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Repost - Earth Rod not taken account in TN systems

fiftyhertz

Hi All,

I realise the function of the earth rod in a TN system is to provide a close reference to true earth for the neutral

The thing that has confused me slightly is the TNCS PNB, which has an earth rod located at the consumer end. When i looked at the old forums there was a debate between whether this was TNCS or TNS, as the neutral carries no current due to the earth rod, and therefore by definition cannot be a combined conductor. If the current is not dissipated into the ground via the rod, why would no current flow in the neutral of this system prior to the rod

Thanks in advance

EDIT: My question wasn’t phrased very well and I’ve tried to clean it up for future readers, but i think this is the correct summary.

Fault current CAN flow between the neutral/earth link and the neutral point of the transformer in a PNB earthing arrangement. The previous forum posters were essentially saying is that even though though the link is remote, fault current will still flow in the CNE cabling, but we can note that it also would in a pure TN-S system but more likely an internal section of busbar within the TX and the neutral bar, instead of external cabling and by that logic TN-S would be a form of TN-C-S if semantics were involved.

Link to thread

 What earthing arrangement is this? 

Parents
  • +1
    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.

      , 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:

  • 0 in reply to gkenyon

    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.

  • 0 in reply to gkenyon

    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

Reply Children
  • 0 in reply to fiftyhertz

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

  • 0 in reply to AMK

    I have been a member sine 2015 ! 

  • 0 in reply to AMK
    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.

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