TN-C-S or TN-S

[JohnW2]

Wouldn't TN-S vs TT depend on whether there is a cable from where the N connects to a local earth rod to the installation, or whether the installation has an earth electrode separate from the one N is connected to?

Yes, of course it would - your diagrams are what virtually everyone presumably expects, and which illustrate/define TT and TN-S.

However, as I've been saying to eric, if he has a TN-C-S installation which suffers a 'CNE failure', there will then be no cable connecting the installation's CPCs (hence exposed-c-ps) to the transformer - so the only path (if any) back to the transformer from L-CPC faults in the installation will be 'through the soil', via a local earth electrode and/or extraneous-c-ps (if any) - and I can't understand why eric thinks that means that his installation would have 'become TN-S'. Usually similar language, I would say that it had 'become TT'.

What am I missing or misunderstanding?

 

[morqthana]

Where's the fault Eric is postulating?

I'm assuming upstream of his inverter. And his inverter is upstream of the N-E link in his installation. So if a fault upstream of the inverter causes its N to connect to its local earth rod, the installation N-E link is still there, and his installation earth is still derived from it.

All that's happened is that the supply source is now his inverter instead of the substation transformer, so isn't he actually still on a TN-C-S supply?

 

[morqthana]

More odd goings on.

I remember replying to this post, saying that my TN-S supply has sections of CNE from repairs.

TN-C-S is the connection at the service head.
PME is what may exist within the network.

Older services can still be TN-S at the service head, but that reveals nothing about what may be connected elsewhere in the network.

The real question is whether the N&E are combined at some point(s) between the transformer and the service head.
In the vast majority of cases they will be, either with CNE cable throughout or sections of CNE where repairs and alterations have been done.

And here's a notification I got after that, but well before my two more recent posts above

But my reply to #8 has also gone.

 

[JohnW2]

More odd goings on. .... I remember replying to this post, saying that my TN-S supply has sections of CNE from repairs. ..... But my reply to #8 has also gone.

Yes, I remember your post - but, yes again, it has vanished.

The posts now visible in the thread bear consecutive numbers - and even that is a bit naughty/potentially confusing, since the numbers of most of the posts (which may have been referred to in other posts) have presumably changed as a consequence.

I can't see that there was anything remotely 'wrong' with the two posts of yours that I have been and which have disappeared (and there maybe more?), so goodness knows why they would be removed - and, in any event, on the very few occasions over the years when posts of mine have been removed (or moved) have had the courtesy to inform me of the reason in an 'alert'.

I think we really deserve an explanation about what has been going on since, on the face of it, it would seem pretty inappropriate!

 

[bernardgreen]

Missing posts may be a result of a problem with Cloudfare

 

[JohnW2]

Missing posts may be a result of a problem with Cloudfare

Who knows - I suppose that 'anything is possible' but I wonder how likely it is that a software problem would result in the missing posts all relating to the same member, and for the remaining non-missing ones getting re-numbered?

 

[JohnW2]

All that's happened is that the supply source is now his inverter instead of the substation transformer, so isn't he actually still on a TN-C-S supply?

Yes, I think I've seen some of the light . I was thinking/writing about 'TN' installations from the viewpoint of the DNO supply - and, as I wrote, there could not possibly be a ('DNO supply') TN earth if the sheath of a TN-S supply or the CNE of a TN-C-S supply had 'failed'. However, as you remind me, I should have been thinking about things with the inverter, not the DNO supply, as being the 'source'.

I'm assuming upstream of his inverter. And his inverter is upstream of the N-E link in his installation.

There is potential for confusion with the terminology, since 'streams' can go in different directions, but I take it that your 'upstream' means upstream of the inverter when the inverter is the source.

Having given more thought to it, I think the situation becomes rather arbitrary (and 'trivial') when the 'source' and 'installation' are in essentially the same location and have only one connection between them to true earth - since, as far as I can see, whether the (inverter) supply is TN-C-S or TN-S depends upon the (essentially arbitrary) decision as to the position of the dividing line (dashed red line in my diagrams) between 'source' and 'installation' - see my diagrams A and B.

However, if the (one) point of connection to earth is on the installation side of the 'dividing line' (my diagrams C & D), I'm less sure what one should call that - but nor do I think that it makes a blind bit of difference what one decides to call it

 

[flameport]

C and D are both variations of TN.

If someone really wanted an inverter or generator installation to be TT it would look like this

but no real installation will be like that.

Inverter / generator installs are almost always TN-S, with the N-E link internal to the generator or immediately after the output terminals, the earth electrode also being connected at that same point.

 

[JohnW2]

C and D are both variations of TN.

Indeed they are but (for what it's worth!) would you call them TN-S or TN-C-S ?

If someone really wanted an inverter or generator installation to be TT it would look like this .... but no real installation will be like that.

Agreed - but, as you imply, there would be no point (yet some reasons for not doing that).

Inverter / generator installs are almost always TN-S, with the N-E link internal to the generator or immediately after the output terminals, the earth electrode also being connected at that same point.

Yes, that's more-or-less what I concluded - although, as I illustrated, I think there are some situations in which one could describe it as TN-C-S (although that would make no difference).