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.
Try figure 13 and 14 here: https://commercial.nationalgrid.co.uk/downloads-view-reciteme/63784 (on page 53)
- Andy.
where the path of the N current shares a conductor with the path between exposed-conductive-parts and the means of earthing.
Ah, so you mean a transformer 'tail' ... the neutral point is the actual star point of the transformer, and therefore the 'tail' is a 'neutral conductor' by definition?
I think a mistake has been made in describing the N tail from the transformer as a PEN conductor.
I'm not sure it's that clear.
Calling it a PEN causes confusion and rapidly reduces the whole situation to absurdity.
I agree, but if we work with existing definitions, and follow your definition of a PEN conductor, then we must reach the absurd conclusion?
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.
I think in real terms, it's more like 'One does not need to conform to PME requirements, the other does ... although as we have seen elsewhere in this thread, at least where the DNO provides TN-S there's not always a guarantee that PME conditions won't apply.
The conclusion being that, only in TN-S and PNB systems supplied from only private transformers and generators, can we dispense with the requirements of BS 7671 relating to PME.
A break in a real PEN conductor is rather different (exposed-conductive-parts can become and remain at hazardous voltage w.r.t. true earth)
That can happen under some circumstances with 'private PNB' if things go awry.
I'm also not convinced that PNB always means 'earth is Earth' in a three-phase system with a broken PEN conductor, because the neutral balance will still be awry - the system won't be earthed at a star point. In private PNB where the transformer case is connected to the neutral, but not solidly to the PE system, you get ground currents that affect EPR.
The only saving grace about the transformer N-stub is it is far less likely to fail than cables are to be damaged.
According to part 2 Earthing is about connecting exposed-conductive-parts to Earth (via the MET) - rather than to the star point
That's simply because this is always the case for each of TN, TT and IT.
Due to debates on this topic at international level, the connection of a live conductor (usually the neutral) to earth is starting to be termed 'system referencing' ... BUT internationally, in single-phase systems not derived from a three-phase system, and split-phase systems, there technically is no 'neutral' ...
Ah, so you mean a transformer 'tail' ... the neutral point is the actual star point of the transformer, and therefore the 'tail' is a 'neutral conductor' by definition?
Yes, it's neutral - but just a neutral - it serves no Earthing purpose (i.e. it plays no part in connecting exposed-conductive-parts to the general mass of the Earth, so therefore isn't a PEN.(It might carry earth fault current, but so other conductors we don't call PEN or protective conductors).
and follow your definition of a PEN conductor, then we must reach the absurd conclusion?
Quite the reverse, I think..
- Andy.
The conclusion being that, only in TN-S and PNB systems supplied from only private transformers and generators, can we dispense with the requirements of BS 7671 relating to PME.
Agreed. Isn't the point here that the owner of the private transformer is also the consumer and that the one entity is responsible for the whole lot? No changes can be made to the network without the consumer's knowledge.
(Save in Downing Street. ;-) )
The conclusion being that, only in TN-S and PNB systems supplied from only private transformers and generators, can we dispense with the requirements of BS 7671 relating to PME.
Agreed (with possible exceptions where the DNO might guarantee or be legally obliged to maintain existing TN-S arrangements - perhaps to campsites or marinas). I think PME conditions should definitely apply to the TN-C-S version of PNB too - as that has all the troublesome aspects of PME - even the multiply earthed feature once consumer's bonds to extraneous-conductive-parts are added in.
For me, what's causing the confusion is that BS 7671 has taken the diagram of the TN-S version of PNB and labelled it TN-C-S. The real TN-C-S version has the electrode upstream of the N-PE link with a PEN conductor joining the too (which is the feature that causes much of the trouble). See the DNO diagrams I referenced earlier.
In any event DNOs generally reserve the right to alter any of their TN-S system into TN-C-S (whether PNB or not) - so the general assumption should be if you're using a DNO's earthing facility then PME conditions will apply.
- Andy.
For me, what's causing the confusion is that BS 7671 has taken the diagram of the TN-S version of PNB and labelled it TN-C-S.
But it can't be TN-S, because it's not earthed at the source, so the neutral and protective functions are combined in part of the system. However, I agree it's not PME.
In addition, it's possible to have TN-C-S systems with a multiple-earthed neutral that does not meet PME requirements.
PME is a specific form of TN-C-S, where the neutral is earthed at multiple points to achieve certain conditions.
Pedantic question:
an Earthing function
What is an earthing 'function'?
Is it a partial or complete function; a physical or conceptual item; is it extensive or point like ?
I.e. Can this single connection be the complete earthing function, or are we "looking for the wires, not the electricity" (*) ?
(*) the phrase was from a BBC interview with E.Powell about whether he was a power broker in politics.
But it can't be TN-S, because it's not earthed at the source, so the neutral and protective functions are combined in part of the system.
I'm not following the logic - the conductor may be connected to Earth, but to my mind it's not providing any earthing functions, since it doesn't connect any exposed-conductive-parts to Earth (the transformer case, is always connected to the HV Earth).
The TN system describes the relationship of the power system to Earth and the relationship of the exposed-conductive-parts to Earth (see 312.2) - rather than the wider earth fault loop.
Even the definition of a protective conductor talks about it connecting exposed-conductive-parts, extraneous-conductive-parts, MET, electrode and the earthed point of the source (my emphasis) - not the star point..
By all the definitions I can find the conductor to left of the N-PE link in Fig 3.9B is neither an Earth conductor nor a protective conductor. To my mind it's simply a neutral conductor. Eveything else then follows on reasonably logically. No rabbit holes required.
- Andy.
But it can't be TN-S, because it's not earthed at the source, so the neutral and protective functions are combined in part of the system.
I'm not following the logic - the conductor may be connected to Earth, but to my mind it's not providing any earthing functions, since it doesn't connect any exposed-conductive-parts to Earth (the transformer case, is always connected to the HV Earth).
The TN system describes the relationship of the power system to Earth and the relationship of the exposed-conductive-parts to Earth (see 312.2) - rather than the wider earth fault loop.
Even the definition of a protective conductor talks about it connecting exposed-conductive-parts, extraneous-conductive-parts, MET, electrode and the earthed point of the source (my emphasis) - not the star point..
By all the definitions I can find the conductor to left of the N-PE link in Fig 3.9B is neither an Earth conductor nor a protective conductor. To my mind it's simply a neutral conductor. Eveything else then follows on reasonably logically. No rabbit holes required.
- Andy.
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