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Topic Title: Metal portacabin and PME (following on from TT obession thread)
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Created On: 05 August 2014 10:05 PM
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 05 August 2014 10:05 PM
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Smith249

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Joined: 09 September 2004

Following on from other thread I am currently installing feeds to 2 new big metal portacabins. Although they are portacabins, they are expected to be in place for the next 20 years and are brand new so the installation is all being done as if it was permanent.

Both cabins have an IP67 32A C form inlet already on the side of them.

I planned to run a 35mm 2 core SWA from the incoming panel board (armour earthed at source) some distance away (incomming supply is TN-C-S) to a local IP65 DB on the wall of the main building just by the portacabins and take 2x 6mm SWAs from there to the cabins by it on 32A breakers from the IP65 DB.

I did plan to put a 100mA Type S RCD in the IP65 board and TT it at that board, not connecting anything to the load end of the main SWA earth.

However looking round the big site today they have another block of 8 portacabins on the other end of the site., these have been wired very neatly but the PME earth has been used.

Just made me wonder if there is any point TTing it, there is incoming water to the portacabins, but everything is plastic.

Thanks.



 06 August 2014 10:34 AM
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davezawadi

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The only thing I would do is add some earth rods near the cabins to keep the cabin to actual ground potential as low as possible. Your equipotential zone is extended to the cabins,which is quite acceptable. Perhaps the next BBB (after BGB) will require TT of metal clad buildings!

-------------------------
David
CEng etc, don't ask, its a result not a question!
 06 August 2014 11:56 AM
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AJJewsbury

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Perhaps the next BBB (after BGB) will require TT of metal clad buildings!

The DNO might already - e.g. from NPG IMP/010/011

"A PME earthing terminal may be provided for use in mobile hom es or temporary site offices which are constructed so that a person in contact with the general mass of earth cannot touch any metalwork of the home or site office which would be connected to a PME earthing terminal. Such mobile homes or site offices may be treated in the same manner as permanent premises and provided with a PME earthing terminal if the installation meets the bonding requirements of the relevant British Standard.

Where the mobile home or temporary site office has exterior metalwork and cannot comply with the above touch criterion, a PME earthing terminal shall not be provided. Supply should be given through a residual current device, and an independent earth electrode sited at least 3 metres away from any PME earth electrode should be provided. Both of these items are the responsibility of the customer."

- Andy.

(edited to remove link that seems to upset the Forum software)

Edited: 06 August 2014 at 01:28 PM by AJJewsbury
 06 August 2014 12:46 PM
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sparkingchip

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Originally posted by: davezawadi

The only thing I would do is add some earth rods near the cabins to keep the cabin to actual ground potential as low as possible. Your equipotential zone is extended to the cabins,which is quite acceptable. Perhaps the next BBB (after BGB) will require TT of metal clad buildings!


Metal clad buildings on a steel frame generally have steel stanchions set in the ground or anchored to steel work in the ground creating sound earth electrodes.

With metal cladding get on a dry timber frame the cladding can get nice and lively if a electrical fault develop's.

Hence if you Portocabin jack legs stand on nice dry pads you could have a problem.

I believe if you add a end of line PME earth electrode it needs a conductor to match the neutral and be at a 100 ohms or less?

Andy
 07 August 2014 05:15 PM
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davezawadi

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And which of these seems applicable to the described situation Andy?

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David
CEng etc, don't ask, its a result not a question!
 07 August 2014 05:23 PM
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AJJewsbury

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And which of these seems applicable to the described situation Andy?

The OP's description of "big metal portacabins" (I'm picturing an adapted shipping container kind of thing) seem a pretty close match to a "site office" that "has exterior metalwork" to me and the situation described wouldn't seem to fit "that a person in contact with the general mass of earth cannot touch any metalwork of the home or site office which would be connected to a PME earthing terminal".
- Andy.
 07 August 2014 09:08 PM
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sparkingchip

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Regards the discussion Metal portacabin and PME (following on from TT obession thread).

Following on from other discussions currently in progress, as this sounds like a large site with a meaty electrical supply, what size earth conductor to you need to the cabins and how does this compare cost wise to making the new cabins TT?

Andy
 07 August 2014 10:18 PM
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UKPN

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PME for steel portacabins! I like it.

You couldnt make it up!

Regards
 07 August 2014 11:14 PM
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sparkingchip

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I was going to point out that David wants a electrode installed regardless of whether it is PME or TT, so you may as well add the RCD upfront and TT it anyway saving messing about running dirty great big earth conductors about the place.

Apart from that steel sheds on legs shouldn't be PME anyway and I'd question why the other cabins are.
 08 August 2014 12:04 AM
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mapj1

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Well one might look at
2012 draft Guidance note 12 version 4


Which contains, on page 23
6.2.3.4 Supplies to other temporary buildings
For temporary buildings, eg temporary classrooms, a PME earthing terminal may be supplied if the installation is constructed so that a person in contact with the general mass of earth cannot touch any metalwork of the temporary installation and the installation complies with the bonding and earthing requirements of BS 7671. In such cases, a temporary building may be treated in the same manner as a permanent building. A PME terminal shall not be offered for a temporary building which is not constructed as above (e.g. metalclad buildings) .


My bold.
This was not always the case however -
in the early version of G12
Version 2-1982

the text contained no such explicit restrictions.

So you might think collective experience has made it seem less wise now than it was considered back in the beginning of PME then

In most cases it will be fine either way, lost neutrals are are still mercifully rare - I would not suggest to change what is already there for example.

-------------------------
regards Mike
 08 August 2014 07:55 AM
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Smith249

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Thanks for all the helpfull replies. UKPN it would be so nice to have some actual contribution from you rather than your usual sarcky comments, I'm sure there is a lot of advice from a DNO perspective you could actually give us if you tried.

I've decided to TT it based on points here, rather have it right.

Thanks again.
 08 August 2014 10:41 AM
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nad

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Hi Andy, may I ask where that PME electrode regulation comes from?
These threads have had me wondering if you could use both the PMEed earth and a local electrode in combination (with or without RCD).

Nad
 08 August 2014 12:12 PM
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sparkingchip

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Mike's the librarian and has the links to the DNO manuals.
 08 August 2014 01:04 PM
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mapj1

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For what its worth the Engineering guidance notes have a long pedigree, going back to the days of a largely nationalised industry and the board of trade and the electricity boards. They were a short libraray of information that ensured among other things that the various area boards were more or less singing off the same sheet technically.

There was a lot of good fundamental measurement and test type work done at the Central Electricity Reseach Labs at Leatherhad which informed many of the early notes- sadly now all defunct, and many perfectly good folk pensioned off early or made redundant.
I suspect indirectly a lot of standards writers in other countries used to benefit from this research as well.

The area boards have morphed into today's DNOs, but it's a much looser structure, and as often as not driven by consideration of commercial rather than pure techical advantages - this sometimes leads to quite divergent policies between areas - hence the different manals, an increasing number of which are in the wild on the web, some of which are available to anyone on request and some that seem to be guarded like the crown jewels and need a freedomn of information request to elecit them..


Luckily the notes are so useful that they are being kept more or less up to date.
Of course the old way was not very efficient by modern standards, but I cant help but wonder what some of the old sweats would make of the current situation, and at least there was a central organisation you could negotiate with for a national policy - at least if your organisation had enough clout.
I'm reminded of the Kissiger comment about diplomacy being easier with the old cold war Russia - now not being able to call any one person to represnt Europe..

Ofgem does not really fulfil the same role at all, although we are still a lot more 'together' in an electrical regulation sense than a great many wilder parts of the world.

I have no formal library of such docs but I do try and keep stuff that will be useful, and also maintain links to the ones that are on the web, and these I have no compunction in linking to or quoting if it seems germane to the discussion.

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

Edited: 08 August 2014 at 01:14 PM by mapj1
 08 August 2014 02:40 PM
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OMS

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Ofgem does not really fulfil the same role at all, although we are still a lot more 'together' in an electrical regulation sense than a great many wilder parts of the world.


Energy Networks Association covers it, Mike - they still publish technical specifications (ENATS) and engineering ecommendations (ENAER) plus the Guidance (ER documents) - eg G59

They are effectively what the Electricity Council was to the Area boards and CEGB

Regards

OMS

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