Farm Outbuilding Office & TT Earthing etc.

gkenyon: 
 

Zoomup: 
 

Thanks Mike.  An office  TT earth electrode will be too close to the electric fencer electrodes for my liking. I will stick to P.M.E. just for the office. The fencer will remain TT supplied on its existing supply.

Z.

Provided there are no buried extraneous-conductive-parts, that is OK, otherwise the electric fence electrode would still be too close.

There is perhaps another solution which hasn't been discussed … export TT from another building (if there's overhead on the demand) … no need for a new electrode for the second building if you also export a protective conductor from the other TT building.

That could be done as well. A nice idea. The proposed new installation is a small wooden building with no extraneous-conductive-parts, so no main bonding is required, Everything will be plastic inside the building, cable insulation, mini-trunking and wiring accessories.

Z.

Zoomup: 
 

Thanks Mike.  An office  TT earth electrode will be too close to the electric fencer electrodes for my liking. I will stick to P.M.E. just for the office. The fencer will remain TT supplied on its existing supply.

Z.

Provided there are no buried extraneous-conductive-parts, that is OK, otherwise the electric fence electrode would still be too close.

There is perhaps another solution which hasn't been discussed … export TT from another building (if there's overhead on the demand) … no need for a new electrode for the second building if you also export a protective conductor from the other TT building.

mapj1: 
 

If the floor is wooden, the tingle risk of PME is largely negated, and by the sound of it I reckon either  will be fine.  

You could confuse the unwary and use the PME earth and still put an electrode in parallel.  If the floor had been concrete you could have asked for a tap point onto the re-bar. (though 2 taps a few m apart is nicer as it allows for a high current through test which is reassuring.)

If you use the PME, I'm assuming the fencers will never be plugged into this building, but supplied from batteries or elsewhere, and more generally that there will not be leads trailing out for any other more outdoors kit in the future.

Given you are fitting an up front  RCD, the change to TT is trivial, perhaps one plastic gland and a link..

Mike.

Thanks Mike.  An office  TT earth electrode will be too close to the electric fencer electrodes for my liking. I will stick to P.M.E. just for the office. The fencer will remain TT supplied on its existing supply.

Z.

If the floor is wooden, the tingle risk of PME is largely negated, and by the sound of it I reckon either  will be fine.  

You could confuse the unwary and use the PME earth and still put an electrode in parallel.  If the floor had been concrete you could have asked for a tap point onto the re-bar. (though 2 taps a few m apart is nicer as it allows for a high current through test which is reassuring.)

If you use the PME, I'm assuming the fencers will never be plugged into this building, but supplied from batteries or elsewhere, and more generally that there will not be leads trailing out for any other more outdoors kit in the future.

Given you are fitting an up front  RCD, the change to TT is trivial, perhaps one plastic gland and a link..

Mike.

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.

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).

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.

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.

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.

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.