Earthing a metal container

[Feel the power]

Evening all, I was asked to price a job today which was putting power into a metal container, the sockets and lights would be installed using plastic conduit as the feller doesn't like galvanised conduit, the supply is already there which is a 10mm twin and earth on catinery and this is fed directly from the house which has a Tn-c-s supply, I am going to install a metal clad cu with rcbos as he doesn't want them all tripping at the same time, all I'm wondering is will it be accepted to install a TT rod on the outside of the container and connect this to the MET on the cu in the container any help would be grateful and much appreciated Thanks

 

[securespark]

Has the container got a water or gas supply via a metallic pipe?

 

[Feel the power]

No it hasn't got anything it's just going to be used as a workshop so just power and lights to be installed also should mention the container is about 10 meters away from the house

 

[Iggifer]

T+E on catenary wire.....I'd walk away if I'd be asked to look at that job

 

[Feel the power]

I know tell me about it not the best but no way of changing it unfortunately, and I didn't install it so that's another query

 

[flameport]

An earth rod will do very little, considering you already have a giant metal box in direct contact with the ground.
RCBOs are good - but only if the supply from the house does not already have an RCD. Even if the supply is just via an MCB, there will still be a problem of discrimination for short circuit faults.
10mm T&E will have a CPC of smaller size, which will be no use for equipotential bonding on a TNCS supply.

The overhead T&E will need to be replaced, and this should be priced as part of the work. Even without the cpc size problem, T&E hanging outside is not suitable.

Another consideration is that metal sheds / containers are magnets for condensation, and the interior is likely to be dripping wet at certain times of the year. Equipment installed in it should be rated accordingly.

 

[Feel the power]

I understand where your coming from, I wasn't particularly glad when I seen the t+e hanging but it was all ready installed prior to me going to the job and the path underneath has just been nicely tarmaced so no chance of digging that up, what are your recommendations with what I've got to work with

 

[Iggifer]

1. Replace the T+E with a suitable cable
2. Don't do the job

It's you that's going to be responsible for that cable once you connect on to it. How high up from the floor is it?

 

[JohnW2]

An earth rod will do very little, considering you already have a giant metal box in direct contact with the ground. ... 10mm T&E will have a CPC of smaller size, which will be no use for equipotential bonding on a TNCS supply.

This seems like an interesting one. Without commenting on the issue of the overhead T+E feed, the container (assuming that it is sitting on the ground) presumably counts as a giant extraneous-c-p, which therefore needs to be bonded to the container's installation. If this were an 'ordinary' outhouse which had extraneous-c-ps, one of the options we would be suggesting would be to not export the TN-C-S earth to the outbuilding but, instead, to TT the container with a local earth electrode. However, in the case of the container, as you say, a rod might be pretty superfluous if the container were in contact with the ground. In that situation, if the measured resistance to ground of the container were low enough, would it be acceptable to regard it as the 'earth electrode' and therefore not bother with a rod? I suspect that one problem might be that, under dry conditions, the 'resistance to earth' of even a giant metal box just sitting on the surface might be quite high.

Kind Regards, John

 

[Feel the power]

It's a good 7 meters high well out the way of anything and nothing can pass under it

 

[Feel the power]

Thanks for your reply John, it it worth installing the rod just as precaution incase of dry spells and surly if the rod is installed it would create a easier earth path if a fault was to occur within the container?

 

[JohnW2]

Thanks for your reply John, it it worth installing the rod just as precaution incase of dry spells and surly if the rod is installed it would create a easier earth path if a fault was to occur within the container?

I agree. I was really responding to flameport's suggestion that the earth rod would/might be superfluous. However, I would think that it would represent a more reliable connection to earth than would a container which could be sitting on bone dry ground.

If you do TT the container, you need to be careful to keep the TN-C-S earth well away from it - i.e. don't connect anything to the CPC of this increasingly infamous T+E feed!

Kind Regards, John

 

[Feel the power]

Thanks John, just wondering tho how would you suggest of keeping the TT away from the CPC of the incoming supply as the CPC of the t+e feed would need to be connected to the MET of my new metal clad board or am I going in the complete wrong direction here

 

[JohnW2]

Thanks John, just wondering tho how would you suggest of keeping the TT away from the CPC of the incoming supply as the CPC of the t+e feed would need to be connected to the MET of my new metal clad board or am I going in the complete wrong direction here

If you were TTing the container, you would not connect the CPE of the T+E feed to anything in the container (and you would make sure that it could not be touched). Only the local TT rod, main bonding to the container and the connection to the local CU would be connected to the container's MET.

... unless someone disagrees with me, that is

Kind Regards, John

 

[Echo the husky]

I'll agree with you John! Was half way through typing the same reply!