Moving on to earthing and bonding at the farm!

[HappyHippyDad]

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This thread is in relation to the job at the farm I am thinking of taking on (3 phase thread).

At present the farm is all PME. I believe it needs to be TT'd as they have now brought 2 x goats in to the main barn (no other animals anywhere). In addition they have some feed for the goats and also some plants may be grown in there. The barn is all stone flooring. I will never be able to confirm if it has a metal grid underneath so I have to assume it doesn't.

Looking at 705.411.4, the note states a TN -C system shall not be used, but a TN-C-S system is not precluded.

705.415.2.1 says that PME is not 'recommended' unless a metal grid is installed.

Now, I have discussed the word recommended with Napit and also the IET and they say it basically means it is up to the installer. It is not mandatory. However, I would have to justify why I have chosen not to use the recommendation.

2 goats, a few sacks of feed and a few plants in a massive (metal) barn (approx 75m x 25m).

What is the actual risk of sticking with the PME? Is it the potential between true earth and the PME earth. The barn will be bonded so it will be at PME earth. The ground will be at actual earth, so is the problem if the goat brushes against the metal barn whilst standing on the ground and there is a potential difference. Or, is it more to do with fault conditions?

Is this an obvious case of needing a TT, or perhaps not so obvious?

I know I have discussed this before, but now it is becoming a reality I thought I would discuss it again/further.

 

[pc1966]

Is this barn with goats the same Barn A that the TPN board is to be installed in?

Personally I would TT any agricultural building to avoid the risk of open-PEN faults from killing livestock and potentially humans as well (given it is often wet and folks in contact with lots of stuff).

If you are going to TT the building you might decide to put in a 300mA delay incomer to the TPN board and just ensure the rod(s) are below 167 ohm by a comfortable margin (old regs would be half that as 25V max in agricultural area). You then really need RCD protection on all circuits but how you go about it can make a difference. for example, it is often difficult to get DP RCBO for TPN boards, but you might put offices one small SP boards with such RCBO and then just MCB & SWA feeding them, etc.

 

[pc1966]

You mention in another post the local transformer is close. how is the main supply delivered? Is it overhead wires, and if so is it separate cables or the ABC (aerial bundled cable) sort?

The open-PEN risk really ought to be considered along with the reliability of RCD disconnection. Of course with both a delay RCD incomer and all final circuits on 30mA RCD/RCBO that risk is low as no single point of failure.

 

[HappyHippyDad]

Is this barn with goats the same Barn A that the TPN board is to be installed in?

Personally I would TT any agricultural building to avoid the risk of open-PEN faults from killing livestock and potentially humans as well (given it is often wet and folks in contact with lots of stuff).

If you are going to TT the building you might decide to put in a 300mA delay incomer to the TPN board and just ensure the rod(s) are below 167 ohm by a comfortable margin (old regs would be half that as 25V max in agricultural area). You then really need RCD protection on all circuits but how you go about it can make a difference. for example, it is often difficult to get DP RCBO for TPN boards, but you might put offices one small SP boards with such RCBO and then just MCB & SWA feeding them, etc.

Thanks for all the posts @pc1966
Yes, it is Barn A.
It has a 300mA time delayed Triple pole RCD at the supply origin

 

[HappyHippyDad]

You mention in another post the local transformer is close. how is the main supply delivered? Is it overhead wires, and if so is it separate cables or the ABC (aerial bundled cable) sort?

The open-PEN risk really ought to be considered along with the reliability of RCD disconnection. Of course with both a delay RCD incomer and all final circuits on 30mA RCD/RCBO that risk is low as no single point of failure.

I think it is overhead cables. Not sure about the ABC.

 

[Lister1987]

Perfect opportunity to try out Condu-disc from Earthing Solutions, they also do conducrete, a conductive concrete to further improve a Ze reading. Much more effective Vs traditional rod/mat, a bit more cost yes but the effectiveness is worth it.

 

[HappyHippyDad]

I've had a good think about this job and I've decided to pass on it. I feel I should have a greater knowledge regarding 3 phase if I intend to start installing it. Disappointing as I have done a few jobs on the farm for the chap and I really like it there. However, I will write him an email now, explaining my reasons and hopefully he'll continue to use me for the smaller jobs.

Thanks again for everyone's help. Especially you @pc1966 who (as usual) has been full of useful knowledge.

 

[bigspark17]

Instead of a rod/matt why not use the huge steels buried in the ground for your TT earth.?
Yes the building needs to be TT or walk away! Open PEN is not fun for anyoby especially if you have 4 bare feet on true earth.

 

[pc1966]

Yes, it is Barn A.
It has a 300mA time delayed Triple pole RCD at the supply origin

Well that also answers another question - the SWA armour is plenty to disconnect if RCD'd at origin!

The remaining question is the earth for the barn and I would do it TT and have the sub-main cable to the TPN board earthed at origin to the TN-C-S part and isolated at the barn. So probably a large plastic gland or similar in stead of the usual armour clamping sort.

 

[pc1966]

You would need to check the cable chosen of course, but for typical 50mm 4C SWA the Prysmian data sheet has 34mm diameter so something like this would do for the isolated end:

50mm IP68 Compression Gland with Locknut Grey (Sold in 1's)

Nylon plastic construction

www.cef.co.uk

Or probably OK if you have 35mm 4C:

40mm IP68 Compression Gland with Locknut Grey (Sold in 1's)

Nylon plastic construction

www.cef.co.uk

Remember 35mm/50mm is hard stuff to wrangle so you will need a couple of cleats fixed to something strong to hold it in place - don't assume such a gland will stop it being pulled out under the cable's weight!