TN-C-S (PNB) versus TN-C-S (PME).

For a domestic or small commercial PNB you probably have to look outside of the building - see whether there's a a DNO transformer for just your property (or occasionally up to 4 close ones) and whether there's LV earthing at the transformer or anywhere else (easier to see with pole mounted ones).

It does beg the question of why bother though? In most cases the DNO reserve the right to change things - and typically that'll mean converting to PME - so rather like an old 'presented as TN-S' situation where we effectively have to treat it as PME anyway.

Big industrial PNB whether they have their own private transformer is presumably TN-S rather than TN-C-S anyway (otherwise they'd be in trouble with the ESQCR).

    - Andy.

AJJewsbury said:

Big industrial PNB whether they have their own private transformer is presumably TN-S rather than TN-C-S anyway (otherwise they'd be in trouble with the ESQCR).

Something interesting to consider.

At what point does TN-S become "TN-C-S (PNB)"?

If you want to be really pedantic, even a short stub of neutral conductor from the transformer, generator, etc., could be considered a PEN conductor?

So, is TN-S actually a thing? And depending on how you answer that, with a private transformer or generator, even with earthing at the source, how do we then consider Regulation 8(4) of ESQCR? No private supplies permitted? No 'island mode'?

At what point does TN-S become "TN-C-S (PNB)"?

In my mind it's quite simple - where the path of the N current shares a conductor with the path between exposed-conductive-parts and the means of earthing.

I think a mistake has been made in describing the N tail from the transformer as a PEN conductor. Earthing is the concept of connecting an exposed-conductive-part to a means of Earthing. In the new Fig 3.9B the N conductor to the left of the N-PE link serves no earthing purpose - it's not connected to any exposed-conductive-parts - just the star point (transformers are earthed to the HV earthing system). Calling it a PEN causes confusion and rapidly reduces the whole situation to absurdity.

As I understand DNO documentation, there are two versions of PNB - the common one is TN-S in our terms, another (typically used where there's >1 customer) does have a PEN conductor and does carry some of the dangers of PME, despite having N earthed at a single point on the DNO side.

   - Andy.

AJJewsbury said:

In my mind it's quite simple - where the path of the N current shares a conductor with the path between exposed-conductive-parts and the means of earthing.

I think a mistake has been made in describing the N tail from the transformer as a PEN conductor.

Now that BOB has arrived, I see what you mean, but I am not sure that I agree.

Suppose that the line conductor in my dishwasher comes into contact with the case (which probably happened a few days ago). The fault path would be back along PE to the neutral-earth link, across the link, and to the transformer along the N, which is now acting as PEN.

gkenyon said:

At what point does TN-S become "TN-C-S (PNB)"?

When the N-PE link is remote from the transformer.

Now you are going to ask what "remote" means.

gkenyon said:

If you want to be really pedantic, even a short stub of neutral conductor from the transformer, generator, etc., could be considered a PEN conductor?

But our friends in wigs would say that is de minimis and accordingly does not count.

The transformer across the road from here sits in its 4.5 m square enclosure. I would say that provided the earth is within that curtilage, the system was (when installed in 1959) TN-S. I have no idea what SSEN did when they changed the transformer a couple of years ago. If the N-PE link is now at the junction box under the highway, it begins to look more like PNB. There must still be separate N and PE conductors in the street main for the older houses which still have TN-S (see my earlier photograph).

That said, the modern houses have CNE service cables so best to treat everything as PME.

gkenyon said:

At what point does TN-S become "TN-C-S (PNB)"?

When the N-PE link is remote from the transformer.

Now you are going to ask what "remote" means.

gkenyon said:

If you want to be really pedantic, even a short stub of neutral conductor from the transformer, generator, etc., could be considered a PEN conductor?

But our friends in wigs would say that is de minimis and accordingly does not count.

The transformer across the road from here sits in its 4.5 m square enclosure. I would say that provided the earth is within that curtilage, the system was (when installed in 1959) TN-S. I have no idea what SSEN did when they changed the transformer a couple of years ago. If the N-PE link is now at the junction box under the highway, it begins to look more like PNB. There must still be separate N and PE conductors in the street main for the older houses which still have TN-S (see my earlier photograph).

That said, the modern houses have CNE service cables so best to treat everything as PME.