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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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    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

    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.

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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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    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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    1034a07b303b91ddbd79480d7e21a78d-huge-101_1275-2.jpg

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    Most of that diagram can be stripped away 

    244b3595432a992c25d7fd45bd8c0f2e-huge-20200426_141234.jpg


    Assuming it is Protective Neutral Bonding, there isn't a PEN conductor or a seperate earth conductor in the DNO cable, the neutral link, source earth conductor and the consumer  installation earth conductor are all connected into the same terminal block in the intake in the meter cabinet on the outside wall of the farm buildings. 


    This set up is really as simple as it can get without an frills or gizmos. 


    Andy Betteridge
  • 0

    AJJewsbury:




    Is the DNO's earth rod connected to the service head; and if so, is the N-E bond made there? If that is the case, with the DNO's agreement, why not sever the N-E bond, retain the rod, and make the whole lot TT?



    I hoping I've mis-understood, If this is a PNB system and the N-PE link is removed then you're removing the means of earthing the supply - it's then not a TT system but an IT system - and given usual DNO transformers likely to float to several kV above true Earth - with some nasty consequences for the consumer's installation.


    Andy, thank you my MISTAKE! I hadn't looked at the picture properly. ?


    So the DNO's rod is the source earth which has been placed in accordance with R.8(3)(b) of ESQCR 2002. It must stay as it is.


    The system is PNB. I must say that it would be helpful if it got a mention in BS 7671, but it doesn't.


    So here is Fig 3.8 (TN-S) with the superfluous bits removed and the two nodes at the extreme left combined into one. If I understand the description correctly, this is the same as the supply to the farm.

    c56306c60e263e476954e0300f4aed76-huge-pnb.jpg



    So this now leaves two questions:


    (1) How far apart are the DNO's source earth and the TT earth?

    (2) Why was the farm building converted to TT?


    I share the opinions that the farm building should have only one earthing system, be it TT or TN.

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    davezawadi:

     I am also not sure why you think an RCBO per circuit is necessarily better, it is several times as likely as a single one to fail, and much less likely they will all be tested regularly, I suppose it does make a trip less inconvenient but also less likely to be noticed.




     

    It is a Crabtree split load consumer unit in the house with the four RCD protected ways all used, the only way to provide RCD protection to lighting circuits and circuits supplying bathrooms that also have cables concealed in walls is to swap the MCBs for RCBOs or replace the complete consumer unit.


    Doing so will upgrade the existing installation by providing additional RCD protection, but the consumer unit will still have a plastic enclosure, so it is still a compromise.


    Andy Betteridge.
  • 0
    Hi Alan.


    I have just emailed WPD to enquire.


    I also have it in mind that you are in a completely PME free network, offering TN-S or the PNB I have described above and understand my concerns about the PV guys not sticking with the earthing system of the rest of the farm buildings and how it influences the house arrangement.


    As you say going all one way or the other is the right answer.


    Come the day someone wants one or more EV chargers the house may have to have its own supply, which will mean replacing the transformer and starting again from scratch.
  • 0
    Andy, why are you upset? Your reply is definitely not "thanks for your assistance" despite getting a DNO reply and suggestion from Alan. All SWA armour below 95mm (from memory) is adequate as the earth connection, and some larger sizes too. There is a paper from JP somewhere on this, ask nicely and he will supply. I am also not sure why you think an RCBO per circuit is necessarily better, it is several times as likely as a single one to fail, and much less likely they will all be tested regularly, I suppose it does make a trip less inconvenient but also less likely to be noticed. Upgraded is a term from the HiFi arena, meaning spend more money for no gain, at least none you can discern! The point of this forum is to provide advice and to allow one to learn, which all of us do all the time. No one is saying it is wrong to ask or be unsure, in fact the opposite is the case. Many find the forum an invaluable resource.

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