Earthing a "Portacabin"

[Sidewinder]

Right,

Can't decide about this one.

Completed a PIR today on a "Portacabin" type building.

Located in a domestic garden.

Conctete base, water supply, fixed phone line installed.

Has been in place for 5 years and will hopefully be there for a long time to come yet.

Will need a road closure and a crane to lift it over the 2 storey house to get it out!

Brief desription of system.

Supply TN-S.

Hed>25mm tails>Henleys>25mm tails>KMF(40A) (& House board, in 25mm but this does not concern us)>25mm tails>NON DNO meter>25mm tails>Polymer enclosure with 40A 30mA instantaneous RCD>Polymer adaptable box>16mm SWA 3 core>DB in "Portacabin).

SWA is part buried part surface clipped.

Main earth 16mm.

Main bonding in house 10mm.

Water in "Portacabin" bonded to local DB in 16mm.

3 cores are LN&E.

Armour earthed both ends in 16mm to MET at one end & local board DB at "Portacabin".

Incoming water in copper.



Now I KNOW that the local DNO network in the area is PME.

HOWEVER the presntation to me is TN-S.

I have not checked the Z's at the origin I was only there to check the cabin out, perhaps I should have but after fitting the EM light & finding it was duff, that p*55*d me off a bit & I lost the plot!

Did all the tests on the cabin, all OK, Z's well low.

Could not isolate the house to do a Ze and this would have been very awkward and time consuming due to the layout.

Could NOT disconnect the DNO end of the earths as they were on a 951 clamp onto the sheath & the DNO guys round here are banned from touching these! To disconnect the 951 is a cable cut outside the property, remove the clamp install new means of earthing and reconnect mains!



Not overly bothered about the export, however, the "Portacabin" is a portable building, though not very in this case!



One thing is the make up of the building, inside all plasterboard walls, outside colour coated steel. This is very like a CARAVAN!



When I had my annual I discussed this with my AE as I had another PIR where by we had a PME supply and exported earths to external steel containers, however, these were within 2m of structural steelwork of the main building. There were other issues that had resulted in the containers being disconnected at the clients agreement. Hence why it was left like that. On the containers one, to split the earth system one would have conductive earthed parts touchable with 2 different earthing systems, so we decided that the local DB

 

[Andy™]

personally, i would rather TT it, but as TNS, its not against any regs, so you couldnt even give a code 4.

even though its TNS, nearest PME point may well be outside the property. i guess if you believe it to be fed from a TNCS even though supply is TNS at head, then you could give it a code 4 for that?

 

[Sidewinder]

Can't TT it that easily as it would mean changing around the cabin DB.

I am just PIR'ing really.

I'd have to give it an "unsatisfactory" then quote on any upgrades but I don't think it is that bad.

As far as the PME bit goes for the last 30 (ish) years, can't remember now all new DNO supplies have been PME no matter what the presentation is I have been told by one of the area engineers at our local DNO.

I'm leaning on the 4 due to the construction of the building like a caravan?

I've just pulled GN1, 5, 7 & 8 & the CDA earthing practice guide as well as the brb to try to get my head around it.

I have coded the single remote RCD a 4 (341.1) it is a real pain if it goes and the user would be highly inconvenienced.

 

[Larnacaman]

Can't really see what your conserned about, from the data you've provided here. Your comparing this installation with past jobs, that bare little resemblance to this set up.

You know and i know, ...That a rod isn't going to provide anywhere near the protection this portacabin style type building is enjoying with it's present set up. Sounds to me like, who ever owns and is responsible for this building, has gone the extra mile and supplied it with a decent and safe electrical supply. ....Leave it alone, all's good with this installation....

 

[ProDave]

The only thing I would check which I don't think you mentioned, is if the metal outer skin of the cabin is bonded anywhere? (if you are treating it like a "caravan" it should be)

As others say, it's been installed to a good standard. If it ain't broke.....

As it's set on concrete foundations, you could argue it's not portable. I once wired a garden shed set on concrete founds, which building control signed off as habitable accommodation. They clearly didn't consider it portable.

 

[Sidewinder]

Larnacaman,

I was not intending make any changes or to propose to pull the existing "earth" merely considering adding a rod at the cabin board as per 411.4.2.

The run is long and the cable well protected really.

My concern really centres about it being a "portable building".

I appreciate your suggestion that I am comparing it to another "job" however there are similarities in that they are both "portable buildings" they are both from PME like it or not even though this one is a TN-S presentation!

I suppose really I have "high integrity earthing" in a way as i have the armour & a core in parallel.

I've gone through the CDA earthing guide, & GN1 and nothing yet!

GN5 does have some ideas such as 411.4.2 I mention above.

My concern is the use of the cabin which I have not yet revealed I know & won't yet for a reason. Sorry guys.

Just going through chapter 14 of GN5 then onto 7 & 8 to see what they say.

Oh & yes they are the 17th versions.

I just could not decide, and working completely alone sometimes it is useful to bounce ideas off others, hence here, which then gets one thinking along a different track e.g. the parallel earthing bit I have just written which did not dawn on me until now.

Prodave,

From my OP:

Armour earthed both ends in 16mm to MET at one end & local board DB at "Portacabin".

 

[Larnacaman]

That's a good point Pro Dave....

Check to see if the outer skin has been bonded to the system if not, give a price to bond it up and all is well again.

By the way, never seen a Caravan with plasterboard internals, normally it's all thin plywood type stuff, ....well it was in my caravanning days!! lol!!!

 

[ProDave]

Prodave,

From my OP:

Armour earthed both ends in 16mm to MET at one end & local board DB at "Portacabin".

But is the metal outer skin of the cabin bonded anywhere, or just floating?

 

[Sidewinder]

Larnacaman,

It's not actually a caravan but it is "similar" construction.

Have just hit a gem in GN7, caravan requirements do not apply to static caravans, whish this is more similar to than a touring caravan, so that don't concern me any more.

721 - Scope, but this does direct me to 717 - mobile or transportable units.

Now it IS transportable!

There is a reason for this thinking out loud!

Thanks guys.

---------- AUTO MERGE Post added at 21:02 ---------- Previous post was at 21:00 ----------

717 out, 717.1 - scope "supply by means of, for example a plug and socket outlet"

This is hard wired with a buried SWA!

Getting there!

---------- AUTO MERGE Post added at 21:04 ---------- Previous post was at 21:02 ----------

Prodave,

Sorry I read your post as the outer metallic skin of the cable!

Doh!!!

 

[Larnacaman]

Sounds to me like it's going to be used as a residence of some kind, ...you wouldn't be going to all this trouble of going through all your codes, Reg's and whatever if it were an occasional workshop or the like.

Just seen your last post about the single RCD protection being inconvenient. This can be easily rectified by replacing the upstream RCD protection with an S type device and providing a local instantaneous RCD device....

Apart from this, ...i think you are privately quite satisfied with the present installation. Don't spend too much time on this one, trying to find a fault with it, ...Safety wise there isn't any, based on what you have told us .....

 

[Sidewinder]

No mate it's not a residence, I've been asked by the client to fully investigate the "requirements" after I quoted the job, when I was doing the site visit with them to another almost identical install.

I have built the "research" costs into the next job, I have the order number for that one, it's just I'm doing them on this job so that I don't end up with egg on my face.

Another clue, the client is not the home owner!

I know this is a little bit of a drip feeding of info, but IF I go the whole hog then you may be blinkered as I was, so I'm using you lot to help me think oustide the box.

Still on GN7 have cleared caravans, caravan parks, transportable units, swimming pools but there is a chapter in there I do need to consider & a document I need the client to give me tomorrow methinks!

Sorry guys & thanks!

 

[Larnacaman]

Transportable?? by means of a heavy duty crane that has to lift it over 2 story houses?? i think it's lost it's transportability 5 years ago when it was positioned in it's present location...lol!!!!

So .... what sort of dimensions are we talking about here for the portacabin???

Go ahead and think out loud, sometimes it's how we find the answers!!!!

 

[Sidewinder]

Thanks larnacaman,

That's why I'm posting it's a lonely life working completely on your own after being part of an engineering team when problem solving in the past!

Not quite as fixed as you may think there a 2 or 3 about to come out if possible.

Mind Mr Cameron et al. may have put a stop on that!

It's about 10' sq (ish).

 

[Larnacaman]

The only other thing i can think of, is some kind of temporary School building, but you say it's located in a domestic garden!!!!! lol!!!

Ah, ....maybe a LSD drug factory, ....or a Cannabis sun room!!! Hmmmmm, .....I think We'll wait until you think it's appropriate to reveal it's intended use

 

[Sidewinder]

Nope not a school building but it is owned by a public sector organisation!

Not trying to be awkward, but I don't want ideas clouded by its actual use, which is nothing illegal!

 

[sparkytim]

Hi

If it presents as TNS then i would not be concerned, however IF TNCS then how about this advise from IET

http://www.theiet.org/publishing/wiring-regulations/mag/pre-2004/pre14-PME-supplies-ESQC-Regs.cfm?type=pdf

 

[ADS]

[quote name='Andy

 

[steptoe]

right, my tuppence,

TNS is as you are perfectly aware Mr Sidewinder acceptable for exporting outside the EZ,

so if I understand this right you are asking for opinions concerning connecting it to a transportable building permanently.?

its a weird one this.

is the building bolted to the concrete base?

if not then I would still say it is portable, unless it is actually fixed in place, then you have the added hassle of it being hard wired and not via plug&play.

assuming it is bolted down then although transportable I would treat it as being a permanent but transportable building,

TNS is perfectly acceptable,

is there any reason preventing you from rodding the portacabin supply cable at both ends?

thereby NOT affecting the supply but keeping you portacabin at the same potential as the supply DB and EARTH at the same time.

just a thought,

I have done this a few times,

I will wait for you(or someone) to give me a reason why this is a big NO NO,

:coat

 

[Sidewinder]

Thanks Tim,

I'll pick up my copy of Cookies commentary on the 16th tomorrow & have a look through.

Which reminds me to check if the 17th one is out yet!

No use now with Amd1 coming so soon though!

However, having trawled the GN's I am pretty happy that it does not come under any of the Special locations in 7671.

My thinking was adding a rod local to the cabin, though it would really need to be down at about 20 ohms, & I'd need to get a 30mA into the local board with a 100 or a 300 mA TD at the other end then.

This has serious cost implications for the client if I get this "wrong" or I end up looking like a t**t!

 

[kme]

I`m pretty much with Albert on this one, mate.

You may KNOW the DNO supply is PME; but YOU have a TN-S - so TN-S is what you`re working with.......

I can`t see a viable reason for beggaring about with it; other than the inconvenient RCD, which can be fairly easily rectified, I would expect?

As for rodding - WHY? Is there a need or requirement? Not as far as I can tell mate.

For what its worth - my twopence worth.

KME