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Topic Title: Oy! Where's my PME earth electrodes gone!
Topic Summary: 542.1.201
Created On: 12 July 2018 05:28 PM
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 12 July 2018 05:28 PM
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kenelmh

Posts: 119
Joined: 17 February 2012

Have I gone blind or has the requirement in 542.1.201 from the DPC been omitted from the final version? You know, the one requiring an earth electrode to supplement the DNO earth in PME installations.

Does this mean that broken neutrals are not such a commonplace risk as was thought?

Looking forward to the discussion, or at least confirmation of my ocular function..

Thanks
 12 July 2018 05:41 PM
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John Peckham

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Joined: 23 April 2005

The IET obviously listened to the responses from the DPC.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 12 July 2018 05:43 PM
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mapj1

Posts: 10778
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That section has indeed gone, I think mainly due to folk pointing out how difficult it was going to be for upstairs flats and so on, and how good an electrode you would really need to do any good against a domestic load.
I suspect the opinions of some on this forum contributed to this, and and maybe a few other last minute changes.

The risk of neutral loss is, as always, outside the scope of BS7671, as it is a DNO matter

-------------------------
regards Mike
 12 July 2018 06:50 PM
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broadgage

Posts: 2798
Joined: 07 August 2007

In case of loss of the suppliers CNE conductor, a consumers earth rod is most unlikely to do any good.
With any likely earth rod resistance, an open CNE would still result in a very dangerous voltage on "earthed" metalwork.
 12 July 2018 06:57 PM
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Zoomup

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Joined: 20 February 2014

Oh ek! I wonder if I can get a refund on that cheap bulk job lot of earth rods?

Z.
 12 July 2018 07:34 PM
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sparkingchip

Posts: 10706
Joined: 18 January 2003

If you have a look through the latest edition of the NICEIC magazine you will find a comment about it being dropped from the final edition of the 18th in the article about the changes.
 12 July 2018 08:26 PM
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dustydazzler

Posts: 2258
Joined: 19 January 2016

Originally posted by: Zoomup

Oh ek! I wonder if I can get a refund on that cheap bulk job lot of earth rods?



Z.


I've got some broken lawn mower engines if you're interested
 12 July 2018 08:53 PM
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Zoomup

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Petrol or diesel?

Z.
 12 July 2018 09:32 PM
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dustydazzler

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2 stroke
 12 July 2018 10:30 PM
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mapj1

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.. look through the latest edition of the NICEIC magazine you will find a comment about it being dropped from the final edition of the 18th in the article about the changes.


on page44 /45 or so of the on-line verison
of course that is NICIEC's opinion, but is presumably a reasonably well informed observation.

If you'd like an estimate of how often an neutral goes AWOL, you may care to look at the
HSL report on the safety of charging cars from PME networks (it is a dropbox link, ignore the adverts, you don't need to join anything, click on the 3 dots top right, to 'direct download' if you want to save a copy.)

-------------------------
regards Mike


Edited: 12 July 2018 at 10:37 PM by mapj1
 13 July 2018 03:26 PM
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kenelmh

Posts: 119
Joined: 17 February 2012

Thanks Mike. Looks like an interesting read!
 13 July 2018 03:28 PM
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kenelmh

Posts: 119
Joined: 17 February 2012

While I'm here.... is anyone finding that there are sidebars on unchanged text? It's a bit worrying, keep thinking "What am I missing here?"..
 13 July 2018 07:28 PM
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Weirdbeard2

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Originally posted by: John Peckham

The IET obviously listened to the responses from the DPC.


Hi John, does that not suggest that they obviously did not listen to responses from the DPC regarding the quite spurious AFDDs are "recommended" "requirement" ?
 14 July 2018 02:19 PM
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gkenyon

Posts: 5094
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722.411.4.1 has been updated to remove the "oh well, go on then as long as it's a dwelling", so perhaps the issue hasn't "gone away" completely.

Option 722.411.4.1 (ii) for the use of PME is for the supplementary electrode, the maximum electrode resistance as per A722.3 being suggested (by the Note).

-------------------------
EUR ING Graham Kenyon CEng MIET TechIOSH
G Kenyon Technology Ltd

Web-Site: www.gkenyontech.com
 14 July 2018 02:46 PM
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mapj1

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Although that is for car chargers, and the annex shows a calculation that in turn seems to permit a touch voltage of 70, and the full installation current including that of the car charger, although it is hidden in the formulae, so its not perhaps so obvious it is higher than the 50V used for normal ADS. Even so , for the 80A house with electric cooker and shower, a 70V touch voltage is going to need some significant earthing network.
I predict whole house TT with an electrode of more like 50 - 100 ohms as the real way forward in such a case, except perhaps for new build with steel piling or rebar to be used.

-------------------------
regards Mike
 14 July 2018 02:47 PM
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sparkingchip

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It wasn't a new idea and could have been considered best practice in domestic installations for many years, what I objected too was the way the regulation had been written in the draft to be all encompassing, without considering how it could be done other than with careful planning and design for new installations and with a retrofit installation being impossible at most properties. I was reassured quite some time ago by an IET representative at a trade association meeting that it was highly unlikely that it could make it to the final edition as it had been written in the draft.

Andy Betteridge
 14 July 2018 03:07 PM
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alancapon

Posts: 7182
Joined: 27 December 2005

One of the benefits of PME is that the "earth" is monitored, as it is also the system neutral. If the earth / neutral fails, the lights go out and the DNO come round and fix it. Inside the equipotential zone (the house, for example), this does not cause a problem if the required bonding is in place, as the potential of everything metallic rises together. Where it starts going wrong is when the PME earth is used outside the equipotential zone, for the garden shed - after all, it is made of wood! In the old PME regulations, this was specifically banned. A similar issue arises with EVs, and we have in one case fitted an isolating transformer to resolve this issue.

If you start adding a myriad of earth rods, you are effectively going to hide the rare broken neutral, so your design must account for the increased current in the bonding, that may be there for a considerable time until someone spots it.


Regards,

Alan.
 14 July 2018 08:24 PM
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mapj1

Posts: 10778
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But actually there are many things that will mask a busted neutral better than earth rods, the obvious one being bonds to water mains on either side of the fault.
Actually even when its working well, to see several amps on the water main is not unknown, as the street neutral and the plumbing may be comparable resistances.

Actually a wooden shed floor is fine - cement or worse still earth, like a barn for animals are far higher risk, I have been known to stand on a dry wooden chopping board to do something nefarious on a damp floor.

-------------------------
regards Mike
 14 July 2018 11:19 PM
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alancapon

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Originally posted by: mapj1
. . . Actually a wooden shed floor is fine . . .

Maybe, but it is at the risk / liability of the electrician, not the DNO.

Regards,

Alan.
 15 July 2018 12:02 AM
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Legh

Posts: 4203
Joined: 17 December 2004

Originally posted by: mapj1

That section has indeed gone, I think mainly due to folk pointing out how difficult it was going to be for upstairs flats and so on, and how good an electrode you would really need to do any good against a domestic load.

I suspect the opinions of some on this forum contributed to this, and and maybe a few other last minute changes.

The risk of neutral loss is, as always, outside the scope of BS7671, as it is a DNO matter


No.... It's not!
Have you seen what is expected from electricians when they fill out both EIC, EICR forms?
The authority for establishing whether or not the PEN conductor at the intake of an installation is suitable for continued used has been dumped onto any electrician who is last in ................

however ........

Legh

-------------------------

http://www.leghrichardson.co.uk

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