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

 

[Devonchris]

You may find that even with overhead supply there's a pme. A lot round here have.

 

[PEG]

Hi,just a quick glance,and i am afraid the SWA is not rodent proof,if they are an issue,they will open the armouring to corrosion everywhere they nibble. Maybe consider some containment of a galvanised variety?

 

[SparkyChick]

Hi,just a quick glance,and i am afraid the SWA is not rodent proof,if they are an issue,they will open the armouring to corrosion everywhere they nibble. Maybe consider some containment of a galvanised variety?

I have considered conduit, I think the price may put them off. To be honest, she mentioned a rodent problem (and there was some evidence a cable being damaged near ground level), but around the barn, the cables already there (not connected) are T+E and I saw no other evidence of meeces, so I'm fairly happy with the choice of SWA. I wouldn't be using the armour for earthing (I prefer not to) just for the additional mechanical protection.

 

[SparkyChick]

You may find that even with overhead supply there's a pme. A lot round here have.

Yeah, I'm seeing overheads converted to PME here too, but 705.411.4 states TN-C system shall not be used (705 is relevant because there are animals present), so unless I'm misunderstanding this regulation I'm required to use TT earthing for this work.

 

[Taylortwocities]

What will the barn be used for? Any possible livestock involved?

 

[SparkyChick]

What will the barn be used for? Any possible livestock involved?

There is a yard with chickens, two pygmy goats, some guinea fowl, a couple of donkeys in the field next door, so I think yes

It was the old milking barn, still got some of the old machinery in there.

This is why I was referring to section 705.

 

[Devonchris]

Hi
I wasn't considering the full implications of your design. Just a general observation on overheads and pme. I'm sure there's no misunderstanding on your part.
Have fun.

 

[SparkyChick]

Hi
I wasn't considering the full implications of your design. Just a general observation on overheads and pme. I'm sure there's no misunderstanding on your part.
Have fun.

That's why I'm asking though... I'm not sure whether my interpretation (and thus my proposed solution) is correct.

In the space of a week, I've got two jobs come up, one on a working farm and the other on this small holding, where I've got the issue of earthing to seriously consider so I'm just wanting to double check my thinking.

 

[ED17]

I've just done a similar job, installed a rod and bonded the structural steel.

Spoke to the bloke in charge of installing the cut out who made it very clear that I was not to use the PME terminal. Then a man came to install the meter and refused to do it because there was no earth in the PME terminal!

 

[westward10]

Yeah, I'm seeing overheads converted to PME here too, but 705.411.4 states TN-C system shall not be used (705 is relevant because there are animals present), so unless I'm misunderstanding this regulation I'm required to use TT earthing for this work.

You can use TT, TN-S or TN-C-S but certain requirements are needed should the latter be employed and some suppliers may not provide TN-C-S for agricultural sites. TN-C is the only system not to be used and I have yet to come across this in the UK.

 

[SparkyChick]

You can use TT, TN-S or TN-C-S but certain requirements are needed should the latter be employed and some suppliers may not provide TN-C-S for agricultural sites. TN-C is the only system not to be used and I have yet to come across this in the UK.

Then I was misunderstanding that as I was assuming it was a short form for TN-C-S... and I must admit I did ponder what other TN-C-? it could have been other than TN-C-S, so I've learned something today

But on a serious note, have been doing a bit more reading about this I don't have a suitably sized earthing conductor that I can connect to the MET at the incoming supply, regardless of earthing system in use, and getting one from one corner of the house to the other will be a problem.

So, I'm going to install a rod (or use the metal structure of the barn if it is in good contact with the earth) and create a TT earthing system for the external electricals, which will include bonding the metal work of the barn.

 

[westward10]

TT is probably the best option saves a lot of "what ifs".

 

[Devonchris]

The "certain requirement" for using a TNC-S could be rather expensive.

This is taken from :

Electrical Contractors guide to wiring Regs 17th edition. (Does not include amendments)

If a bonded metal grid cannot be laid in the floor of animal houses during construction or refurbishment, it is recommended that a TN-C-S (PME) electricity supply is not used. A TN-S supply is preferred. If a TN-C-S supply is offered or is the only type available, it is recommended that a separate local earth electrode be installed.

I think your ok with your design but don't take my word for it.

 

[Wilko]

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.
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