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Topic Title: Bonding
Topic Summary: Assume PME
Created On: 26 May 2017 08:10 AM
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 26 May 2017 08:10 AM
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NMBelectrical

Posts: 56
Joined: 28 February 2017

In a recent article in Professional Electrician, Stroma have advised where a DNO earth is provided, unless confirmed otherwise it should be assumed to be PME. Earthing and bonding to be sized accordingly.
Also that UKPN have declared their entire network to be PME, does that spell the end for reduced size main bonds in any UKPN areas?

Nick
 26 May 2017 08:23 AM
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leckie

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Hi Nck

If you look at page 42 of this document, it says that apparent TN-S is to be treated as TN-C-S, so your bonding requirements will be to conform with their requirements for TN-C-S for public supplies.

http://library.ukpowernetworks...on+Earthing+Design.pdf
 26 May 2017 08:38 AM
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NMBelectrical

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So regarding Ze readings and identifying earthing arrangements on paperwork. Ze readings on a system that looks to be TN-S if they are higher than 0.35, they should be flagged to UKPN? All installations other than TT would be identified as a TN-C-S on the EIC?

Thanks

Nick
 26 May 2017 08:50 AM
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AJJewsbury

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It also begs the legal question over originally TN-S supplies and things that are prohibited from using PME - caravans and boats. Have the DNOs actually checked that none are connected before converting their systems or should we expect prosecutions of DNOs under regulation 9(4) of the statutory Electricity Safety, Quality and Continuity Regulations? http://www.legislation.gov.uk/.../uksi_20022665_en.pdf

- Andy.
 26 May 2017 09:01 AM
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leckie

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Well I don't think the DNO's "guarantee" a figure of 0.35 ohms Ze. I think its more about treat bonding requirements as being compliant with TN-C-S requirements.

On the EIC, personally I put a note that the system is TN-S treated as TN-C-S as per the DNO's requirements. Whether that is technically correct or not I'm not sure, but it saves others flagging up that I have made an error in describing the earthing arrangement.

I think the only true TNS now would be if there is an on site transformer.

Not sure if DNO's made any checks on existing TNS systems - but I doubt it as I have never heard of that happening.
 26 May 2017 09:12 AM
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John Peckham

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Andy

I don't think UKPN would ever connect their supply directly to a boat or caravan. If you read on in their policy document they will not provide a PME supply to a building site (a TBS) although this is allowed in BS7671.

The IET panel presentation covered this topic over the last 2 days at the Elex show. If you missed it they will be covering the same topic at the Elex shows in Coventry and Sandown Park later in the year. The topic was chosen as the result of the input from this forum before the 1st show in January this year.

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

http://www.astutetechnicalservices.co.uk/
 26 May 2017 09:15 AM
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mapj1

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I'd not be holding my breath over prosecution. There have been very few legal actions of significance against any DNOs, even when their inaction has caused a problem, there have however been a couple of deaths in the last year or two that have lead to notable fines, one where a fallen HV cable electrocuted a walker in 2012, and another a few years back, but we are not talking many. A technical non compliance with the ESQR is not likely in itself to be anything more than a cease/desist warning.
Actually longer term a change in the ESQR may be more likely if it becomes a problem, or more TT supplies.
The advice is only reflecting the situation that has existed on the ground, or at least under it, for some years now, that in effect at many repairs and additions to the network, cable with 3 phases and PEN are being used to augment what used to be 4 core plus CPC armour.
It is indeed very unclear, even at a service with an apparent TN-S presentation, to be sure exactly where the nearest NE bond is, and fault current may not flow back to the substation in the way you may imagine.
Not so long ago I did have a rather jokey conversation with someone with inside contact at a well know DNO and it was suggested it was because it allowed them to employ folk as installation contractors unable to count past four, but he was having a chunter about something that had been spectacularly badly done, and I suspect it is not the official view, and it has far more to do with accountants and the cost of cables.

-------------------------
regards Mike
 26 May 2017 09:31 AM
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leckie

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To the OP.

If the Ze was in excess of 0.35, but was up to 0.8 ohms then I wouldn't be too concerned. As long as you are within the max ZS requirements for you OPD's I wouldn't think there is an issue.

If you wanted to you could requires a PME terminal. Then then would fir the link to the neutral and probably connect lead from the existing Hepworth clamp, or whatever has been used, to the new earth terminal block. And charge you of course!
 26 May 2017 11:49 AM
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broadgage

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I suspect that most urban supplies are now PME in all but name.

More than once I have observed failed service joints being replaced, or new services being connected in a way that in effect converts to PME.
if a short new piece of main is required to replace a blown up joint, this is invariably 3 core plus combined n/e.
Strictly speaking this could be called TN-S, but from a practical point of view I am not convinced that combined neutral and earth up to a joint outside the front door is much different from combined up to the service cut out.
 26 May 2017 12:00 PM
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John Peckham

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If the DNO comes along and digs up the road and repairs a failed PILC cable with a couple of metres of CNE cable by definition what earthing system is in use if the DNO supplies the earthing to the consumer's premises?

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

http://www.astutetechnicalservices.co.uk/
 26 May 2017 01:03 PM
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mapj1

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what earthing system is in use if the DNO supplies the earthing to the consumer's premises?


perhaps something like
TNS-C-S-C-s

or maybe

TNS-C-S-C-S

depending on the number of changes between CNE (combined neutral and earth) and SEN (Separated neutral and earth)on the line on the way from the transformer to the consumers, and if it is presented as a PME head or not.
Always strikes me as funny how critical it is in TNC-s that N and E never touch again once into the separated stretch of the final leg, being a 3 line whip in BS7671 never to do this, but the equivalent occurs at DNO street joints all the time .

-------------------------
regards Mike
 26 May 2017 01:29 PM
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leckie

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Well section B3, page 42 of the UKPN document I linked explains exactly what they consider one of their earthing arrangements to be TNS, and everything else is TN-C-S, as the OP also quoted from the PE magazine. Any form of CNE, even short links, make it TN-C-S, so you then bond and install earthing conductors to comply with that arrangement. So to my mind, if the supplier say their provided system is TN-C-S you should tick that box, not TNS.
 26 May 2017 02:15 PM
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geoffsd

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Do people think that 544.1.1 is correct in stating "except where PME conditions apply"?

Should it say TN-C-S?
 26 May 2017 05:21 PM
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leckie

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Ah ha, this is the debate you need! Have a read through this and you tell us

http://www.theiet.org/forums/f...AR_FORUMVIEWTMP=Linear
 26 May 2017 08:04 PM
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geoffsd

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Oh, right. Thanks.

Don't want to start that again.
 26 May 2017 09:58 PM
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leckie

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LOL. Actually, I think it's a great post.
 26 May 2017 11:12 PM
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mapj1

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I must have missed that one. Got quite heated . Probably as well I was not there, as I'd be out on a limb twice, once in considering split phase not to be two phases, as you cannot make anything rotate with it, and secondly on PNB being closer to TNS then TNC-s.
It is very important in electronics not to confuse an inversion of the waveform with a 180 degree phase shift. Of course for continuous sine waves you cannot tell, but for anything more complex, with either switching transients, or any asymmetrical feature, it is very obvious there is a difference between a half cycle time delay, and no time delay, but a reversing of the sense of 'suck' and 'blow' on the electrons.

-------------------------
regards Mike
 27 May 2017 02:55 AM
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spinlondon

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Lol, live life on a limb, I always do.
 27 May 2017 09:54 AM
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AJJewsbury

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I don't think UKPN would ever connect their supply directly to a boat or caravan. If you read on in their policy document they will not provide a PME supply to a building site (a TBS) although this is allowed in BS7671.

I was thinking more of a house with a blue socket on the side of the garage used to feed the caravan when parked on the drive - originally supplied by TN-S, then the DNO come along and convert the entire street to PME. Hasn't the DNO then effectively connected the caravan's metalwork to their PME system?
- Andy.
 27 May 2017 12:24 PM
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weirdbeard

Posts: 3116
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Originally posted by: NMBelectrical

In a recent article in Professional Electrician, Stroma have advised where a DNO earth is provided, unless confirmed otherwise it should be assumed to be PME. Earthing and bonding to be sized accordingly.

Also that UKPN have declared their entire network to be PME, does that spell the end for reduced size main bonds in any UKPN areas?



What's a stroma?

The Advice from the NICEIC is :



Non-compliances with the current edition of BS 7671 that do not give rise to danger and do not require reporting


. Inadequate cross-sectional area of main protective bonding conductor provided that the conductor is at least 6 mm2 and that there is no evidence of thermal damage




See the third item of page 18 of 20:



http://www.electricalsafetyfir...ce-Guide-4-Issue-4.pdf

-------------------------
:beer)
IET » Wiring and the regulations » Bonding

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