How good are you at telling the difference?
How good are you at telling the difference?
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.
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.
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.
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