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,
[automerge]1574632328[/automerge]
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.