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

 

[driverman]

I would of thought....TT cable at mains end with stake ( or more depending on readings obtained ) with RCD and that should be Ok

 

[Doomed]

Depends on the out building, if it has other earth paths in the out building (water pipe, structural steel etc) then will need separating from the incoming earth and staking again.

 

[davesparks]

Why would it need a seperate stake in that situation?

If you can get a good enough Ra at the intake then all is ok. It may be the case that a second earth electrode at the second building may be required to get the Ra down for the whole installation.

 

[SirKit Breaker]

I wont try to expand and complicate things. As much as i don't like to quite the regs, you need to read the bit in Part 7 of the BGB about Agricultural and Horticultural premises. Earthing systems and what system you can and can't use in different parts of those locations are well explained. This may swing your proposal to some degree.

Cheers...........Howard

 

[Engineer54]

Every building you intend converting from TNC-S to a TT system should be rodded. In fact i'd prefer to see two linked rod positions for each building. There is absolutely no need to isolate the rod positions, in fact it is far better to link the TT'd building rods, it will greatly bring down the overall Ra value. And don't TT the main farm house, that is far better being left on the DNO's TNC-S earthing system...

As SB above suggests, in fact i'd go as far as saying it's imperative you make yourself familiar with the requirements for Agricultural and Horticultural premises. Providing certain criteria are met, TNC-S/PME can still be utilised in buildings housing farm animals...

 

[atm84]

TT the outbuilding but not the house. Why would you put a rod in at the house? That should not be TT'd. Run an SWA, no additional earth is required.

At the outbuilding end, isolate the armouring from the outbuilding DB. This is there to earth the sub main. Knock a stake in and connect an earth conductor from the stake to the MET at the new DB.

 

[Simon-0116]

Sorry to jump in, Might be me been thick but do you meen remove bonding from gland / earth tag to sub bd, which I would do. But I was once told by a guy that you need to take swa gland out and put a piece of insulation under so no earth path present back to met, obviously put gland back in

 

[atm84]

Sorry to jump in, Might be me been thick but do you meen remove bonding from gland / earth tag to sub bd, which I would do. But I was once told by a guy that you need to take swa gland out and put a piece of insulation under so no earth path present back to met, obviously put gland back in

Yes correct as above

 

[davesparks]

The earthing and bonding guidance note has some pretty clear explanations and pictures on the subject.

But basically any metal parts connected to the earthing system of the origin (armouring and gland) should be completely isolated from the sub board and in theory not enter the enclosure or be available to touch.

 

[Engineer54]

Sorry to jump in, Might be me been thick but do you meen remove bonding from gland / earth tag to sub bd, which I would do. But I was once told by a guy that you need to take swa gland out and put a piece of insulation under so no earth path present back to met, obviously put gland back in

You can get special SWA glands to isolate the armour, or you can nip off the exposed armouring and use a good quality stuffing gland at the isolated end of the SWA cable. Another point to make when talking about farm installations, is to always use brass locknuts/lockrings and tight fitting shrouds on SWA cables, or corrosion will become a real problem. ...What with all the chemicals now used on farms, and the legendary neglect associated with general farm installations!!!....

 

[shanky887614]

You can get special SWA glands to isolate the armour, or you can nip off the exposed armouring and use a good quality stuffing gland at the isolated end of the SWA cable. Another point to make when talking about farm installations, is to always use brass locknuts/lockrings and tight fitting shrouds on SWA cables, or corrosion will become a real problem. ...What with all the chemicals now used on farms, and the legendary neglect associated with general farm installations!!!....

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

 

[AJO]

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?

 

[atm84]

So does the house have a separate DNO supply? Is the cattle shed a steel framed building?

 

[AJO]

So does the house have a separate DNO supply? Is the cattle shed a steel framed building?

Not quite sure to be honest. I could not gain access. I am assuming so. yes the cattle shed is steel frame which needs bonding correctly

 

[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??