Farmhouse supplied from farm buildings

[stuartsjg]

Hello,

The normal PME etc query i see on the forum relates to the farm buildings supplied from the house where the house runs on PME and the farm on TT with local earth rods.

In this case, the supply is terminated to the the farm which has a PME supply, should the farm CU & final loads be connected as a TT system and run the house from a PME for which i would run in a 2C+E.

Are there then not issues with having the farm house (which is about 3m from the buildings) on a different earth system to the farm? Would it be best to run the whole site as TT?

Any advice would be good.

Thanks,
Stuart

 

[malcolmsanford]

There is nothing wrong with having a farm building on a TNC-S system but there are certain criteria you have to make sure and that is installing a earth mesh for the farm buildings and having it all bonded together.

 

[stuartsjg]

Hi, That was the impression i was under. Its a re-wire which is required so there probably is no opportunity to pickup an existing, or to lay a mesh under the floors and i cant see it being practical to get the whole thing bonded to meet these requirements so i would think TT is the way to go. Gut feeling is to also put the house on TT each with their own earth rods.

Have put a rough sketch together. I would protect the cable & equipment before the TT RCD's with the TN-C-S earth and i believe this will be OK provided under fault, the voltage between the two doesn't exceed 50V so nobody gets a nip between the CU on the TT and the isolators the TN-C-S. Unless ive missed something - i beleive this should be OK.

Everything is easier offshore - its all metal!

 

[Engineer54]

I wish you luck if your going to be satisfied with your stated 200ohm Ra on a farm set up !!

That typical farm installation in pic 2 looks as if it could bite at anytime!! ..lol!!

 

[Knobhead]

Fingers crossed they don't have livestock.

 

[stuartsjg]

It appears i was optimistic about getting a professional response. There is a reason why i tend to avoid these forums and stick to the black and white text of regs. Its fortunate that these outlets exist to fill in the areas where things are not black and white. Its unfortunate that people feel the need to make senseless comments which waste time and achieve nothing.

They do have livestock, and i would be very interested to engage in a meaningful discussing as to how i can deal with the various issues.

I appreciate this is not what im used to and obviously you are hence you would not have been able to make such a comment. I would also appreciate if you would care to enlighten me as to your thoughts on the issue.

Thanks.

 

[Engineer54]

It appears i was optimistic about getting a professional response. There is a reason why i tend to avoid these forums and stick to the black and white text of regs. Its fortunate that these outlets exist to fill in the areas where things are not black and white. Its unfortunate that people feel the need to make senseless comments which waste time and achieve nothing.

They do have livestock, and i would be very interested to engage in a meaningful discussing as to how i can deal with the various issues.

I appreciate this is not what im used to and obviously you are hence you would not have been able to make such a comment. I would also appreciate if you would care to enlighten me as to your thoughts on the issue.

Thanks.

You could TT the lot, if you have farm animals, or leave the farm house only on PME. If you DO decide the TT the lot, link all the buildings rod positions, so that at least you'll get a decent Ra value at each of the TT'd farm buildings Do Not use those short thin 3/8'' rods for this farm installation, use at least 2 extensible 5/8'' at each rod position. And, .....do not rely on a 200ohm Ra value as noted on your sketch...


If you are going to have the farm house on PME, (which is recommended) then you would HAVE to use a 3 core SWA cable, if only to meet bonding requirements, the wire armour will comply to and satisfy your CPC needs.


What is it your re-wiring, the house or that god awful mess in pic 2??

 

[GLENNSPARK]

Fingers crossed they don't have livestock.

its neigh bother tony....lol.....
its a point of contact after all....

 

[Rockingit]

I see worse than pic 2 every time I go to my inlaws farm - and they have a very large amount of livestock. The problem with BS7671 is that it is written from the perspective of design and 'starting the job from scratch' rather than evolution and development. Earthed cattle grids and meshes etc etc are all very well but how is anyone going to install them under a stone barn that's been there for 200 years?

So yes, I do have some sympathy for the OP. I refuse to do any work for the inlaws now, it's just not 'commercially safe' for me to even do a quick family favour.

OP - farmers like making holes in the ground: suggest you get them to dig you some nice deep 2 - 3m ones and then drop some chunky rods in, backfill with something suitably conductive then put a concrete cap over the top with an inspection cover. You might also want to consider running 1c SWA to them instead of normal PVC.

 

[stuartsjg]

You could TT the lot, if you have farm animals, or leave the farm house only on PME. If you DO decide the TT the lot, link all the buildings rod positions, so that at least you'll get a decent Ra value at each of the TT'd farm buildings Do Not use those short thin 3/8'' rods for this farm installation, use at least 2 extensible 5/8'' at each rod position. And, .....do not rely on a 200ohm Ra value as noted on your sketch...


If you are going to have the farm house on PME, (which is recommended) then you would HAVE to use a 3 core SWA cable, if only to meet bonding requirements, the wire armour will comply to and satisfy your CPC needs.


What is it your re-wiring, the house or that god awful mess in pic 2??

Hi, thanks for your reply,

I was thinking towards TT on the whole installation, including the farm buildings & house. The issue i feel is earthing of the switch gear between the supply and the RCD's. This would be earthed on the PME and as such would be at a different potential difference to the earthed TT equipment which would be within reach and i wouldn't want to have two earthing systems within each of each other.

Whats your thoughts about this?

If i was running PME to the house, i would be using a 3C 35mm.sq.

For the TT earthing, i was going to fit a main lattice and also drive thick & deep rods around the buildings. I was aiming for a much better Ra value, 200Ohm is the max by the regs and i would want better in practice from the system to allow for soil variations etc.

The electrics in the farm are very bad - they recently found cloth insulated cable and they are worried about it so want to get it sorted out. Curiously, just now the farm is setup on PME earthing however there is no bonding anywhere!

This whole thing came about as their electrician was just going to replace "like for like to be on the safe side" but i didn't think things would be compliant if he done that.

Thanks again,
Stuart

 

[stuartsjg]

I see worse than pic 2 every time I go to my inlaws farm - and they have a very large amount of livestock. The problem with BS7671 is that it is written from the perspective of design and 'starting the job from scratch' rather than evolution and development. Earthed cattle grids and meshes etc etc are all very well but how is anyone going to install them under a stone barn that's been there for 200 years?

So yes, I do have some sympathy for the OP. I refuse to do any work for the inlaws now, it's just not 'commercially safe' for me to even do a quick family favour.

OP - farmers like making holes in the ground: suggest you get them to dig you some nice deep 2 - 3m ones and then drop some chunky rods in, backfill with something suitably conductive then put a concrete cap over the top with an inspection cover. You might also want to consider running 1c SWA to them instead of normal PVC.

Hi, this is kind of the same with me - wishing i didnt get involved, i didnt know what the mess was and how non-compliant things were and probably how they were going to be.

Agree that there isnt much in the regs which cover for what to do in this situation, hence my feeling of "TT the lot". Will arrange a hole digging competition

Thanks, Stuart

 

[Rockingit]

With your comments re switch gear being at a different potential - doesn't have to be, surely? Just because the DNO have left you with an earthing point doesn't mean you have to use it. Make 'that point' divorced and your first MET with a rod.

 

[stuartsjg]

With your comments re switch gear being at a different potential - doesn't have to be, surely? Just because the DNO have left you with an earthing point doesn't mean you have to use it. Make 'that point' divorced and your first MET with a rod.

Thats the discussion i had with the electrician, he felt the earth terminal must be used if it was provided. This is where im leaning towards leaving it out all together and perhaps have a 100mA time delayed RCD after the fuse head to cover the farm and cable to house, then 2 core to the house, with 30mA RCD for the house.

 

[Engineer54]

Thats the discussion i had with the electrician, he felt the earth terminal must be used if it was provided. This is where im leaning towards leaving it out all together and perhaps have a 100mA time delayed RCD after the fuse head to cover the farm and cable to house, then 2 core to the house, with 30mA RCD for the house.

Just isolate the suppliers earth point in a small insulated earth block, or use a small Henley block. Then provide your own installation MET, that each buildings EMT is taken back to (as well as any local bonding), either by separate sub-main SWA cable core (Preferred), or by the armouring of the SWA. If the latter is to be used, make dam sure that the cable sheath has not been damaged during installation/cable laying!! Personally i Always perform sheath tests on direct buried SWA cable installations....

I'd be inclined to use several earth rod positions on a farm installation (rather than a single point), linking them out would also give you a dam good final Ra value too as well as a stable system, ...if you ''Don't'' use the popular thin twigs, that they have the cheek to call an earth rod!! lol!!