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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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    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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  • 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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