DNOs forbid PME earth being exported???

[Pretty Mouth]

Exporting A PME Supply To Another Building

In this article the technical team at Stroma Certification offers expert guidance on exporting a PME supply to another building. One of the most common queries to the Stroma help desk concerns running

professional-electrician.com

In the 1st paragraph of the above article, it states "some Distribution Network Operators (DNOs) may forbid their PME earth being exported from the supplied building.".

I hear this repeated a lot, or "you must gain permission from DNO before exporting the earth". I don't believe it though, I can find no evidence of any DNO having a blanket ban on "exporting" the earth, or requiring permission.

Do any other forum members have any evidence to back up the claim made in the article?

 

[SparkyAndGeorge]

That link does not work

 

[Pretty Mouth]

How do I make the link work? You can google 'stroma export pme" and the article should show up top of the list

 

[westward10]

It is poorly worded, you do not export a PME earth you extend an earth from one building to another whereby the source has a PME earthing system. In general installations the DNO cannot quote any Regulation it is nothing to do with them. The ESQ&R Regulations however do forbid a PME derived earth in some situations.

 

[snowhead]

Professional Electrician say copy the content, don't link directly.

One of the most common queries to the Stroma help desk concerns running a supply to a garden building where the supply to the house has a PME earthing system. There’s no regulation that says you can’t export a PME earth to another building. However, some Distribution Network Operators (DNOs) may forbid their PME earth being exported from the supplied building.

 

[Pretty Mouth]

It is poorly worded, you do not export a PME earth you extend an earth from one building to another whereby the source has a PME earthing system. In general installations the DNO cannot quote any Regulation it is nothing to do with them. The ESQ&R Regulations however do forbid a PME derived earth in some situations.

Thanks westward, I thought as much but always good to hear other opinions.

My DNO is western power, for them to provide a TN-C-S earth they want compliance with BS7671 and Engineering recommendation G12. If ESQR forbid an PME earth for a particular situation, is it also likely to be forbidden by these two sets of regulations?

 

[westward10]

ER G12 is for network operators and if any part of this is applicable to the consumer it suggests compliance or reference to BS7671. Nevertheless it is a good document to peruse.

 

[Midwest]

Isn't more likely, for larger or commercial properties, rather than providing a supply to someone's potting shed?

 

[Cid]

Dno will always err on the side of caution. That is why they will never agree with you touching their stuff. You just have to use your best judgement.

 

[Deleted member 122525]

It is very simple to deal with a situation like this and it is common sense, don’t take PME outside the supply building, unless you are extending the equipotential zone. Using the same size Earth as the property is bonded in. PME is a horrible supply, should it go open circuit, within your property or the DNO supply, if you are going to take it outside, TT it, it’s the safest option.

 

[DPG]

, if you are going to take it outside, TT it, it’s the safest option.

Really? For every outside building, shed, etc?

 

[Pretty Mouth]

Thanks for all your comments. It appears to me that this is a rumour that has done the rounds for so long that even one of the schemes believes it!
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It is very simple to deal with a situation like this and it is common sense, don’t take PME outside the supply building, unless you are extending the equipotential zone. Using the same size Earth as the property is bonded in. PME is a horrible supply, should it go open circuit, within your property or the DNO supply, if you are going to take it outside, TT it, it’s the safest option.

Not sure I agree with all of this Tony. There are certainly times when a TT earthing arrangement would be preferable (or required by regs), but each situation should be risk assessed to decide if TN-C-S or TT is appropriate.

TT comes with its own risks - if the RCD fails (we've all seen it) and there's a L-E fault, then unlikely that the OCPD will operate. Now we're in the same situation that we were trying to avoid, ie all exposed/extraneous metalwork going live.

Suppose you TT an outbuilding, bonding the extraneous metal water pipe that originates (and so is bonded) in the main, TN-C-S supplied building. If we lose the DNO neutral, that pipe is going live along its length, both in the main building and the outbuilding, along with everything else bonded/earthed out there. Now we have the worst of both earthing arrangements, doubling the risk.

So perhaps a little more to it that simply TT'ing everything outside the main building

 

[westward10]

Equipotential zone, when was that last in the Regs.

 

[Deleted member 122525]

“Suppose you TT an outbuilding, bonding the extraneous metal water pipe that originates (and so is bonded) in the main, TN-C-S supplied building. If we lose the DNO neutral, that pipe is going live along its length, both in the main building and the outbuilding, along with everything else bonded/earthed out there. Now we have the worst of both earthing arrangements, doubling the risk.”

That’s assuming that you are taking a metal pipe out there, and not poly, so in that case you would precautions to not let this happen.
The main reason why I am so strong for not taking PME outside, is when I did my 18th update, I was taking to an engineer who works for SSE, his job is to locate and fix, floating neutral faults on PME supply cables, he was very knowledgeable about this supply type. He could not state enough how dangerous it is under fault conditions, he even went on to say if his house had a PME supply, he would disconnect it and make his house TT.
I was also taught this when I was doing my 2330 lev3, my NVQ3 and my inspection and test.
I would put PME down as a C2 on an EICR if I saw that it was taken outside.

 

[westward10]

Maybe you need to reconsider your own understanding of earthing systems and base each case on its own merits as opposed to C2ing everything on the hearsay of some SSE individual. In #10 you speak of equipotential zones, Regulations have not referenced this for some time.

 

[Pretty Mouth]

I would put PME down as a C2 on an EICR if I saw that it was taken outside.

So, a metal outside tap, connected to the inside bonded pipework - C2?
An outside class 1 light fitting - C2?
A wooden summer house, wooden floor, no extraneous parts, with a double socket and a light in it - C2?

What regulations would you reference to justify these codes?

 

[Deleted member 122525]

Look on the ITE website,
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You guys sound dangerous, glad you don’t work with me or around me,

 

[westward10]

Look on the ITE website,
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You guys sound dangerous, glad you don’t work with me or around me,

Not sure who the ITE are but insulting other members without sound technical reasoning is not the way forward.

 

[Deleted member 122525]

This is one of the worse, forums I have ever joined, the lack of knowledge and understanding here is unreal, everyone seams to be trying to take a short cut to getting work done.
I don’t really give a ---- what people think. My knowledge and understanding is sound, and my work is safe, I don’t ever take risks, and it is my opinion and I don’t have to justify a single thing to anyone here,

 

[Pretty Mouth]

Well, he's not here anymore to defend himself, but he kind of shows why I started this thread in the first place.

I'm on a FB group for sparks trained through a certain centre, most members are relatively new entrants to the industry such as myself, but some old (and apparently very knowledgeable) hands as well. The prevailing attitude in the group is "for outbuildings, DNO forbids TN-C-S, must TT it", which as you've probably gathered I disagree with. When I challenge this, at best I get referred to the professional electrician article I tried to link to in the first post, at worst I just get it repeated slightly more firmly.

It's not just in that group, I see it all over the internet too.

 

[Dan]

I think you'll always get plonkers in trades and on forums. It's life.

But you can hold your head up high if you crack on and do the job right.

If it's 'the done thing' it doesn't mean it's right.

 

[Cid]

The regs do say you can't install a car charging point outside with exported PME.

From that we can infer we shouldn't be taking PME outside. Perhaps?

 

[Simon47]

My knowledge and understanding is sound, ... I don’t have to justify a single thing to anyone here,

Lets examine the facts. You've made (bkanket) statements that other people who understand the issues didagree with (professionally). You've failed to provide any reasonable justification as to why your professional opinion is more valid than anyone else. You've also failed to provide any references to back up your position.
You then resort to insults, and an "I know thus stuff" attitude rather than answer reasonable questions.
I've made my mind up what sort of sparky you are - not one I'd employ ! Others can make their own minds up.

The regs do say you can't install a car charging point outside with exported PME.
From that we can infer we shouldn't be taking PME outside. Perhaps?

It's not that simple - and you can use an exported PME earth with the right considerations. In fact, the limitations are such that few will be able to use a TT earth and must use the PME earth.
It's interesting to observe that for a long time the rules has been "thou shalt not switch the protective earth, ever", yet now we have a subset where the earth must be switched

 

[Pretty Mouth]

The regs do say you can't install a car charging point outside with exported PME.

From that we can infer we shouldn't be taking PME outside. Perhaps?

TBH I don't know enough about EV chargers to say exactly why PME isn't allowed. At a guess I would assume the same reason as caravans - a large metal object that is touched frequently by people standing on bare ground. If you lose the dno neutral the odds of someone getting a shock are high.

Compare that to a wooden summer house, wooden insulating floor, no extraneous parts. The risk here is no higher than in the main building, possibly less.

Assess on a case by case basis

 

[EddieSparks]

It is mainly for the risk of open circuits as I understand it, Regarding EV charging sockets. I was always told PME outside was risky mainly due to the possibility of the potential difference between the supply earth and the general mass of earth. Due to the volt drop if the run is long, from the transformer to the out building, but a risk assessment is always the best way to go,

 

[davesparks]

“Suppose you TT an outbuilding, bonding the extraneous metal water pipe that originates (and so is bonded) in the main, TN-C-S supplied building. If we lose the DNO neutral, that pipe is going live along its length, both in the main building and the outbuilding, along with everything else bonded/earthed out there. Now we have the worst of both earthing arrangements, doubling the risk.”

That’s assuming that you are taking a metal pipe out there, and not poly, so in that case you would precautions to not let this happen.
The main reason why I am so strong for not taking PME outside, is when I did my 18th update, I was taking to an engineer who works for SSE, his job is to locate and fix, floating neutral faults on PME supply cables, he was very knowledgeable about this supply type. He could not state enough how dangerous it is under fault conditions, he even went on to say if his house had a PME supply, he would disconnect it and make his house TT.
I was also taught this when I was doing my 2330 lev3, my NVQ3 and my inspection and test.
I would put PME down as a C2 on an EICR if I saw that it was taken outside.

You cannot code an observation on an EICR based on your opinion, you can only code it according to bs7671. Currently there is nothing in bs7671 to support your suggestion of a C2 for a PME earth used for an outbuilding.

Yes PME has its downsides, but it is a very reliable earth connection for the vast majority of the time. You talk about it as if supplies to installations suffer neutral faults on a regular basis, they don't.

 

[Simon47]

You could always add your own earth rod, so if there's a PEN failure you can earth the whole street
Wasn't there talk of that being made mandatory in the last update to the regs ?

 

[DPG]

You could always add your own earth rod, so if there's a PEN failure you can earth the whole street
Wasn't there talk of that being made mandatory in the last update to the regs ?

I think it got removed luckily.

 

[EddieSparks]

Not totally removed, I found this the other day in the OSG chapter 4.

 

[davesparks]

You could always add your own earth rod, so if there's a PEN failure you can earth the whole street
Wasn't there talk of that being made mandatory in the last update to the regs ?

Yes you can, that's how it's done in a lot of other countries and is a good thing in my opinion.
It was going to be included in the 18th but I think it has been delayed for an ammendment.
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The regs do say you can't install a car charging point outside with exported PME.

From that we can infer we shouldn't be taking PME outside. Perhaps?

The regs don't say this at all, they have certain requirements for car chargers connected to a PME supply.

 

[Midwest]

Yes you can, that's how it's done in a lot of other countries and is a good thing in my opinion.
It was going to be included in the 18th but I think it has been delayed for an ammendment.

Thats alright, until you come to do a CU change on the top floor flat.

 

[davesparks]

Thats alright, until you come to do a CU change on the top floor flat.

Being on the top floor, or any other floor, is irrelevant. The PME earth terminal and MET will still be at ground or basement level wherever the DNO cutout is.

 

[Midwest]

Being on the top floor, or any other floor, is irrelevant. The PME earth terminal and MET will still be at ground or basement level wherever the DNO cutout is.

Not necessarily, some establishments are not so simple. I recall from some of the comments placed on the draft, asked this conundrum and allegedly why the committee put the reg change on hold for the time being.

 

[Pretty Mouth]

Excuse my ignorance, but how does installing an earth electrode in addition to a PME earth mitigate the effects of an open PEN conductor? Is it something to do with a gradual voltage drop between that electrode and the 1st (or all?) earth stake of the PEN?

 

[Ian1981]

Excuse my ignorance, but how does installing an earth electrode in addition to a PME earth mitigate the effects of an open PEN conductor? Is it something to do with a gradual voltage drop between that electrode and the 1st (or all?) earth stake of the PEN?

It will be a similar requirement to that of EV installations where an additional electrode connected to the PME arrangement, during an open pen circuit the touch voltage from exposed parts to true earth must not rise above 70v (hand to foot for EV but may require to be 50v hand to hand contact between exposed parts)
This in reality is hard to achieve as per examples in section 722 annex and guidance note 5 and 7 examples.
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Being on the top floor, or any other floor, is irrelevant. The PME earth terminal and MET will still be at ground or basement level wherever the DNO cutout is.

That’s a sweeping statement considering there are many set ups where flats have their own DNO supplies complete with metering and are situated on many floors, be it older type accommodation/schemes.

 

[EddieSparks]

Excuse my ignorance, but how does installing an earth electrode in addition to a PME earth mitigate the effects of an open PEN conductor? Is it something to do with a gradual voltage drop between that electrode and the 1st (or all?) earth stake of the PEN?

As I understand it, say the PEN breaks in the road, the current from let’s say house 1 (brown phase) They are using 50 amps, it a family of 4 at dinner time. That current is still going to try and flow back to the transformer but can’t, so it will find the shortest path, which will happen to be up the neutral of house 2 (black phase) House two may only be a little old lady and she only has a lamp and her Tv on so say she is using 5 amps. What is start to happen is she is going to get 400v turning up at her, appliances. And things starting to go bang, it will add a picture of this as well. So if there was an earth electrode at each house, it might help dump some of that stray current to earth, and trip some protective devices.

 

[Pretty Mouth]

It will be a similar requirement to that of EV installations where an additional electrode connected to the PME arrangement, during an open pen circuit the touch voltage from exposed parts to true earth must not rise above 70v (hand to foot for EV but may require to be 50v hand to hand contact between exposed parts)
This in reality is hard to achieve as per examples in section 722 annex and guidance note 5 and 7 examples.

Thanks Ian, I'll have to take a closer look at that at the weekend with a fresh mind, there's no way I'll get my head around it now!

 

[Ian1981]

Some examples some examples

 

[Pretty Mouth]

Ian1981, thanks again for that. I also found this John Ward youtube vid where he talks you through the calculation from about 13mins in.

View: https://www.youtube.com/watch?v=q87H7aIujjA