DNOs forbid PME earth being exported???

[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