Earthing for an outdoor install in a barn

[SparkyChick]

Hi all,

I'm in need of a bit of clarity on this decision.

So, long story short... have been asked to quote for the installation of some electrical services. Property is a small holding, my bad, but didn't check existing earth as there was a lot of items discussed (given it's location, I suspect it's TT as it's an overhead supply), but key ones are provision of various bits outside in a barn and a shed. Barn is attached to property, shed is not. So, I'm planning a small plastic CU inside a weatherproof steel cab inside the barn, fixed wiring will be SWA to try and reduce rodent problems.

My plan is to provide an earth in the vacinity of the barn by way of a rod or structural steel that penetrates the ground (if there is any, not sure there is). If the structural steel doesn't penetrate the ground, I'll be bonding it anyway. Thus creating a TT installation for the outdoor power.

Supply to barn is from main CU by way of a 30mA RCD protected circuit, so I don't need to worry about localised RCDs. I know it would be preferable to have the RCDs local to the board, but the cable is already in place, it's T+E and runs through the building so I'm inclined to leave the RCD at the source.

Am I on the money or way off? If you guys need more info, please let me know and I'll be happy to provide

Thanks

 

[Richard Burns]

TT earthing for the barn sounds like a good plan in this case.
Remember to ensure that all extraneous parts are bonded and consider if supplementary bonding would be useful for reduced touch voltages if there is a lot of metalwork in the barn.
I would read and apply section 705 for this installation, even though it is not too onerous.
Keep cables high and out of reach where possible and consider appropriate protection if they are in reach of animals.

 

[SparkyChick]

I like the TT solution for remote agricultural buildings. The barn is attached to the dwelling and so it should have a metal containment for the CU as you say. But there's a 5 way IP rated metal one from BG that's good value, if it was big enough.
Bg - Search Results | Screwfix.com - https://www.screwfix.com/search?search=BG

View attachment 38887

See, I was planning on separating the barn and out house/shed circuits (which means 4 in total). There will be a buried SWA segment linking the two buildings and I thought I'd separate the circuits to allow that segment to be turned off completely in case of a fault whilst maintaining the services in barn. If I combined lighting circuits I could make do with that CU, but then there is no room for expansion.

That's where the cost is coming from as I'm looking at a small customisable plastic CU inside a metal weatherproof cabinet.

I did actually consider two of the BG units... one for the main switch for this section and then the other for the OCPD, but I think that looks a little bad.

If anyone knows of a nice cheap 6 way weather proof CU that is Amd 3 compliant, I'd love to know about it

 

[The Ghost]

If the structural steel doesn't penetrate the ground, I'll be bonding it anyway.

I think I may be being a bit thick here but why would you bond it? So how do the supply to the (three?)other sheds work? Are you daisy chaining it or supply to each from main supply? Sorry if this is all obvious.

 

[SparkyChick]

I think I may be being a bit thick here but why would you bond it? So how do the supply to the (three?)other sheds work? Are you daisy chaining it or supply to each from main supply? Sorry if this is all obvious.

I think what I should have said, is I'll be doing some checks to establish if the metal work is considered extraneous and bond it accordingly. There is metal work in the barn that will need to be evaluated, old equipment and such like.

The supply goes like this:-

SUPPLY -> HOUSE CU -> T+E -> ATTACHED BARN

From the barn, it's a short hop (about 2m) to a detached shed/out house where they want some lighting, power and an outside socket for a pond.

Currently there is a T+E in place and terminated at the CU, it's just not turned on which is a good thing as the other end is just coiled up with a bit of PVC tape over the end from what I could see.

The plan is to put a CU in the barn on the end of the T+E and run SWA to the other places... 1.5mm 3 core for the lighting, 2.5mm 3 core for the power with a 2.5mm 4 core for the hop to the shed/out house so I'm only running one cable.

 

[The Ghost]

So if the Barn or outhouse develop an earth fault and you are relying on the CU RCD in the house then all the house goes out with the outside buildings if I am understanding this correctly?

 

[SparkyChick]

So if the Barn or outhouse develop an earth fault and you are relying on the CU RCD in the house then all the house goes out with the outside buildings if I am understanding this correctly?

It's a split load board and I agree, it's not ideal... but the cable is T+E and it runs an unknown route through the house, so to be sure it complies, it really should be RCD protected at the origin.

I could convert the board to include a non-RCD protected section and put the barn supply on an RCBO, but that just gives me the chills.

Rock and a hard place is my view.

 

[SparkyChick]

I should add I could also add a stand alone board, but there is no room for that.

 

[The Ghost]

So I was thinking of a stand alone supply with discrimination at the supply and TT at the barn and on. Keeping it separated from the residential. But then I suppose it depends on what your supply is as well. I guess along with you it is TT.

 

[Richard Burns]

I but the cable is T+E and it runs an unknown route through the house, so to be sure it complies, it really should be RCD protected at the origin.

If the earthing is TT then all circuits need to be protected by an RCD

Just out of interest what size T&E is supplying all these various loads required? Just wondering about max demand and MCB sizing.

 

[SparkyChick]

So I was thinking of a stand alone supply with discrimination at the supply and TT at the barn and on. Keeping it separated from the residential. But then I suppose it depends on what your supply is as well. I guess along with you it is TT.

Ideally, I'm with you a separated supply would be grand, but there literally is no room.

If the earthing is TT then all circuits need to be protected by an RCD

Just out of interest what size T&E is supplying all these various loads required? Just wondering about max demand and MCB sizing.

The T+E that runs to the barn looks to be 16mm, from the looks of what's in the barn it used to feed the milker. In terms of capacity, we're looking at two LED bulbs, three 5ft twin fluorescent battens, tumble drier and pond pump.

 

[The Ghost]

I suppose I would at least suggest emergency lights so they can see when everything goes out then. But what about RCBO residential sockets and main switch 100ma then RCBO outside? Oh just re-read and see its a split load maybe that wouldn't work then.

 

[SparkyChick]

I suppose I would at least suggest emergency lights so they can see when everything goes out then. But what about RCBO residential sockets and main switch 100ma then RCBO outside? Oh just re-read and see its a split load maybe that wouldn't work then.

No, it's a royal pain in the posterior. Personally I wouldn't have put the barn on one of the split load RCDs, but then I wouldn't have installed the supply using T+E either. But based on the fact it's red/black I'm guessing it was a farm install, so whatever worked.

 

[Richard Burns]

The T+E that runs to the barn looks to be 16mm, from the looks of what's in the barn it used to feed the milker. In terms of capacity, we're looking at two LED bulbs, three 5ft twin fluorescent battens, tumble drier and pond pump.

Sounds like you have enough csa there!
I was a bit worried it was 2.5mm T&E and they wanted full power supply in barn for lots of equipment and then even more stuff in the outhouse.
Sounds like it is almost going to be reasonable.
Not the ideal situation but workable and compliant.