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Topic Title: ground currents in building with multiple TNCS supplies
Topic Summary: magnetic field problems are interfereing with equipment
Created On: 09 July 2013 06:53 PM
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 09 July 2013 06:53 PM
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jrvdp

Posts: 26
Joined: 10 October 2006

I'm working on an old, relatively recently converted building with 3 REC supplies into it, all TN-C-S. The REC supplies are to 3 different points within the shell, separated by many tens of metres. From the TN-C-S heads there is some haphazard bonding and earthing, and the heads then feed a load of about 25 local and remote electricity meters (which look to be DNO / REC meters) for the occupiers of different parts of the building.

The occupiers generally look like they're treating their incoming supply points as installations in their own right and each appear to have their own main bonding. The building has a fair amount of structural steel, as well as a lot of metallic services and local steel structures within.

Imbalance/residual currents in the incoming REC/DNO cables are in the tens of amps (if you clamp around the outside of the armour - much smaller if you clamp around the cores and neutral tail). After the heads, the imbalance currents are to a great extent flowing in the various earth and bonding conductors connected to the TN-C-S supply heads, and can be measured readily by clamping on to this heap of green and yellow spaghetti.

We are going to carry out detailed checks for neutral earth faults and mixed supply returns in due course. However, so far, it appears that neutral return current is making its way across the (over) bonded structure from one supply to another via the bonding and earthing. Obviously a single main earth terminal and supply at just one point would be a help.

These currents look like they're the cause of 50 Hz magnetic fields in the 100s of microTeslas, which are interfering with equipment in my client's area of the building. They're also well above what some people regard as acceptable for human occupancy, but lets not go there, here.

Any thoughts and contributions would be much appreciated. In particular, also, can anybody please shed any light on any UK regs requirements and guidance for multiple PME supplies being redistributed within single buildings which are multiple occupancy.

All the best. RvdP

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Rupert van der Post BSc CEng MIET
 09 July 2013 08:06 PM
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AJJewsbury

Posts: 11295
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Not uncommon - we had similar problems quite a few years ago in a multi-occupancy unit - the image on CRT screens would "wobble" in certain parts of the building; only later did we realise that the multiple PME supplies & steel structure were probably to blame.

I gather than some DNOs are now declining to install multiple PME supplies into steel framed buildings, insisting on TT instead (for at least all but one) - although possibly on account of one of their CNE conductors becoming overloaded rather than any particular compassion for the effects on the consumer!

- Andy.
 09 July 2013 08:16 PM
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MrOther

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What does REC stand for?
 09 July 2013 08:51 PM
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jrvdp

Posts: 26
Joined: 10 October 2006

Andy, Thanks for your sympathy - were you able to deal with it and if so how ? Rupert

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Rupert van der Post BSc CEng MIET
 09 July 2013 08:52 PM
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jrvdp

Posts: 26
Joined: 10 October 2006

MrO - its old school for Regional Electricity Company - guess they don't really exist any more, but... best regards. Rupert

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Rupert van der Post BSc CEng MIET
 09 July 2013 08:56 PM
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MrOther

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Thanks for that, I'll slip it when talking with some of the older more knowledgably folk round the breakie table
 09 July 2013 09:01 PM
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jrvdp

Posts: 26
Joined: 10 October 2006

Older, maybe, but more Knowledgeable? Shame on you, Steve. Yank their beards hard, and pull their pointy hats over their eyes after tying their shoelaces together - you ought to be able to get to them somehow. At worst,butter both sides of a slice of toast and watch it hover 6 inches (152.4 mm) above the floor. All the best. R

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Rupert van der Post BSc CEng MIET
 09 July 2013 09:18 PM
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AJJewsbury

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Unfortunately we didn't get to the bottom of it at the time (probably nearly 20 years ago now (gulp!) - the problems of PME were less well understood at the time) - and we settled for swapping devices around until we found one that was sort of tolerable in that position. These days I think the usual suggestion is to loose the PME earth and TT the installation so that all the supply Ns become independent again - although I can appreciate that's probably not exacltly convenient in an exiting multi-occupancy building, espcially if the other occupiers aren't noticing the problem.
- Andy.
 09 July 2013 10:35 PM
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John Peckham

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Rupert


PM me your email address I may have a useful document for you?

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

http://www.astutetechnicalservices.co.uk/
 10 July 2013 09:10 AM
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jrvdp

Posts: 26
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Andy, Thanks for the input. I suspect TT would be a nightmare of leakage from variable speed drives and trying to get a low enough earth impedance. Overall about 1000A / 3ph of supply capacity . All the best. Rupert

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Rupert van der Post BSc CEng MIET
 10 July 2013 09:12 AM
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jrvdp

Posts: 26
Joined: 10 October 2006

John, I'm having hiccups with the PM system, so I've sent a hello email to info at astute technical. Best regards. Rupert

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Rupert van der Post BSc CEng MIET
 10 July 2013 09:12 AM
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Parsley

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UKPN now seem to have realised the potential issues with multiple PME supplies, and their latest earthing manual states the following.

Multi occupancy buildings

It has been common practice for many years to provide connections to individual premises from a PME system and for individual premises to be given a PME earth terminal. However this is no longer acceptable due to the issues outlined below:

The provision of a suitable end-of-main electrode which may be impractical, particularly in multi-storey buildings.

The problems caused by the flow of neutral current (refer to Appendix F) through the building steelwork.

Having to apply continuous ratings to steel wire armour on cables used for the building network to cater for diverted neutral current.

The need for equipotential bonding between the intake positions.
The following sections only apply to multiple occupancy buildings that have a steel-frame or shared metallic services. Where these criteria do not apply all services may be provided with a PME earth terminal, subject to the relevant planning standards.

App F
Neutral current diversion can occur within the structure of a steel-framed building (or a building with shared metallic services) supplied with multiple services from a combined neutral and earth network due to a broken neutral or unbalanced loads. The natural passage of neutral current through the structural steelwork can cause:

Magnetic field problems both close to the steelwork and at the source which may cause interference to VDUs etc.

Equipotential bonding conductors to carry neutral current resulting in overheating and consequential fire risk.


As Andy has commented the normal way to avoid the problems is to only allow one service to be connected to the PME earthing terminal and TT the others, which may not be acceptable to some consumers.

I was in a large new shed a few weeks ago in the SSE area, split into 2 tenants with separate PME (TNC-S) supplies, according to the EIC the main earthing conductor csa was 185mm2 and so was the bonding conductors, it made me wonder whether the csa of the bonding conductor (well above the 54.8 requirement) was a specific requirement of SSE to prevent overheating etc.

Regards
 10 July 2013 09:25 AM
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AJJewsbury

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My only other thought is that diverted N currents are a result of having a N current in the first place - if it's all 3-phase then in theory balancing the load across the phases will reduce the N current and hence the diverted N. Probably not that practical if you've a lot of single phase loads that can be switched independently or electronics etc that generate 3rd harmonics (hence some N current even when perfectly balanced), but might be worth a consider.
- Andy.
 10 July 2013 09:33 AM
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jrvdp

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Hi Parsley,

That is a very helpful document - thank you so much. I found the full text of it online at

http://library.ukpowernetworks...n+Earthing+Design.pdf

Section 7.8.5 is particularly interesting:

7.8.5 Existing Buildings

Wherever possible the requirements for new buildings detailed in Section 7.8.2 or Section 7.8.3 shall be applied to existing buildings, however, if the requirements cannot be satisfied then the requirements for multiple services detailed in Section 7.8.4 shall applied (sic)

I need to follow this down, somewhat.

Thanks again, all the best. Rupert.

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Rupert van der Post BSc CEng MIET
 10 July 2013 09:48 AM
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jrvdp

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Hi Andy,

The documents Parsley has suggested are really quite positive (see the link in my previous post, also the library at UKPN has a lot of other good stuff in it of which I was not previously aware - that's a couple of evenings "ruined" learning something new...)

UK Power Networks are pushing towards a proper engineered solution - nice to see that. I agree that the neutral current results from imbalance. The issue for me in a multi occupancy situation is that one doesn't have control of the other occupants' loads in any reasonable way. Its part of the reason TTing is'nt practical - as 20 odd other parties would have to sign up to have their installations messed around with.

Thanks for all your help. Rupert

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Rupert van der Post BSc CEng MIET
 10 July 2013 01:23 PM
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AJJewsbury

Posts: 11295
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Sometimes, just sometimes, I get this urge to start a campaign - CAMRE - the Campaign for Real Earthing - with the aim of getting PME replaced with a proper TN-S system.
- Andy.
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