Farm earthing arrangements.

So is it 411.3.1.1 that concerns us? 


Z.

Jump forward.


I will say that the earthing arrangement is considered to be TN-C-S, but is PNB not PME. So is considered by people on a far higher pay grade than me as effectively TN-S.


So as an electrician I have to consider the DNO system as TN-C-S, which is not banned on farms, but actually this version of TN-C-S may not have the risks that are normally associated with it because it is effectively TN-S, but I cannot declare it as that.


Until the PV system was installed there was no interconnection between the house and buildings by protective conductors or extraneous conductive parts, but now there is.


So where to from here?


 Andy Betteridge

Drawing not to scale, but is the farmstead the only customer from the transformer?


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?

Yes it is the only installation supplied by the transformer and unless the farm building and house supplies are separated will always be so, but doing that probably means having a new transformer and digging the fields up so is completely re-engineering the system anyway.


It is not difficult to drop the house tails out of the Henly block and fit an upfront RCD for the house, then swap the house earth conductor onto the TT earth terminal, there is a main switch I can just turn off to work.


But if I do that I need to assess the PV and storage batteries system and move the earth conductor from them onto the TT earth terminal as well having confirmed they are adequately protected by an appropriate RCD, so effectively redesigning and re-engineering the PV and batteries installation.


The PV guys could not see a problem or any issues regarding connecting to the DNO earth terminal when they designed their installation, so is it appropriate or a requirement for me to alter their installation, indeed should I consider altering the farm building installation and reconnecting it to the DNO earth terminal, which I am not considering at the moment? 


However I also need to consider if the 25mm two core SWA armour is suitable as a main earth conductor for the 60 amp house supply.


Andy Betteridge

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.

 

If TN-C  (earthed concentric wiring) is normally forbidden for use in the U.K. then why make a special mention of it in reg. 705.411.4?

TN-C need not be concentric - ABCs and some open wire overheads aren't concentric but still can be TN-C-S.


BS 7671 doesn't forbid TN-C and the ESQCR only prohibits it in consumer's installations and then only for systems to which that legislation applies (which I suspect means things connected to the grid). So private networks that aren't consumer's installations (like BNOs) could be TN-C - likewise (I suspect) off-grid or private generation systems. Plus of course BS 7671 can apply in regions where the UK law doesn't.


   - Andy.

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 

And why is the SWA armour not suitable? You seem to be suggesting that the N-E bond and therefore the Earth rod are used by the DNO in the house as the only supply Earth connection? If so (and I doubt it) what is the problem with TTing the house as well? The supply to the house is NOT TNC-S as you seem to think and you are not importing a TNC-S supply. You need to talk to the DNO because you seem mighty confused and are wanting to do changes that probably are not justified. A necessary requirement for a PME supply is multiple points of earth bonding, and if these are not present an open neutral is not particularly dangerous (as the current is limited by the leakage capacitance) and will not be protected by RCDs very well if at all. It is the fact of multiple other loads which makes TNC-S with an open neutral more dangerous, not a single isolated supply. Try drawing out the shock circuits carefully and you will see why.