consumer unit schematics for the outbuilding

[ebee]

I suppose that depends upon what you mean by "only". As I said, in my version of an "!only" statement, in a situation such as we are talking about ()i.e. per the photo we have seen) a definitive answer can 'only' be achieved by someone' looking inside the cutout.

In my limited experience, DNOs are not usually able to answer such a question 'from their records' {which many people seem to think they can} - so even the DNO would have to look inside the cutout in a situation such as we are talking about..

Kind Regards, John

What I meant John was that only the DNO can declare what it is as far as they are concerned and that definition must stand as the authorative one .
Only they know what it was, how it has been changed, what nearby connections are affecting it so to what extent it falls into which category and therefore what it now is as far as they are concerned and therefore we can only guess, however good our guess may or may not be. They have (should have) all the info and we do not.
My own property was originally TNS and that is plain to see with the PILC and all of the service heads for this town origin, indeed it was the locasl councils who originally owned it. But so much has altered over the last few years with repairs just a few yards away and the Ns and Es joined so many times it can not be considered as anything but TNC-S (or at the very least, TNC-S like in pretty much every respect in the way it operates) but each time we have to ask for the DNO to actually declare what it is.

 

[JohnW2]

What I meant John was that only the DNO can declare what it is as far as they are concerned and that definition must stand as the authorative one .

Yes, I understood what you meant but, as I said, in the case we are discussing (and many/most other cases) anyone who looks inside the cutout can determine with certainty what sort of DNO-provided 'earth' (if any ) it is.

That 'anyone' who opened the cutout should, strictly speaking,' be DNO personnel or other 'authorised' people (like 'meter changers'), but the answer to the question could be ascertained by 'anyone' who looked.

Only they know what it was, how it has been changed, what nearby connections are affecting it so to what extent it falls into which category and therefore what it now is as far as they are concerned and therefore we can only guess, however good our guess may or may not be. They have (should have) all the info and we do not.

The history of the situation, and issues like "nearby connections" are surely irrelevant to the question of what sort of DNO-supplied earth (if any!) there currently is - which, above, simply requires determination of what (if anything) of the DNO's supply is connected to the MET (i.e. the incoming CNE or the cable sheath).

I don't know whether, when they 'convert' TN-S to TN-C-S they ever leave the cable sheath connected to the MET. I somehow doubt it but, if they did, I'm not sure what they would 'call it'. However, particularly given that it would presumably most commonly be done because the sheath had 'failed', I imagine it would probably be regarded as simply TN-C-S.

Kind Regards, John

 

[ebee]

yes indeed they do John, old TNS worked upon or works done nearby and they redeclare it as TNC-S, in some cases say they make a repair or two at or nearer similar houses in the same street then they redeclare from TNS to TNC-S but they do not go around informing everyone that their supply is changed, you only find out when you ask them what it is, and the answer is often TNC-S whereas a few years ago their answer would have been TNS and there was no work on your sightline of supply and you happen to find out that there has been a splice repair around the corner/in the next street etc. It is pretty much a given now that you mostly get the answer TNC-S in the towns I live in/near so the easy way is to assume that it is TNC-S or ask them, further out from town you get a few TT but rarely a TNS answer.

 

[JohnW2]

yes indeed they do John, old TNS worked upon or works done nearby and they redeclare it as TNC-S, in some cases say they make a repair or two at or nearer similar houses in the same street then they redeclare from TNS to TNC-S but they do not go around informing everyone that their supply is changed, you only find out when you ask them what it is ...

I don't really understand. "What it is" in a given installation is solely dependent up[on what is connected to the MET of that installation.

If the MET is connected only to the CNE or only to the cable sheath, or to nothing provided by DNO, the situation is straightforward and "what it is" is totally clear. Only if the MET is connected to both the CNE and sheath is there potential confusion, but, even if the sheath is at least partially intact, that really is TN-C-S/PME, since the path to earth (and transformer neutral) really is just one of the "M"s of the PME.

It would seem a little 'naughty' for them to leave the cable sheath connected to MET when they 'convert to TN-C-S by connecting the MET to the CNE, since that unnecessarily increases the number of installations that would experience 'high earth p[potentials' in the event of a 'lost CNE' conductor.

If I understand (maybe not?) what you are suggesting,you seem to feel that if that happens adjacent properties that are still 'wired as TN-S' should be regarded/declared by the DNO as TN-C-S, because of the connection between sheath and CNE in some other property. Is that what you are suggesting? If so, do you think that the DNO should regard/declare my (TT, as far as I am concerned) installation as TN-C-S because my MET appears to be connected via bonding and a metal water supply pipe to my neighbour's (undeniably) TN-C-S 'earth' ?

Kind Regards, John

 

[ebee]

Not exactly,
Imagine adjacent/nearby properties all TNS then two or three get a solid bolted connection close by linking N & E together so they are parallel effectively an infinitesimal impedance between N & E for the last few metres into a property and all the way back to transformer Starpoint

The S in TNS is no longer S but effectively C to all intents and purposes, thereby now TNC up to the property or nearly so.

 

[JohnW2]

Not exactly, ... Imagine adjacent/nearby properties all TNS then two or three get a solid bolted connection close by linking N & E together so they are parallel effectively an infinitesimal impedance between N & E for the last few metres into a property and all the way back to transformer Starpoint

Do you mean with the cable sheath ()as well as now, the N) still connected to the MET in the 'converted' installations?

The S in TNS is no longer S but effectively C to all intents and purposes, thereby now TNC up to the property or nearly so.

Well, for a start, the 'conversion to TN-C-S' is commonly done because the sheath (providing a TN-S 'earth') has 'failed, and if that failure is 'complete', then the conversion to TN-C-S will have absolutely no affect on any other TN-S installations.

If the 'conversion' is undertaken despite the sheath (hence TN-S 'earth') still being intact, then I suppose what you say is true - but I think it would be of pretty limited relevance to anything.

... but is still leaves the question of my (TT) installation - since it is "TN-C" all the way back to the transformer other than for abit of bonding cable and a bit of water pipe, should the DNO regard/declare it as "TN-C-S", even though I regard and use it as TT?

Kind Regards, John

 

[ebee]

If you use it as TT then the DNO would declare it as TNC unless last time they checked you were using it as TNS or TNC-S is my understanding.

Imagine this.
Your supply is TNS and has been since the year dot.
say 30 houses away down your street a repair was done and N/E linked together, in the next street on both sides of you a similar repair has been done at some time.
So, not far from your property there are a link or 3 between N & E, your original TNS back to the transformer can no longer be considered TNS but is most like TNC in it`s actual working properties.
So under classifications you would have a TN supply that is not TNS but could be TNC-S or TT depending upon the way your installation is connected , if you look at your service head whether opened up or not it will present as TNS, it could meter out similar to TNS or not quite, how would you know?
If you ask the DNO they would look at their records and tell you and/or they would make a site visit.
If you happen to see one of their linesmen in a hole doing a repair and explain your credentials they will often say "Treat it as if it were PME" in other words it is TNC-S at your installation unless you are using it for TT.

PS, in a neighbouring town I was working at a house I I checked Ze, both with and without bonding in place, with bonding I got expected results for TNS, without bonding I got a very high Ze. Oooh Deary me!
I contacted the DNO and the next couple of days I noticed that they had that garden and some adjoining properties gardens dug up plus a little on the pavement and street areas.

I returned months later and so I rechecked, all ok.

So, obviously a rotted PILC then after rectified there was no change to be seen on the service head.
Had they repaired by connecting E & N with a new piece of cable? No idea.

I suspect that a few properties in nearby streets had similar problems due to info I received from one or two other installers, they had noticed similar but not reported it. I assumed that all the bonding in those streets had provided an effective earth path to all properties

 

[JohnW2]

If you use it as TT then the DNO would declare it as TNC unless last time they checked you were using it as TNS or TNC-S is my understanding.

As far as an installation (which is what we've been discussing) is concerned, what surely matters is what earth IS being used, rather than what earth could be used, isn't it?

From the DNO's perspective, I suppose they could 'declare' any supply that was PMEd to be TN-C, and 'declare' any supply to be TN-S if it had an intact/adequate sheath - and, I suppose, declare it as ';both' if it were PMEd and had an intact/adequate sheath !

However, if (as in my case) one chooses to ignore what is provided (or potentially provided) by the DNO and, instead rely on one's own TT electrode, the installation is surely TT, regardless of anything the DNO might say. Similarly, if the DNO supply is both PMEd and has an intact/adequate sheath (which could be the case for me, if my supply were 'underground'), then if one uses just the DNO's sheath as one's earth, then one's installation is TN-S, even though it could have been wired as TN-C-S, isn't it?

However, it';s a little more complicated than that since, although one is free to ignore everything to do with "DNO earths" and configure one's installation as TT (as I do), one cannot (should not!) oneself create connections to the incoming cable sheath or CNE. It thereore comes down to 'what the DNO have done' and, as I've said, in the case we are discussing that really requires a look inside the cutout to be sure.

Returning to your comment above, the one thing my supply cannot be is TN-S, since it's an overhead supply consisting of just four singles (3p+N) with no separate 'earth' conductor of any sort. However, I understand that it has been PMEd, so the DNO could possible regard it as a TN-C supply (even though I don't use it as such.

The added complication with my installation is, of course, that although it is configured as TT, it's earing system seems to have a low impedance path to the neighbour's TN-C-S earth, via bonding and a water pipe! ... so I suppose one can debate what I should call the earthing system. However, since I can't guarantee that the water pipe will always remain metal, I think I have to assume that the connection to a TN-C-S earth will not necessarily always be there.

I'll respond to the rest of your message once I've given it a little more thought!

Kind Regards, John

 

[ebee]

As far as an installation (which is what we've been discussing) is concerned, what surely matters is what earth IS being used, rather than what earth could be used, isn't it?

yep go on then John, you are correct.

I do disagree with your TNS statement however, Any joint after the "Star Point" connection of N & E (no matter how many metres that star point connection is physically form the transformer) can not allow it to be TNS, that separation is lost at the second (and subsequent) connection.

 

[JohnW2]

yep go on then John, you are correct.

As I'm sure you understand, what I'm trying to learn is what is the relevance of what a DNO 'declares' their supply to be (in terms of provision of an 'earth'), when what actually matters is what 'earth' the installation actually uses.

Kind Regards, John