By the way Adrian, great to have you on the forum. New recruits are always welcome! You will find this forum a very useful sounding board with some of the best in our industry prepared to take time to respond to all sorts of queries in a friendly and measured way.

I think the pros outweigh the cons in this situation.If i mention it on my Eicr then the client will feel he needs to act on it, and he is very willing to keep his farm up to date and to a good standard. But say i did take that route and we decided to convert system to TT there is many drawbacks with this .Main one is Rcd protection on the sub mains. 3 sheds are supllied on 1 sub mains, 2 off another and finally one shed on its own along with other out buildings. So a 300ma Rcd feeding 3 seperate sheds .Each of these sheds have got 2 variable speed drives which require type b rcd and will almost certainly from experience give us nuisance tripping of the up front rcd.This along with a lot of other motors and equipment. This i would say is more unsafe for the welfare of the birds than a Tnc-s earthing arrangement. Does anyone think that it would be benificial to add an earth electrode to each submains whilst also keeping the tncs earth. Then in the event of a loss of neutral from the DNO there would still be an earth path?  Or maybe this is a pointless exercise.

My understanding on the subject is that for remote buildings, any extraneous-conductive-parts must be isolated from the metalwork that is connected to the PME earthing terminal. If isolation is not possible, then earth electrodes and rods for the entire installation must be installed (TT) as already discussed. Another option is to use a dedicated transformer to supply the premises and then protective neutral bonding like you have may be used. For dairies, a metal mesh must also be embedded in the concrete floor and connected to the earthing system to prevent the harmful effects of a small voltage difference on the animals’ feeding and milking behavior and productivity. 

that is the best practice  in the current standard, and if a new chicken shed  was being designed from scratch as  a new build, or in any new farm building for that matter, then studs can be welded to the floor slab re-bar, and the TNC-s retained. 

However, as the thread above, this is pre-exisitng, and actually this not a high risk category like a dairy farm, but actually only involves  animals whose feet are closer together than those of humans, so the whole  reason for that advice really does not apply.

Worse there is a credible risk to animal welfare if the additional RCDs for TT conversion are   applied.

Ergo, those who can see it, and weigh up the balance of  risks in that situation, may or may not decide the TNC s is not an immediate danger, or indeed a realistic potential danger - which sounds like the way it is heading - actually  I think it is moving towards 

'not to current standards but quite satisfactory for continued use and no action required'

But of course the caveat is that  neither of us have actually seen it....

M.

Hi mapj1. Do you agree the earthing can be transferred to the remote buildings if the supply is PNB from a dedicated pole transformer that supplies the premises.

In this case, as the PME risk is low - chickens, certainly, and in any case quite possibly..  It rather depends how the PNB has been done, and where the LV earth electrode connection  is made in relation to the N-E link (transformer side or load side).

There are in effect both TNC-s and TNS-s flavours of PNB, although unless it is clear the advice is usually to treat PNB as being a bit like PME, but as there is only one electode that is PME without the ME...

The thing to consider is could a single break leave the transformer star be left connected to terra-firma earth  but not the neutral and earth wiring of the instalation, or would the electrode remain connected to theCPC of the installation remain on the load side of the NE-split.

Most DNO documantation does not differentiate, and only really talks about SNE (separated Neutral and Earth) or CNE (combined Neutral and Earth) so it is often not clear what exactly is being presented.

But yes, PNB, done with the electrodes on the installation side, can be as solid as TNS and suitable for all situations,

Mike.

Thanks Mike. So Both PNB and PME are variations of the TN-C-S earthing system but In PME, the PEN or CNE conductor is earthed at the source and at multiple points in the supply network. In PNB, the neutral earth connection is only made at or near the customer’s premises (The Earth connection and the customer’s intake should be within 40 meters of each other) So would you agree this is probably the most common practice in scenarios where dedicated transformers or pole-mounted transformers in isolated locations supply individual customers ? I realise it’s best to confirm with the local DNO.

Exactly so- now electrically your 3 drawings are the same as the NE link slides along and are what might be considered TNS PNB i.e. the safer variant where there is no PEN.

But if  NE link is as your lower pic, but the electrode connection is as per your upper one, like this hack of your images, then it is stll PNB but a more TNCs-like version.

Here the risk become as per TNCs as the single lower conductor is actually both CPC and neutral, i.e.  a PEN, and now, like PME,  the users CPC  voltage uplift relative to terra-firma becomes  load current dependent, and a break in the PEN is now more dangerous.

In reality on a private transformer  the common PEN may be as short as one shared bolt or brass jumper link, and if it is shared or not depends how the LV earth  electrode has been wired. It is hard to tell without actually looking, and as I said above, the DNOs descriptions do not really differentiate. (and if it is all bolted up tight and idelaly doubel screwed, then no-one needs to care too much)

The key distinction is if during that fault, the electrode (and terra-firma voltage) goes with the customers CPC or the transformer star point - if that conductor is interrupted..

Mike

Please excuse my deviation from the main subject, but I find this topic very interesting and I would like to learn more from your expertise if you don’t mind. In the case of marinas, do dedicated transformers or pole-mounted transformers fall under the category of TN-S systems in terms of supply characteristics and earthing arrangements? I have not had any experience working on a marina, but I was intrigued by the other post on the forum.

It would be good  if they always did but the correct answer  is 'it depends' Certainly if the transformer is being put in and will only supply one premises, then getting it configured to be TNS used to be something that could be sorted by talking nicely to the chap who came to bolt it in, at least until a few years back. - I have not had to worry about it for a while but at one point I was involved with advising the installation of  mobile phone base stations, that have external masts and so on that we did not want to export the PME earth on.  At that time a TNS was easy to arrange at installation time or a few days before, but not easily changed after HV was connected.

As your diagrams illustrate it all depends where the NE link is and where the LV earth electrode attaches. Sites where the earth resistivity is high need separate HV and LV earths anyway, so even if the ~NE bond is in the building, the LV earth can be as well, so TNS is quite practical then.

Other things that get messy are when the site has been beefed up and there is more than one transformer- as on the consumer side of things you cannot have more than one NE bond, but as there often has to be, then part of the LV has to be considered as 'distribution' to meet the ECSQR.

Mike

It would be good  if they always did but the correct answer  is 'it depends' Certainly if the transformer is being put in and will only supply one premises, then getting it configured to be TNS used to be something that could be sorted by talking nicely to the chap who came to bolt it in, at least until a few years back. - I have not had to worry about it for a while but at one point I was involved with advising the installation of  mobile phone base stations, that have external masts and so on that we did not want to export the PME earth on.  At that time a TNS was easy to arrange at installation time or a few days before, but not easily changed after HV was connected.

As your diagrams illustrate it all depends where the NE link is and where the LV earth electrode attaches. Sites where the earth resistivity is high need separate HV and LV earths anyway, so even if the ~NE bond is in the building, the LV earth can be as well, so TNS is quite practical then.

Other things that get messy are when the site has been beefed up and there is more than one transformer- as on the consumer side of things you cannot have more than one NE bond, but as there often has to be, then part of the LV has to be considered as 'distribution' to meet the ECSQR.

Mike

do dedicated transformers or pole-mounted transformers fall under the category of TN-S systems in terms of supply characteristics and earthing arrangements?

It also depends on who owns the transformer - if it's owned by the customer, then yes they can arrange for it to be TN-S. If it's owned by the DNO they tend to want to keep their options open. E.g. if someone was to build a bungalow next to the site and wanted a supply (as they most likely would) then it often makes sense to tap into the exiting LV system (whether the transformer needs upgrading or not) and with multiple customers things tend to be TN-C-S (either true PME or PNB with the electrode at a common point before the split, and PEN conductors out to each customer) so most DNO supplies get slapped with "PME conditions apply" from the start regardless.

   - Andy.