AMK said:

PME and PNB are both variants of the TN-C-S earthing system, where the PEN conductor is split into separate neutral and earth conductors at the consumer’s premises.

 AMK , this isn't quite true.

Even in some public PNB systems, the distributor provides an SNE cable.

WPD, at least, used to publish this openly.

The key differentiator, is that the transformer star point (or mid-point) is NOT earthed directly in PNB systems (whatever the variant), but is in true TN-S and TN-C-S, but the N conductor is connected elsewhere from the transformer.

You can then argue whether the "N" conductor (I will not call it a Neutral at this juncture, because strictly, according to the International Electrotechnical Vocabulary, it's perhaps not a Neutral in single-phase or split-phase secondary transformers) in a PNB system acts as a PE conductor for the transformer itself ... but the wiring (which is what BS 7671 is concerned with) can be SNE, and therefore it's possible for PNB to be TN-S (but PME conditions would still apply).

The following diagrams might help illustrate what I'm talking about:

There are further (minor) variations in the above between the DNO's (and before that the old 'Electricity Boards').

A small number (typically 4) of consumers may be supplied by each of the PNB variants.

In 'variant 2', sometimes each customer has their own electrode, sometimes only the first, or first and last.

Those are very helpful diagrams Graham, your commentary on multiple potential pole consumers each with their own earth rod explains the ENA logic of applying PME bond sizes to PNB supplies

Hello Graham, I have drawn my conclusions from the following reliable sources, which support my statement that PME and PNB are both variants of the TN-C-S earthing system, where the PEN conductor is split into separate neutral and earth conductors at the consumer’s premises:
The [Electricity Safety, Quality and Continuity Regulations 2002] stipulate that a TN-C-S system is “an earthing system having a common conductor throughout the system which is used to carry both earth fault current and the neutral current of that system, and in which the voltage of that conductor is kept substantially at earth potential by connecting it to earth at one or more points”. This stipulation applies to both PME and PNB systems, as they both employ a common conductor (PEN or CNE) that carries both earth fault current and neutral current, and is connected to earth at one or more points.
The [IET Wiring Regulations (BS 7671:2018)] specify that a TN-C-S system is “a system having one or more points of the source of energy directly earthed, the exposed-conductive-parts of the installation being connected to that point by protective conductors”. This specification also applies to both PME and PNB systems, as they both have one or more points of the source of energy directly earthed (the neutral of the source in PME and the PEN or CNE conductor in PNB), and the exposed-conductive-parts of the installation being connected to that point by protective conductors (the metallic sheath or armour of the service cable in PME and the separate protective conductor or CNE conductor in PNB).
[The Electrician’s Guide To The Building Regulations] state that PME and PNB are both variants of TN-C-S systems, where “the distributor provides a combined neutral and protective conductor (PEN) up to a point near to the consumer’s installation. At this point, separate neutral (N) and protective (PE) conductors are provided for connection to the consumer’s installation”. This statement corresponds to both PME and PNB systems, as they both have a PEN or CNE conductor up to a point near to the consumer’s installation, where it is split into separate neutral and earth conductors. Therefore, based on these sources, I uphold that my statement is accurate and consistent with the definitions and descriptions of TN-C-S earthing systems. However, I highly respect your opinion and suspect you are right. I also invite any further evidence or arguments that contradict my claim.

Agreed, but regards your first point:

AMK said:

Hello Graham, I have drawn my conclusions from the following reliable sources, which support my statement that PME and PNB are both variants of the TN-C-S earthing system, where the PEN conductor is split into separate neutral and earth conductors at the consumer’s premises:

That is true of 'variant 2' in my diagrams, but NOT 'variant 1' which is split earlier than the consumer's premises.

And the last point.

AMK said:

[The Electrician’s Guide To The Building Regulations] state that PME and PNB are both variants of TN-C-S systems, where “the distributor provides a combined neutral and protective conductor (PEN) up to a point near to the consumer’s installation. At this point, separate neutral (N) and protective (PE) conductors are provided for connection to the consumer’s installation”. This statement corresponds to both PME and PNB systems, as they both have a PEN or CNE conductor up to a point near to the consumer’s installation, where it is split into separate neutral and earth conductors.

Variant 2, the distributor provides a separate N and PE, not a PEN.

It certainly is not that clear cut at all.

EXCEPT that G12/5 says that PME conditions apply where PNB is used (that would be for Variant 1 and Variant 2 - although strictly only variant 2 is always TN-C-S).

There's a little more to this, being the fact that the Neutral (as we use it in most of BS 7671) isn't always a "neutral" ... especially in single-phase systems (i.e. where the LV supply transformer is single-phase).

How could you! Defamatory allegations on the internet. I have purchased numerous IET publications

I have been a member sine 2015 ! 

AMK said:

How could you! Defamatory allegations on the internet. I have purchased numerous IET publications

The simplified explanations work most of the time, and certainly won't lead you down the wrong path (unless you call the private version where the . That doesn't make them incorrect.

As I said, G12/5 says that PME conditions apply where PNB is used (that would be for Variant 1 and Variant 2 - although strictly only variant 2 is always TN-C-S).

Have a look at 14.1.2 of IET GN5 (page 118 in the 9th Ed) which discusses the point ... we need to be very careful in writing guidance, NOT to lead people to believe that 'variant 1' in my diagram above, is actually "TN-S" because "PME conditions" are more important for safety.

One difficulty in writing the guidance, is that BS 7671 all but says "TN-C-S = PME = TN-C-S". In fact, what's more important is the safety and protective provisions n BS 7671 for PME, that should be applied to PNB when used in the public supply. that would not be an issue for a private PNB system.

Older versions of GN8 (pre-2018) also discuss this.

Those diagrams are helpful, but I know of a supply to two cottages which fits none of them. The sole earth appears to be close to the transformer's pole (I have not trespassed to get a closer look) as in PNB variant 1; but the CNE cable divides at the service head rather like PNB variant 2.

Life can be confusing!

Chris Pearson said:

Those diagrams are helpful, but I know of a supply to two cottages which fits none of them. The sole earth appears to be close to the transformer's pole (I have not trespassed to get a closer look) as in PNB variant 1; but the CNE cable divides at the service head rather like PNB variant 2.

Yes, agreed ... that's a little more like "traditional TN-C-S" ... I think it's been said a few times in my posts in this topic, that there are more permutations of the variants. It also depends on the DNO (and the old "board" before privatisation) as to which exact arrangements you're likely to have in a given area.

Chris Pearson said:

Those diagrams are helpful, but I know of a supply to two cottages which fits none of them. The sole earth appears to be close to the transformer's pole (I have not trespassed to get a closer look) as in PNB variant 1; but the CNE cable divides at the service head rather like PNB variant 2.

Yes, agreed ... that's a little more like "traditional TN-C-S" ... I think it's been said a few times in my posts in this topic, that there are more permutations of the variants. It also depends on the DNO (and the old "board" before privatisation) as to which exact arrangements you're likely to have in a given area.