Portacabins and Earthing

[il78]

Evening all, looking for a reasurance hug really..

Carried out an inspection on a commercial installation having two metal containers with a 4.0mm SWA supply.

The installation within thses cabins was poor and needs to be redone or disconnected completely.

I'm presuming that the sytem type is TNCS

Had a quick look into exporting earths and it seems to be mixed audience out there on what should be done.

For me I'm looking at doing these cabins TT or with the installation to these cabins being so poor do I upgrade the supply cable to them and tie the earth into the earthing terminal?

Also there is a metal perimter fence about 2feet away from the cabins, should I also be providing an earth to this?

Be gentle I've been away for sparking for a couple of years..

Cheers Ya'll...

 

[17thman]

Hi.

Why are you presuming it is a tncs?

 

[il78]

Sealed unit and I haven't enquired.

 

[ricicle]

If it is PME then it will be necessary to ditch the exported earth in favour of a TT set-up.

 

[RF Lighting]

The first thing you need to do is find out for certain what the supply type really is. Untill then theres no point us just guessing what might or might not be the correct soloution.

 

[il78]

Ok it's TNCS

 

[RF Lighting]

How come you didn't know that half an hour ago?

 

[il78]

I had a little nap and it all came rushing back in a dream..

 

[RF Lighting]

Don't waste our time.

 

[il78]

Grow up...

I asked a question based on the situation being a TNCS setup...if you didn't want to answer then keep your nose out...

Anyway thanks ricicle for you confirming what I wanted to know..

 

[RF Lighting]

But you don't actually know that to be the case, and until you confirm what the supply type is, you are just wasting both ours and your time.

Sorry if this isn't the answer you wanted to hear, but you absoloutely can NOT design electrical systems by supposition and guess work.

 

[il78]

So you've got TNCS and TNS earthing arrangements and you wouldn't be able to design an installation based on them two scenarios, not even erring on the side of caution.

So "erring on the side caution" I've presumed that the earthing arrangement is TNCS because this woud be worse case for the portacabin setup.

Even if it was a TNS setup I could still convert the portacabin supply into TT system, could I not?

And for designing installations by guess work...there the jobs I look forward to..

 

[Adam_151]

Have you got a pitcure of the service intake for the building?

If its three phase and you have just a black sheathed pvc/xlpe cable comming up into the sevice position with no separate earth then its almost certainly TNCS, as I do no beleive any version of wave-form cable exists with a separate earth inside it.

If its older lead and paper, then the TN-S if fitted is normally obvious as you can see the earth sweated to the sheath.

It can get a little more complicated if the 11kv transformer is on site, but most of the time its TNS (depending how you want to class PNB )

 

[ericmark]

My first thought was we are looking at Section 708 Electrical installations in caravan / camping parks and similar locations. But then looked at SECTION 717 MOBILE OR TRANSPORTABLE UNITS which seemed to better fit.

However 717.411.4 TN System and 708.411.4 TN system are very similar and it would seem TN-C-S is not going to be permitted.

TN-C-S system. A system in which neutral and protective functions are combined in a single conductor in part of the system. This is a problem since as the user we often don't have a clue as to what system the DNO is using away from our own head.

Now to feed a caravan on a site where fire regulations mean 6 foot between units with a TT supply is not really a problem. But both with boats and Portacabins the distance between units can be a lot less than 6 foot so some one could touch the metal shell of two units at the same time.

With boats the water makes a reasonable earth bond between units but with Portacabins it could be quite dry. So one could easy have two adjacent units both with phase to earth faults on different phases but still not enough current to trip a RCD.

I actually had this working in Algeria I will admit in UK normally not dry enough but still possible. It's all down to the earth spikes being maintained and the problem was the 32A three phase and earth sockets were not tested frequently enough to realise there was an earth fault before some one had a shock. It was back in 1980 so 100 mA RCD's if any.

So if we go for TT and not only knock in an earth rod for each unit but also link the units together. However we then produce rather a good earth mat between all the units. Now we have a different problem. The earth is too good. So a fault on the DNO supply can cause huge currents to run. I have seen 4mm earth cable melted as a result of a really good earth.

There is really no one size fits all answer. One must look at the installation and decide what is the most appropriate method for that installation. In the main we used WMDU with an earth rod (in fact always two so if any one damaged there was a second) and RCD often 1A at 500 seconds which in turn fed the cabins with a TN-S supply from the WMDU. I am not saying this is the correct method but it was used on many sites and we were never told off for the method used.

Larger sites we were guaranteed a TN-S supply as on out own dedicated step down transformer.

717.411.4(i) would have applied to all the sites I worked on so I suppose we could have used the supplied earth. With the amount of muck shifting on large sites earth rods are not really to be trusted. If I could get away without using TT then I would.

I would forget the BS7671 and use some common sense and a risk assessment as once one has separate earth cables one has a chance that earth is lost without a loss of supply.

Do keep records of RCD and earth rod tests to show you took reasonable care.

I don't know who decided on the value but where we installed earth rods before we had a supply 8 ohms was the figure we were told was required. I have needed 8 x 1.2 meter earth rods to get that reading another good reason for earthing WMDU rather than each unit.

Oh WMDU = Weatherproof Main Distribution Unit big orange boxes with 300A moulded breaker and adjustable RCD two knobs one for time other for current. We could easy connect 30 units to each WMDU.

 

[il78]

Adam it is a sheathed PVC coming into the cutout with no separate earth, however you never what the LSA have done elsewhere...

It's my own fault for writing "presumed"

So that in mind, let's take for granted that the system is a TNCS.

My preference would be to ditch the exported earth and make the cabins a TT setup.

By the way these are metal containers/cabins.