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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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  • 0
    Bye heck! You’ve gone off piste there.


    Just because the DNO supply an earth terminal you don’t have to use it, indeed you have to decide if it is appropriate to use it.


    However in this particular instance both the DNO transformer and the underground DNO cable are  located entirely on the customers land, as is the one and only DNO earth which is connected to the supply cable at the building end of the cable, not at the transformer, you can see the connection coming out of the bottom of the head. I have not walked over to the pole lately, but the connections are all clearly visible and the LV earth is kept well away from it by putting the rod  at the building end of the cable.


    So if I disconnect the earth from the DNO terminal within the meter cabinet or within the workshop the DNO earth rod will still be within the vicinity of the steel portal frame of the farm building, I have no way of knowing how far back up the trench it is located, it could be immediately adjacent to a stanchion.


    That TN-C-S PNB is effectively TN-S has been debated at length on this forum, but I still have to treat it as TN-C-S PME as far as I know, I suppose I could try having a discussion with Western Power but need to decide what questions to actually ask!


    A few years ago I was reasonably happy with the farm building buildings being TT and the house being connected to the DNO earth terminal, but then the PV installers created a hard wired connection between the two earthing systems, this  however this is the first time I have had to address the issues that this may create as the house needs an EICR and I also need to consider that the use of the 25mm two core SWA armour was acceptable thirty years ago but maybe not acceptable now.


    The easy option is to make it all TT, but I’m not sure if that can create an issue with the two PV array inverters and the batteries inverter, also I need to determine that the PV/batteries consumer unit and RCD arrangement is suitable for TT, I need to have another look at it.


    But before making it all TT it is appropriate to actually determine it there really is anything wrong with the house using the DNO earth terminal.


    There are several options:
    • Reconnect the farm buildings to the DNO earth terminal, so everything is connected to it.

    • Disconnect everything from the DNO earth terminal and make it all TT.

    • Just move the PV and batteries installation earth conductor to the farm buildings TT MET and leave just the house connected to the DNO earth terminal.


    I am leaning towards the second option, make everything TT as the day goes on, it just means taking responsibility for altering the PV and batteries installation earthing.


    Andy Betteridge 

Reply
  • 0
    Bye heck! You’ve gone off piste there.


    Just because the DNO supply an earth terminal you don’t have to use it, indeed you have to decide if it is appropriate to use it.


    However in this particular instance both the DNO transformer and the underground DNO cable are  located entirely on the customers land, as is the one and only DNO earth which is connected to the supply cable at the building end of the cable, not at the transformer, you can see the connection coming out of the bottom of the head. I have not walked over to the pole lately, but the connections are all clearly visible and the LV earth is kept well away from it by putting the rod  at the building end of the cable.


    So if I disconnect the earth from the DNO terminal within the meter cabinet or within the workshop the DNO earth rod will still be within the vicinity of the steel portal frame of the farm building, I have no way of knowing how far back up the trench it is located, it could be immediately adjacent to a stanchion.


    That TN-C-S PNB is effectively TN-S has been debated at length on this forum, but I still have to treat it as TN-C-S PME as far as I know, I suppose I could try having a discussion with Western Power but need to decide what questions to actually ask!


    A few years ago I was reasonably happy with the farm building buildings being TT and the house being connected to the DNO earth terminal, but then the PV installers created a hard wired connection between the two earthing systems, this  however this is the first time I have had to address the issues that this may create as the house needs an EICR and I also need to consider that the use of the 25mm two core SWA armour was acceptable thirty years ago but maybe not acceptable now.


    The easy option is to make it all TT, but I’m not sure if that can create an issue with the two PV array inverters and the batteries inverter, also I need to determine that the PV/batteries consumer unit and RCD arrangement is suitable for TT, I need to have another look at it.


    But before making it all TT it is appropriate to actually determine it there really is anything wrong with the house using the DNO earth terminal.


    There are several options:
    • Reconnect the farm buildings to the DNO earth terminal, so everything is connected to it.

    • Disconnect everything from the DNO earth terminal and make it all TT.

    • Just move the PV and batteries installation earth conductor to the farm buildings TT MET and leave just the house connected to the DNO earth terminal.


    I am leaning towards the second option, make everything TT as the day goes on, it just means taking responsibility for altering the PV and batteries installation earthing.


    Andy Betteridge 

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