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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? 

  • 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.

  • 0

    The reply wasn’t intended for you Graham.

  • 0 in reply to AMK

    I have been accused of using AI by Fiftyherrtz. in reference to IET documentation. I do reference online content/articles but doesn’t everyone? Just a thought, maybe AI is referring to my answers ? Slanderous accusations on the internet is a dangerous game to play.

  • 0 in reply to AMK

    Hi AMK, apologies, i did delete the reply immediately after replying, after thinking about it as i did not want to cause offence. The reason i asked was the format of the references in your post, which i have seen before generated by AI, and was genuinely curious as the information was helpful. I also saw that you submitted a post on AI recently.

  • 0 in reply to AMK
    AMK said:
    The reply wasn’t intended for you Graham.

    No problem ... all part of a healthy debate.

    This is not a clear-cut subject, as I alluded to. You are correct that the current suite of guidance from the IET does not want to confuse the issues of 'PME vs TN-C-S vs TN-S vs PNB' and, if you do have a PNB supply (unless you have what might by some be termed as PNB from a private transformer) you should treat the supply as PME.

  • 0 in reply to fiftyhertz
    fiftyhertz said:
    Hi AMK, apologies, i did delete the reply immediately after replying, after thinking about it as i did not want to cause offence. The reason i asked was the format of the references in your post, which i have seen before generated by AI, and was genuinely curious as the information was helpful. I also saw that you submitted a post on AI recently. Apologies, hopefully this can keep constructive even with my error

    Also remember this is a discussion forum, and not a source of free consultancy ... i.e. you need to make your own conclusions regarding the discussions here, not use them as "the answer". See Community Rules and Guidelines, especially item 6.

  • 0 in reply to gkenyon

    Thanks for the pointers Graham, much appreciated. I have updated the post for anyone else that comes across my question.

  • 0

    Of course 'treat PNB as if it were PME' really means 'take the same precautions, apply the same assumptions about fault currents' not 'is in all aspects equivalent'.

    There may be an element of 'lost in translation' about that difference.

    As PME requirements for bonding are the most onerous, all this means is 'if in doubt assume it may be as bad as  the worst case'

    M

  • 0 in reply to fiftyhertz

    I appreciate your apology  and your post deletion. You wondered about my sources because they looked like AI did them. I did not use AI for sources, but I took text from the web and gave credit. I have all the books I talked about and I studied them a lot. I might have overreacted but it was Saturday night and I had, had a few Blush  So I apologise for that. I got my knowledge from books, web, tools, and college up to HND level. I hope we can be mutually respectful and polite to each other in future posts.

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