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Farm earthing arrangements.

Sparkingchip
009a7dbbf62372ab469269cf2662d114-huge-20200425_145756.jpg


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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    The farm buildings were changed to TT when a log cabin was erected by a small lake several hundred metres away from the intake and a supply to a farm building that was demolished reused. The cabin has an oil fired boiler, log burner and lpg cooker, so just needed a 16 amp supply, the biggest breaker is a B16 and the biggest load a washing machine, other than that there’s lights and tv, etc.


    With several hundred metres of cable including some which is thirty year old relying on the DNO earth at the intake end did not seem a good plan, so I made it TT with a rod at both ends to reduce the risks of a CPC failure, if the CPC fails both sides of the point of failure are still earthed and everything is RCD protected.


    Then it just went from there, another rod was added at another building, so there’s three rods plus fortuitous earthing from steel stanchions on buildings., getting a low Ra definitely is not an issue.


    The house remained untouched and as original without any interconnections between the house and buildings earthing, until the PV guys did their work.


    Andy Betteridge
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    Measured in a direct line on Google Earth the installation is 243 metres long and takes in eight buildings, it’s just grown as time as gone on, as farms do. The longest run measured in a straight line from the intake is 140 metres. Hence my preference for localising the earthing a bit with rods at each end as well as in the middle, rather than just one point in the middle.


    Andy Betteridge
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    Sparkingchip:
    1034a07b303b91ddbd79480d7e21a78d-huge-101_1275-2.jpg

     




    To avoid confusion, just because you may see a DNO earth connection like this does not always mean it is PNB, it could be the last DNO earth electrode in a PME system.

  • 0

    Sparkingchip:

    The farm buildings were changed to TT when a log cabin was erected by a small lake several hundred metres away from the intake and a supply to a farm building that was demolished reused.



    Several hundred metres - difficult to keep Ze low enough!

    ... so I made it TT with a rod at both ends to reduce the risks of a CPC failure, if the CPC fails both sides of the point of failure are still earthed and everything is RCD protected.

    So what is the separation between the rod at the supply end and the DNO's rod?
  • 0
    There  is no reason why you cannot put as many rods at various places if you like on a TT installation. The only requirement is that the total Ze from the CPC meets the 50V criterion for the RCD. In general several spaced rods and some structural steel will reduce the Ze significantly which is what you want. There is no sense whatever in making the various TT sections separate from the others CPC, in fact this is often difficult anyway. Have you contacted the DNO yet?
  • 0
    On the subject of PNB, it seems there are a number of variants.


    In the simple case - one transformer feeding one consumer with the electrode connected on the earth side of the N-PE link at the consumer's cut-out it is a nice simple TN-S system.


    There do seem to be other possibilities however. Engineering Recommendarion G12 (which is what think most DNO's policies are based on) seems to suggest that up to 4 consumers may be permitted to share the same transformer - but as there is still only one point where the N is deliberately earthed - there can't be an electrode at every one of the consumer's cutouts - but must presumably be at some common point upstream. There's no suggestion that there needs to be separate N and PE conductors downstream of the point the N is Earthed, so there would be a CNE or PEN between that point and individual consumers which then have both their N and PE connected to the supply CNE. This would seem to be TN-C-S (but not PME as there's no multiple earthing). Some DNOs seem to limit PNB to a single consumer - but not all it seems.


    Even when there is a single consumer there is also the possibility of the N earthing point being upstream of the consumer's cutout where the N-PE link is - which again would be TN-C-S rather than TN-S.


    WPD's document https://www.westernpower.co.uk/downloads/3293 seems to give a pretty comprehensive overview (especially appendix B) and seems to sum things up nicely with "Depending upon whether the earth connection to the customer is taken from the transformer neutral earth or from the neutral direct this can be viewed as either TN-S or TN-C-S.".


       - Andy.
  • 0
    And that is the nub of the problem I think - when is TN-S not TN-S ?

    In terms of the broken PEN risk, and all the attendant  'not suitable for livestock, caravans, building sites, cars, or anything else that worries regulation writers  of a nervous disposition '  , the answer is very simple, when there is a multiplicity of NE links, instead of just one.

    Sadly in terms of sensible definition, this is not always the one used, so we find we can have.

    PNB -TNC-s or

    PNB TN-S


    To me in terms of the true risk, this sounds more like TNS and until another property is added to the transformer with an NE link at the cut out, it will remain immune to the rise of earth potential risk associated with PEN failures.


    You can have add many and various buried electrodes to TN-S of course, so long as you do so to the CPC, and not to the neutral.

  • 0
    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?
  • 0
    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, so any ideas of reconnecting the farm buildings to the DNO earth terminal have been blown away and I need to disconnect the PV installation earth conductor from the DNO earth and drop it into the farm buildings TT MET after I have checked the PV consumer unit out.


    There wasn't an opinion offered on what to do with the house one way or the other. 


    Having declared TN-S was maybe a bit of a long shot, but would have made life easier, but the fix isn't too complicated.


    Andy Betteridge
  • 0



    He has declared it as a TN-C-s PME system, so any ideas of reconnecting the farm buildings to the DNO earth terminal have been blown away and I need to disconnect the PV installation earth conductor from the DNO earth and drop it into the farm buildings TT MET after I have checked the PV consumer unit out.

     


    Why is that then? Is it a farm with animals or just an arable farm? I did ask before but got no answer.


    Z.


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