeds 10 properties spread over half a mile and there is a 20 ohm or so DNO spike at the base of every other pole. (we get a lot of that here but mostly the houses are TT. )
Mike.
In effect the planet is at the mid point of two resistors straddling the fault.
I can see this being OK in a street of terraced houses and lots of plumbing and so on, I am less sure about a country lane with overhead singles zigzagging over the road from pole to pole, where one modest pole-pig transformer feeds 10 properties spread over half a mile and there is a 20 ohm or so DNO spike at the base of every other pole. (we get a lot of that here but mostly the houses are TT. )
Agreed ... but that raises more questions - like what voltage do you choose for protecting yourself, which we've not delved into yet either?
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.
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