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Farm Outbuilding Office & TT Earthing etc.

Zoomup

The wild geese are migrating now. We can see them overhead flying south as they appear out of the morning mists, calling to each other as they fly in large numbers in a large V formation. Then they disappear back into the high level mists. Some say that they will fly to S. Africa.

Wild Geese Flying - Bing video

Meanwhile, a local smallholder who keeps four footed animals, has built a new wooden building to be used as an office and farm storage unit. The new proposed office is only about 25 metres square.

He asked me about installing a few sockets and lighting in the new building.

Presently the nearest supply is a plastic cased TT earthed consumer unit.

Near to the new build there are two earth electrodes used solely by the electric fencer unit. These are run in parallel. How far away from any TT L.V. installation earth electrode should they be?

Will the electric fencer interfere in any way with I.T. equipment?

Shall I supply the office with P.M.E. earthing, or shall I TT it?

I have my own thoughts on these matters, but wish not to have missed any safety issues.

There is a miniature Shetland pony that can't eat regular grass. It is allergic to it. Its hay has to be “sterilised” in a hay steamer rated at just under 3kW.

Thanks,

 

Z.

 

 

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    Farm = TT. Do not use the fencer electrodes, put in new one(s) at a distance. A pony allergic to grass is a serious problem, let it eat cake! (Horse nuts). The fence should not cause an interference problem, provided you don't introduce one with the Earthing. How will the phone line and Internet get to the building?

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    davezawadi (David Stone): 
     

    Farm = TT. Do not use the fencer electrodes, put in new one(s) at a distance. A pony allergic to grass is a serious problem, let it eat cake! (Horse nuts). The fence should not cause an interference problem, provided you don't introduce one with the Earthing. How will the phone line and Internet get to the building?

    But does a farm office have to be TT earthed as well? Is there a remote risk of extension leads being used outside of it? I have no intention of using the fencer electrodes as earthing for the office.

    Z.

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    Yes. The purpose of a building is not in section 705. 705.411.4 applies

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    Zoomup: 
     

    davezawadi (David Stone): 
     

    Farm = TT. Do not use the fencer electrodes, put in new one(s) at a distance. A pony allergic to grass is a serious problem, let it eat cake! (Horse nuts). The fence should not cause an interference problem, provided you don't introduce one with the Earthing. How will the phone line and Internet get to the building?

    But does a farm office have to be TT earthed as well? Is there a remote risk of extension leads being used outside of it? I have no intention of using the fencer electrodes as earthing for the office.

    Z.

    Section 705 does NOT prohibit PME or TN-S? Agricultural premises are not (always) caravan sites.

    If you are talking about 705.415.2.1, that only has a note recommending PME is not used unless a supplementary -bonded metal grid is laid in the floor of locations intended for livestock … but the normative text before this note recommends concrete reinforcement is part of the bonding in any case.

    I agree, however, that in areas for livestock, TT (separated from PME) is preferable, and you might as well because RCDs are necessary anyway even in distribution circuits - 705.411.1.

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    davezawadi (David Stone): 
     

    Farm = TT. Do not use the fencer electrodes, put in new one(s) at a distance. 

    BS EN 60335-2-76 (as referenced from BS 7671) says "A distance of at least 10 m shall be maintained between the energizer earth electrode and any other earthing system such as the power supply system protective earth or the telecommunication system earth."

    There are recommendations for physical separation of electric fence wiring from other wiring systems and overhead power lines too.

    Electric fence systems powered by separate energizers also need at least 2.5 m between electrodes of each system.

    Interesting aside … For those who have been reading about EV charging installations, and the distances discussed in the IET Code of Practice for EV Charging Equipment Installation for separating TT earth electrode systems from PME, the distances 2.5 m and 10 m will come as no surprise … they are based on physics plain and simple.

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    Graham, whilst it might be possible to find a farm with a TN-S supply, such would be considerably unusual. A private transformer might do this (I have seen a few of these for grain installations) but most distribution outside of towns is from pole-mounted transformers without a distributed Earth. TN-C is not permitted, and this I understand from 705.411.4, not that it may be not used for final circuits which are not permitted anyway elsewhere, but the supply. 705.411.1 then specifies that RCD protection must be provided everywhere on all final circuits. Zoomup asks if he can export a PME supply, which is not permitted on a farm anyway for obvious reasons. The conclusion is that his new building should be TT, as it has no particular disadvantage. This could be an ambiguity with the exact wording of a regulation, but I would not want an area with animals with a PME supply, even if they seem isolated from possible fault voltages, which may even be external to the farm.

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    davezawadi (David Stone): 
     

    Graham, whilst it might be possible to find a farm with a TN-S supply, such would be considerably unusual. A private transformer might do this (I have seen a few of these for grain installations) but most distribution outside of towns is from pole-mounted transformers without a distributed Earth. TN-C is not permitted, and this I understand from 705.411.4, not that it may be not used for final circuits which are not permitted anyway elsewhere, but the supply. 705.411.1 then specifies that RCD protection must be provided everywhere on all final circuits. 

    705.411.4 does not mean TN-C-S (PME) is not permitted.

    You could have an installation with its own generator and no public supply, in which case ESQCR wouldn't apply and it may be possible to use TN-C … the purpose of 705.411.4 is to prohibit Tn-C from agricultural and horticultural premises. It has nothing to do with PME.

    Zoomup asks if he can export a PME supply, which is not permitted on a farm anyway for obvious reasons. 

    ???

     

    The conclusion is that his new building should be TT, as it has no particular disadvantage. This could be an ambiguity with the exact wording of a regulation, but I would not want an area with animals with a PME supply, even if they seem isolated from possible fault voltages, which may even be external to the farm.

    I don't disagree with the conclusion, just not sure about the “not permitted” bit (unless the DNO has particular conditions) - overall, I think it is permitted, but not recommended … two different things.

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    I have just had to do some research. The definition of TN-C is “A system in which Neutral and protective functions are combined in a single conductor throughout the system”. So it is forbidden.

    The farmhouse is fed by a P.M.E. supply. Currently all outbuildings are TT earthed.

    I can see no reason not to supply the new wooden office building with a P.M.E. supply. It is a self contained unit away from any animals. That would save having an earth electrode close to the fencer electrodes.

    Z.

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    I have to agree with you on that last, Z, exporting PME for an office outbuilding is fine if no animals present. I have done just that on my farm (which, BTW, is supplied from a pole transformer but is PME). Exporting PME requires a 10mm earth (or sufficient CSA on the SWA).

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    MrFox: 
     

    I have to agree with you on that last, Z, exporting PME for an office outbuilding is fine if no animals present. I have done just that on my farm (which, BTW, is supplied from a pole transformer but is PME). Exporting PME requires a 10mm earth (or sufficient CSA on the SWA).

    Thanks. There will be no extraneous-conductive-parts in the new wooden office, so no bonding with 10.0mm2 will be required. If a new S.W.A. supply is installed I would install a new 100mA S type R.C.D. at the origin of the supply to limit fire risk, and all final circuits would be 30mA R.C.D. protected.

    Z.

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