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Farm earthing arrangements.

Sparkingchip
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Some years ago I started doing electrical work at a farm, originally the farm buildings and house were both connected to the DNO earth terminal.


I altered it so that the farm buildings are TT , but left the house connected to the DNO earth terminal. The possible issues are that there is a steel clad switch fuse enclosure for the house inside a wooden cupboard in the farm workshop and the SWA cable for the house passes through the ground immediately adjacent to the farm building. But there was no physical connection between the house and farm earthing arrangements.


It had been like that for a few years as the house has not had any alterations at all, just a few repairs. So the house installation is effectively exactly the same as it was when the house and the farm buildings were thirty years ago and connected up as a new installation by the DNO.


However (you knew there was going to be a however!) a couple of years ago PV panels were installed on the farm building roof and the installers used the DNO and house earth terminal rather than the farm buildings earth terminal, despite the PV system being in and on the farm buildings and it having storage batteries that feed back into the both installations as they share a meter; and the storage batteries act as a supply to emergency lighting in the farm buildings when the installation is off-grid.


I am now reviewing the earth arrangements, the house is empty and needs tidying up, thirty two lights need replacing and odd repairs, there won't be any alterations it is just replacing fittings and replacing MCBs in the split load consumer unit with RCBOs to give additional RCD protection in the house. 


I will leave it at that and not express my thoughts, as it will be more interesting to see what your thoughts are rather than trying to get you to consider mine.


Comments please. 


Andy B
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    A farm with animals, does BS7671 differentiate between livestock and arable farms? How many animals are grown on horticultural premises?  


    There’s no distinction to be made.


    Andy B
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    There’s no distinction to be made.


     


    Well if there are animals there are different risks as compared with just an arable farm. That is why I asked. A four footed animal is more likely to be shocked, and the loss will be greater, than a sack of potatoes being shocked in a farm building. Is farm TT earthing just a traditional thing or a risk reducing solution. Does B.S. 7671 prohibit a TN-C-S supply on a farm?


    Z.


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    PME is not banned, but the supplementary bonding requirements are too onerous, I would be there running bonding all over the place, then there is the steel shipping container used for secure storage to consider in addition to the various requirements for the eight buildings.


    Andy B.
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    I am really late on this but anyway, will cast a stone in the ocean just in case it helps anyone out.


    As per the ESQC Regulations, a consumer is not permitted to combine the functions of neutral and PE within their installation- the DNOs however can and it is a different story.
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    8.3 looks like what I have been chuntering about.
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    Sparkingchip:

     A WPD engineer phoned this morning and said they don't have records for this property, so he drove there this afternoon to check it out.


    He has declared it as a TN-C-s PME system ...


    This really does seem to be a deficiency of DNOs. They don't keep records so have to send somebody out. Then they say "PME", but if you ask them where the multiple electrodes may be found, they cannot answer.


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    Chris Pearson:
    Sparkingchip:

     A WPD engineer phoned this morning and said they don't have records for this property, so he drove there this afternoon to check it out.


    He has declared it as a TN-C-s PME system ...


    This really does seem to be a deficiency of DNOs. They don't keep records so have to send somebody out. Then they say "PME", but if you ask them where the multiple electrodes may be found, they cannot answer.




    Standards have slipped dangerously in this modern world.


    Z.


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    Sorry but I'm completely confused now this all seems overly complicated am I right in thinking that if the neutral between the intake and the transformer breaks then the whole lot will go off but if the earth come off of its rod at the house then things will continue to work and be protected because of the link at the head also the barn supply will be ok because it's still connected properly. Most of the houses in my area are earthed via the metal cable sheath but at 2 property's rewired a few years back there is a neutral earth bond which is connected to the MET and also to a metallic water pipe is this what the farms got?

    Think of the electrode connected at the cut-out as the supplier's source earth (the one at the transformer in the textbook diagrams), if that goes AWOL then the LV system isn't (reliably) referenced to Earth any more and (via capacitive coupling from the HV side of the transformer) may drift up to some nasty voltage above true earth, but there will still be 230V between L and N so things will still function (if more likely to go bang as the insulation to true Earth is over-stressed).


    Yes if the supply N breaks then things will stop working. Depending on whether the system is PME or PNB (and which version of PNB) that might also mean the consumer's earth terminal being pulled up to something approaching line voltage.


       - Andy.
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    They inherited the records from Central Networks and many originated from the MEB, maybe even the original companies from before nationalisation, such as the Corporation of Worcester Electricity Department. What has been digitalised and what remains on paper or disappeared over the years is probably one of life's mysteries.


    Presumably they now have a record for this installation.


    Andy Betteridge
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    I've seen that DNOs can have rather poor detail about parts of their network, and I realise that they've inherited the network, moved to computers, etc.  But I'd have thought there'd be occasional inspection (10yr?) to check the basics at LV transformers, e.g. soundness of main electrode, wire to it, pole, etc., which would be the opportunity to check that there's a record and that it fits the fact.  Do they have a chosen policy or obligation of checking through the system, or is it only a reactive service after complaints?


    Regarding PNB vs PME vs etc:  one could go on forever ... so I won't get into several of the aspects that tempt me - e.g. about mutual exclusivity or not, of PNB/PME. But I will just observe (not relevant to the WPD system in the OP, but to mapj1's point)  that I find it a strange idea to consider a system with a combined N+E downstream of the nearest-to-source earth electrode to be other than PME, even if it's deemed PNB by virtue of some distance from the source to that earth.  Even if the supplier has only the one electrode, then as soon as they've provided an "earthing terminal" to this combined N+E at an intake,  that customer could then or later end up bonding it to things that in effect are good electrodes. The broken-conductor shock-danger aspect of this multiple earthing of an active conductor is what mapj1 mentioned. And even without a break, farm animals outside a very well bonded area may be upset by small voltages.  I'd want complete separation of N+E for all points downstream of the first earthing, in order not to consider PME 'issues' to be relevant. (When did they stop having to get the home secretary's or suchlike's permission for more than one earth on a neutral? Some countries have been very fussy about this: for Germany I've heard the power companies felt very unfortunate for having come after the telephone/telegraph ones, which then complained about any earth currents. I think that was the main concern with getting permissions in the early days of PME.)

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