DNOs forbid PME earth being exported???

[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