TT in a barn..

[grumpyjohn01]

Have been asked to 'sort out' the wiring in a largish collection of buildings of varying construction on a farm . There is a separate supply with a DB all covered by 30mA mainswitch & TT earthing. Most is straightforward as it is within the confines of the main building , a steel framed barn however there are lights etc in various separate but built up against buildings and they have asked for lights and a socket in a new wooden building separated by a metre or 2. For this was intending to run a 6mm radial in swa on a B20 (have worked this out should be fine) and then put a few lights on FCU. What is the best earthing approach for the 'separate ' buildings? Stake down but connect CPC's to form a network or separate TT earthing for anything outside the main barn? The latter approach would be more awkward as I don't intend on fitting any further DB's if it can be avoided. also there is a solar PV install on the roof, there is a stake associated with this (I assume on DC side) which has been clumsily shoved in right next to new wooden building. Any advice appreciated. cheers John

 

[bigbaddave]

Google Exporting PME supply.. There weillbe 1000's of leads to this forum..

 

[Guitarist]

Can I just asked how you came to 6mm swa on a B20 breaker, when the building is only a metre or 2 away?

 

[grumpyjohn01]

were it exporting PME I would be very clear on what to do!! however it is TT 'networked' as it were that I am less clear on. thanks tho....

 

[grumpyjohn01]

Sorry not clear on that one, the new building is at the far end of the barn from the incomer/DB so the total run to the new socket would be approx 75m

 

[Engineer54]

If you want to do things properly, Install small CU's at each of the buildings with at least one rod position connected to each of them. You don't separate TT systems, as there is generally little point in doing so, the more rod positions you have, the lower the overall Ra value will be!! They can be interconnected via a separate core in the SWA, or via the steel wire armour of the sub-main distribution cables back to the MET. If the latter, ensure that the PVC outer sheath of the SWA cable(s) is unbroken, or corrosion will quickly set in, especially in a farmyard environment...

I'd also look into that main switch that only has a 30mA RCD covering all circuits. Look into providing a 300mA S type up front RCD and standard arrangement 30mA CU/DB's


I don't think much of your FCU route to supplying electrical installations in a working farm situation.

 

[grumpyjohn01]

Thanks! exactly the reply I needed! I doubt I can get them to change the DB as it is newish and serviceable and the 30mA covers all bases. On a fresh install your approach is obv the best but getting tight farmers to fully rewire etc is a struggle!
The FCU I agree again not ideal but it saves a separate DB at the remote building to run lights and a socket from one final circuit. So should I just stake down the accessory that the swa terminates in? (socket or FCU depending on the layout) also testing...is Ra taken with all stakes connected but no bonding or with everything connected? or is each stake separately measured?

 

[Engineer54]

Thanks! exactly the reply I needed! I doubt I can get them to change the DB as it is newish and serviceable and the 30mA covers all bases. On a fresh install your approach is obv the best but getting tight farmers to fully rewire etc is a struggle!
The FCU I agree again not ideal but it saves a separate DB at the remote building to run lights and a socket from one final circuit. So should I just stake down the accessory that the swa terminates in? (socket or FCU depending on the layout) also testing...is Ra taken with all stakes connected but no bonding or with everything connected? or is each stake separately measured?

That's up to the farmer!! But it far from covers all bases, any fault from the various outbuildings is going to trip that single 30mA RCD. And more importantly if it fails for any reason, their will be Zero earth fault protection (depending on the overall Ra value of the TT system)!!


A small garage type CU is all that is needed, they aren't expensive and will provide a far superior installation!! I just would not ever contemplate the use of an FCU in a working farm situation!!


Any new rod position should be tested separately in the first instance, so you have a record of it's Ra value. Then test as you have stated for the over-all Ra value. Also useful to take a reading of the system with everything else connected to the MET, that will be the working Zs value that is available at the main DB!! lol!!

 

[Guitarist]

Agree with Eng here. State that you will not accept one upfront RCD covering the entire installation. They can and do indeed fail, with the result that nothing then has protection, let alone the inconvenience factor.
They might not want to spend the money, but I bet the kids go to private school and the skiing holiday is booked...

 

[grumpyjohn01]

I just would not ever contemplate the use of an FCU in a working farm situation!!

Not arguing but what would your specific reasons be?

 

[Engineer54]

A small CU will provide both single point local isolation to that buildings installation and local OCPD's for both sockets and lighting circuits etc!!

 

[grumpyjohn01]

thanks guys will consider....