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Topic Title: Steel frame buildings and PME
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Created On: 13 June 2012 02:21 AM
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 13 June 2012 02:21 AM
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jleltd

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We are about to carry out an installation in a new steel frame building in the western power reagion (not are normal area).
The building is next to but not touching an existing steel framed building but the buildings can be touched at the same time.
The new building is to have a new supply from western power but they wont give us PME saying its got to be TT but the existing building is TNCS.
Both buildings have 200A supplies and are to be fed from the same transformer.
Seems a bit strange to me anyone got any idea why?

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James
 13 June 2012 08:22 AM
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OMS

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Sounds like the DNO have got themselves a bit confused. If there is only one service into the new shed then it's highly unlikeley that thier fear of diverted neutral currents through the structural frame and through other other adjacent supplies will be realised.

I think I would challenge the statement and ask specifically why a single steel shed with only one service warrants a prohibition on PME. I suspect you'll get a convoluted answer regarding neutral currents for both buildings returning through the neutral of a single service - but when you look at it, it won't stack up.

Regards

OMS

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Failure is always an option
 13 June 2012 07:11 PM
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sparkingchip

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I know to two houses which both had a new supply run in sharing the same joint into the cable in the street and Central Networks, now Western Power TT'd one and PME'd the other, I fitted the same type of dual RCD consumer unit in both houses and added a rod to one. I didn't both to ask why.

Andy
 16 June 2012 06:52 AM
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jleltd

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Sorted had a word with the engineer, and he then had a word with his technical guys and hey presto PME.

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James
 13 August 2012 09:32 PM
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sparkingchip

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Just looking at a Western Power application form, it specifcally says it would be a problem if the steel building is multi occupancy.

http://www.westernpower.co.uk/...tion-4-dwellings.aspx

Andy
 14 August 2012 09:16 AM
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OMS

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For multiple services in a steel framed building, I can see the potential problem - whilst each unit has a supply delivered down the line conductor, add in some bonding etc and there is no gaurantee that current will return along the relevant neutral to the PEN conductor - it could all end up in the service cable to one unit (in theory) - causing significant overheating of the DNO pen conductor.

In a single occupancy building though (or one with a single service), there isn't usually any problem.

regards

OMS

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Failure is always an option
 15 August 2012 12:02 AM
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sparkingchip

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Brick building, two separate three phase intakes and a common 15mm copper pipe earthed to both intakes, could be the same issue?

Andy
 15 August 2012 09:52 AM
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AJJewsbury

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Brick building, two separate three phase intakes and a common 15mm copper pipe earthed to both intakes, could be the same issue?

Or a terrace of houses with a common steel water supply pipe running through the cellars...

Sometimes I think this TN-C-S idea hasn't been fully thought out. For all the trouble it causes and extra copper it demands I can't help thinking that TN-S wasn't so bad after all.

- Andy.
 15 August 2012 10:30 AM
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ArthurHall

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I agree with Andy TN-S is a better system. TNC-S does save the DNO's money on their cable, the customer is left to pay for all the extra bonding.
I cant see PILC cables coming back but some DNO's use 4 core waveform.
 15 August 2012 12:20 PM
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daveparry1

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With the advent of almost everything being rcd protected I think TT has a lot going for it! (i'm talking mainly of domestic, as usual!)

Dave.
 15 August 2012 01:15 PM
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OMS

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Originally posted by: sparkingchip

Brick building, two separate three phase intakes and a common 15mm copper pipe earthed to both intakes, could be the same issue?


Andy


Bonded at both intakes -

Yes, a similar problem, but much less in the way of common multiple steel connections I guess - Neutral CSA sized conductor run between the intakes would solve the problem

Regards

OMS

-------------------------
Failure is always an option
 15 August 2012 01:36 PM
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UKPN

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still struggling? PNB now PME!

Regards.
 15 August 2012 01:37 PM
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AJJewsbury

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I guess - Neutral CSA sized conductor run between the intakes would solve the problem

Perhaps from the installation's point of view, but presumably the supplier is still left the problem of a significant proportion of N currents from several installations trying to return through one installation's supply cable?
- Andy.
 15 August 2012 01:38 PM
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UKPN

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never been a problem for us! or our customers who utilise it after correctly
following our advice--which is still there.

i think the problem is various guidance books making up their own mind
and a happy band of followers being led on.

PME-the networks preferred earthing system.

Regards.
 15 August 2012 01:44 PM
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OMS

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Originally posted by: AJJewsbury

I guess - Neutral CSA sized conductor run between the intakes would solve the problem


Perhaps from the installation's point of view, but presumably the supplier is still left the problem of a significant proportion of N currents from several installations trying to return through one installation's supply cable?

- Andy.


But from the network POV, the loads from multiple consumers are likely to present with some degree of balance so the neutral current will be "low" ?

regards

OMS

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Failure is always an option
 15 August 2012 01:53 PM
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OMS

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Originally posted by: UKPN

never been a problem for us! or our customers who utilise it after correctly

following our advice--which is still there.

i think the problem is various guidance books making up their own mind

and a happy band of followers being led on.

PME-the networks preferred earthing system.

Regards.


LoL - just like a public service provider

Suggest you go back and look at the OP and comment on the DNO advice given in the first instance and then how that advice modified based on a challenge informed by this forum.

It's always a bit worrying when you see that happen - makes you wonder just how good the advice was in the first place

Regards

OMS

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Failure is always an option
 15 August 2012 02:15 PM
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AJJewsbury

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If the installations are spread evenly over a three-phase supply you mean? Yes, I can see that. You do still sometimes see a pair supplier's cables clipped along the front of a terrace though (Tee'd off for each house with a trip loop through the front door frame), although I can see that the sum N is unlikely to be massive even then.

PME-the networks preferred earthing system.

I don't doubt it. It must save them a fortune - not only can they avoid the cost of an entire conductor on new works, they can avoid replacing old cables with rotting/damaged metal sheaths by just strapping it to the N here and there. And they never bother mentioning it to the consumer that they've altered their earthing characteristics either.

Whereas it's the consumer that's left with the cost of upgrading bonding and TT'ing outbuilding etc and suffers from unnecessary EMI problems, not to mention the risk of electric shock when the supply CNE goes open circuit (especially around the boundaries of the "equipotential zone").

- Andy.
 15 August 2012 02:21 PM
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marclambert

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Just a quick question re the following quoute "never been a problem for us! or our customers who utilise it after correctly
following our advice--which is still there."

Are you, UKPN, actually an accredited spokesperson for all the DNOs?
Just read "us" and "our" and the ascertation that it's "never been a problem for us" is that a fact?
My apologies if you are indeed such an accredited person, I wish no offence. But if you're not, it's a bit like me saying my interpretation of the regs is right 100% and everyone else is wrong just because I'm an IET member.
All I seek is clarification of the "officialness," (not a proper word I know), or otherwise of your posts on this and other topics.
Thanks
Marc
 15 August 2012 06:03 PM
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alancapon

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Originally posted by: marclambert
. . . Are you, UKPN, actually an accredited spokesperson for all the DNOs? . . .

I don't believe so. I have tried on several occasions to enquire whether "UKPN" is an accredited spokesperson for UK Power Networks, but have yet to have a reply.

Regards,

Alan.
 15 August 2012 09:47 PM
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sparkingchip

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Thinking about it the mass of steel in a steel framed building the resistance could end up lower than the DNO cabling, so the major part of the neutral current could follow through the building frame in preference to the cables, which isn't going to happen with a 15mm copper pipe so readily is it?

Andy
IET » Wiring and the regulations » Steel frame buildings and PME

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