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Topic Title: Earthing for flats
Topic Summary: I'm having a thick day!
Created On: 10 June 2013 04:25 PM
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 10 June 2013 04:25 PM
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Avatar for dickllewellyn.
dickllewellyn

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Good afternoon All

I'm having a bit of a thick day today and I'm looking for some reassurance from my trusted bretherran.

I popped into some flats today built 12 years ago. There are four flats per floor, with cut-outs and meters situated in cupboards on each landing.

In typical house bashing style, the supply from the meter cupboard to the associated consumer unit in each flat is a 16mm T+E protected by one of those dinky little MEM 80Amp switch fuses about the size of a 20 pack of fags. My first complaint here is the single insulated cores outside an enclosure where the T+E terminates. I guess their argument at the time was that a locked cupboard creates an enclosure accessible only with the use of a tool.

The flats themselves have electric heating throughout, so there are actually 2 of these T+E submains in parallel. Both supplies are in the same dual tariff consumer unit, with both Earth wires terminated in the same terminal on the Earth bar.

Plumbing in the flats is entirely copper pipe with electric immersion, heating tapped off a copper riser which presumably is common to all the flats. This pipe is bonded in the flat with a 10mm jobbin. There are no other services entering the flats.

There is PME Earthing throughout the building.

I'm having a bit of a hard time thinking about these parallel T+Es, trying to decide if we have a 12mm Main Earth to each flat and weather everything is in order. I've scratched my little bald spot raw thinking this over, I think I've confused myself beyond all reason, so I decided to open it up to the forum while I have a brew, relax, and crack on with some other paperwork for a little while.......

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Richard (Dick)

"Insert words of wisdom and/or witty pun here"
 10 June 2013 04:40 PM
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AJJewsbury

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I guess first question is is there any bonding at whole-building level? (which might make the bonding in the flats good practice supplementary, rather than main) and where's the N-PE link? at each flat's cutout (TN-C submains) or at the main intake with TN-C-S submains?

those dinky little MEM 80Amp switch fuses about the size of a 20 pack of fags. My first complaint here is the single insulated cores outside an enclosure where the T+E terminates. I guess their argument at the time was that a locked cupboard creates an enclosure accessible only with the use of a tool.

I think you can get little covers for those for use when using T&E rather than separately insulated tails.

trying to decide if we have a 12mm Main Earth to each flat

Sounds plausible, although if it's acting as a PME main bond it could be carrying some significant current in normal service, which might affect the overall current carrying capacity of the 16mm2 T&E.

- Andy.
 10 June 2013 04:41 PM
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kj scott

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Originally posted by: dickllewellyn

Good afternoon All

I'm having a bit of a thick day today and I'm looking for some reassurance from my trusted bretherran.

I popped into some flats today built 12 years ago. There are four flats per floor, with cut-outs and meters situated in cupboards on each landing.

Are you intending to carry out additional work or EICR?

In typical house bashing style, the supply from the meter cupboard to the associated consumer unit in each flat is a 16mm T+E protected by one of those dinky little MEM 80Amp switch fuses about the size of a 20 pack of fags. My first complaint here is the single insulated cores outside an enclosure where the T+E terminates. I guess their argument at the time was that a locked cupboard creates an enclosure accessible only with the use of a tool.

Not acceptable to rely on a locked cupboard for this, unsatisfactory workmanship.Observation or note dependant on your remit

The flats themselves have electric heating throughout, so there are actually 2 of these T+E submains in parallel. Both supplies are in the same dual tariff consumer unit, with both Earth wires terminated in the same terminal on the Earth bar.

If the conductors originate from a common point and terminate at a common point, I don't see any real issue, but it is worth recording it either way.

Plumbing in the flats is entirely copper pipe with electric immersion, heating tapped off a copper riser which presumably is common to all the flats. This pipe is bonded in the flat with a 10mm jobbin. There are no other services entering the flats.

So bonding within dwellings is to standard?

There is PME Earthing throughout the building.

PME will surely end at the isolator and then TN-S?

I'm having a bit of a hard time thinking about these parallel T+Es, trying to decide if we have a 12mm Main Earth to each flat and weather everything is in order. I've scratched my little bald spot raw thinking this over, I think I've confused myself beyond all reason, so I decided to open it up to the forum while I have a brew, relax, and crack on with some other paperwork for a little while.......


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http://www.niceic.biz
 10 June 2013 05:09 PM
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dickllewellyn

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Hmmm..

There is a seperate cutout and meter for each flat with what looks like an ordinary PME terminal on the side. There doesn't seem to be a main intake as such, just a top to bottom riser with cupboards on each landing containing cutouts and meters.

I confess I didn't look for building wide bonding, I'll see if I can pop by in the morning for a look. Judging by the layout of the one flat I could enter today, and the riser cupboards I have seen, I don't recall seeing any common pipework.

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Richard (Dick)

"Insert words of wisdom and/or witty pun here"
 10 June 2013 05:25 PM
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AJJewsbury

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PME will surely end at the isolator and then TN-S?

The only way to get rid of PME is to TT the installation. In BS 7671's view it can only be TN-S if N & PE are separate all the way back to the substation's earth electrode.
- Andy.
 10 June 2013 09:37 PM
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leckie

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Well it's PME to each floor, then it's a sub-main, distribution circuit, to each flat. But x 2 because heating supply in parallel.

So 2x6sq.mm cpc's in parallel.

I suppose it's down to the design as to whether they are considered 12sq.mm and so could be considered a bonding conductor. A bit like having say a 6sq.mm swa and using a core as a cpc and tying on a 4 sq.mm parallel cable coloured green/yellow and saying "that's 10 sq.mm".

I must admit I would run in a separate 10sq.mm as a bonding conductor and say the cpc's are just that, cpc's. However, we must be careful critisising on EICR's so a few more opinions might be welcome.
 10 June 2013 10:06 PM
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Parsley

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The dno's have now realised that this type of arrangement is not ideal and would now
install just one pen cut out and run separate sne submains to each flat to avoid potential issues with diverted neutral currents where there are common metallic services or the building is steelwork construction.

regards
 10 June 2013 10:50 PM
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leckie

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So does that make it TNS at the flat?

It's pme at the incomer but TNS at each floor?
 11 June 2013 08:47 AM
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kj scott

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Originally posted by: AJJewsbury

PME will surely end at the isolator and then TN-S?


The only way to get rid of PME is to TT the installation. In BS 7671's view it can only be TN-S if N & PE are separate all the way back to the substation's earth electrode.

- Andy.


That's not what I meant Andy. If you have PME in the supply, the consumers part is TN-S; as ESQCR doesn't allow the consumer t o combine neutral and earth functions in their installation.
So we have the combination TN-C and TN-S, abbreviated to TN-C-S.

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 11 June 2013 08:53 AM
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kj scott

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Originally posted by: dickllewellyn

Hmmm..

There is a seperate cutout and meter for each flat with what looks like an ordinary PME terminal on the side. There doesn't seem to be a main intake as such, just a top to bottom riser with cupboards on each landing containing cutouts and meters.

I confess I didn't look for building wide bonding, I'll see if I can pop by in the morning for a look. Judging by the layout of the one flat I could enter today, and the riser cupboards I have seen, I don't recall seeing any common pipework.


So you have a reasonably short parallel 6mm² earth conductors, can't see a major issue with that as long as one doesn't get disconnected; so a note and a label perhaps.
Are there no landlords common parts to the building or accessible services?

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 11 June 2013 08:54 AM
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kj scott

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Originally posted by: leckie

So does that make it TNS at the flat?

No, TN-C-S

It's pme at the incomer but TNS at each floor?


So TN-C supply, TN-S installation = TN-C-S.

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 11 June 2013 11:13 AM
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leckie

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I have seen this exact set up recently. TN-C-S INCOMER into a distributed box then out as TNS to each floor with several meters to each floor. The supplier label each floor and the distributer box as SNE

My NICEIC assessor said it was TNS as my tails were connected to the SNE connections. Bit confusing!
 11 June 2013 11:20 AM
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AJJewsbury

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So TN-C supply, TN-S installation = TN-C-S.

I know "other standards" do refer to "TN-S installations", but that isn't logical in the BS 7671 context. The TN-whatever notation describes the overall earthing system, starting at the source, not just one downstream part of it. The first "T" refers to the connection at the source - so it's not logical to refer to just describe just the installation as "TN-S" as the source isn't part of the installation - the best you can really claim for the installation alone is the -S (or possibly N-S) bit. So I'd be happier describing system up to the supplier's cut-out as TN-C, the installation as "-S", so overall = TN-C-S.

The point being that the dangers of PME are present everywhere that's connected to a PME derrived earth - my worry is that describing an installation as "TN-S" might suggest that PME precautions could be ignored.

The ESQCR has no objection to a consumer using a TN-C-S system, provided the -C- bit is outside of the consumer's installation.

- Andy.
 11 June 2013 11:45 AM
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Parsley

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Personally I think it's TNC-S.

What's the difference between a large building with one intake and lots of DB's and a building with lots of individual consumer units in separate flats, all be it with individual cutouts and meters?
The earthing system is TNC-S PME. The DNO will probably want each flats water/gas service to be bonded and they will state must comply with BS7671 although this would actually require a conductor sized in relation to the neutral of the incoming supply to the building, so likely to be bigger than 10mm2. the meter operator won't realise this and as long as the 10mm2 in place will connect their meter. This isn't really protective equipotential bonding it's more like supplementary bonding re-establishing the equipotential zone in each flat.

Regards
 11 June 2013 12:44 PM
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leckie

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So the SNE label fitted on my job do not means its TNS then?
 11 June 2013 12:59 PM
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AJJewsbury

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So the SNE label fitted on my job do not means its TNS then?

I thought that SNE and CNE referred to the cable/conductor, rather than the overall earthing system - although I can't vouch for what the DNO had in mind. It might be just to reassure themselves that it's not TN-C to the flat, which has caused them problems in the past.

- Andy.
 11 June 2013 04:07 PM
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leckie

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I'm not sure to be honest. Separate neutral and earth seems like TNS?

On the job I described I put it down as pme, tn-c-s, but was told it was TNS. Now I'm not really sure! Keith seems to be certain it's still pme.

On reflection I think it is still pme as you still have all the associated risks and, you say Andy, the SNE bit is basically just descriptive for what happens between each floor.

Edited: 11 June 2013 at 10:00 PM by leckie
 11 June 2013 09:46 PM
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kj scott

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Originally posted by: AJJewsbury

So TN-C supply, TN-S installation = TN-C-S.


I know "other standards" do refer to "TN-S installations", but that isn't logical in the BS 7671 context. The TN-whatever notation describes the overall earthing system, starting at the source, not just one downstream part of it. The first "T" refers to the connection at the source - so it's not logical to refer to just describe just the installation as "TN-S" as the source isn't part of the installation - the best you can really claim for the installation alone is the -S (or possibly N-S) bit. So I'd be happier describing system up to the supplier's cut-out as TN-C, the installation as "-S", so overall = TN-C-S.

Andy have a look at fig 3.9 which describes the system as a combination of TN-C and TN-S.

The point being that the dangers of PME are present everywhere that's connected to a PME derrived earth - my worry is that describing an installation as "TN-S" might suggest that PME precautions could be ignored.

The overall system is still derived from a PME source so would be descibed correctly as TN-C-S.

The ESQCR has no objection to a consumer using a TN-C-S system, provided the -C- bit is outside of the consumer's installation.

That is what I said, neutral and earth can not be combined within the consumers installation.

- Andy.


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 11 June 2013 09:58 PM
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kj scott

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Originally posted by: leckie

I have seen this exact set up recently. TN-C-S INCOMER into a distributed box then out as TNS to each floor with several meters to each floor. The supplier label each floor and the distributer box as SNE

My NICEIC assessor said it was TNS as my tails were connected to the SNE connections. Bit confusing!


It sounds like TN-C-S with some misleading labelling; I had something similar a month or so back, after some discussions with the supplier and an additional neutral earth joint it was relabelled PME.

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 11 June 2013 10:02 PM
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leckie

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Now that makes sense.
IET » Wiring and the regulations » Earthing for flats

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