Earthing systems with agricultural premises

[AJO]

Hello all

I've just been recently asked to do some remedial work in a farm for insurance. The supply is 3 phase and the existing earthing system is TNC S and I am going to convert this to a TT system which is in a cattle shed. From the main consumer unit I intend to resupply a consumer unit in some adjacent outbuildings which also 3 phase. My questions are would I be correct in installing two separate earth rods? 1 at the mains end and 1 at the out build?? Run a separate earth with the swa to the out build if I was enable to put a stake in? Also 300mA incomer at the main consumer unit?

your help would be greatly appreciated

 

[Engineer54]

surely it would be better to use adhesive shrouds.

ive done an install on a farm and the cleanest part was the septic tank lol

the problem you will have most of all is the farm hands most wouldn't think twice about fishing things out of septic tanks etc with bare hands or using plastic conduit to climb up etc

We generally use adhesive Heat shrink on all our external SWA glands, but if you can get adhesive shrouds then yes, better all round. Trouble is, will the installer pay the extra or even be bothered, that's the main problem i can see in the threads and posts here, doing the minimum work for minimum cost.... Which is why i stated a tight fitting shroud above...

 

[Engineer54]

thanks for the replies guys.... the main supply is in the cattle shed where the distributor has provided a PME terminal, which isn't labelled, has existing equipments earth connected to it. I have read that TNC S supplies may be installed. would be best to use this for the main db, earth armourings,disconnect the armourings at sub main and try find somewhere to stake it that end?

If the cattle shed has a concrete floor with a mesh rebar reinforcing, then providing you can bond to that re-bar mesh, PME would be allowable for this building. I'd still drive in a earth rod or two adjacent to the incoming DNO supply and connect to the MET. This will act as local TNC-S/PME N-E connection, but it will need to be a low single figure Ra value, certainly sub 10 ohms, NOT the numpty 200 ohms mentioned in BS7671...

 

[AJO]

If the cattle shed has a concrete floor with a mesh rebar reinforcing, then providing you can bond to that re-bar mesh, PME would be allowable for this building. I'd still drive in a earth rod or two adjacent to the incoming DNO supply and connect to the MET. This will act as local TNC-S/PME N-E connection, but it will need to be a low single figure Ra value, certainly sub 10 ohms, NOT the numpty 200 ohms mentioned in BS7671...

Many thanks mate. Not got much experience with agriculture installations. would a 300mA incomer with time delay and obviously 30mA rcbos for the socket outlets be the way to go?? Ive got a sub main coming from the main db in the shed also so could use an additional core from the armoured as a CPC??