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Neutral earth links in feeder pillars

Lee Petherick

So, we have a private HV network on site that feeds feeder pillars and LV switchboards as a TN-S system.

I have multiple locations where I think there are neutral earth links that shouldn't be there, but even though it doesn't "feel" right to me, especially when I see 26 amps going down the earth conductor and only 16 amps going down the neutral in one particular place.

Example one - TP+N from TX into new feeder pillar which then feeds onto an old feeder pillar - neutral earth links in both, and they are only 2 metres away from each other.  One of these fuseways then feeds a building that has another neutral earth link in the switchpanel.

Example two - TP+N from TX into LV switchpanel (ACBs and MCCBs) which then feed two separate feeder pillars - both which have neutral earth links in them.  Although I haven't seen it myself, I am guessing that the LV switchpanel has it's own neutral earth link too.

I hope it's not correct, as I just don't see how it can be, but always willing to learn!! Slight smile

  • 0 in reply to gkenyon

    How far from 'the source' does earthing for TN-S have to be before it is classified as "TN-C-S PNB"?

    I suspect the DNO think about things in a different way - not so much in terms of lengths, but number of electrodes - if the system doesn't have multiple electrodes, it can't be PME, but N is still earthed, so it must therefore be PNB. Even on "ordinary" PME system, if you look a rural overhead installations, the 1st electrode is sometimes a whole span (a few tens of metres) away from the transformer (to keep HV and LV earthing separate).

    Is "PNB" actually a thing?

    In DNO land, certainly. It doesn't always map nicely onto BS 7671 nomenclature (TN etc) nicely though. I've seen some PNB arrangements that look like textbook TN-S to me (especially where there's only a single customer on a transformer). But I've seen other diagrams that are definitely TN-C-S (but not PME as there's only one deliberate electrode). The water seems to get muddied further by many DNOs seemingly insisting on the right to change supply arrangements in the future (perhaps not unreasonably) and so to cover themselves say that TN-S PNB customers treat the supply as if it were TN-C-S anyway (even saying "PME conditions" apply, when clearly the physics and maths of the initial setup screams otherwise). Customers with their own private transformers can make up their own minds of course.

       - Andy.

  • 0 in reply to gkenyon

    "Is "PNB" actually a thing?"

    Round here, SSE in Hampshire and Wiltshire it certainly is and is labelled as such on the main intake as either PNB TNS or PNB TNCS
    Usually isolated farms and campsites with one or 2 buildings and their own pole-pig transformer.

    mike

  • 0 in reply to mapj1
    mapj1 said:
    Usually isolated fams and campsites with one or 2 buildings and their own pole-pig transformer.

    I have seen 2 overhead cables feeding 2 cottages about 100m from the "pole pig". Definitely not TN-S, nor TT.

  • 0 in reply to mapj1
    mapj1 said:
    Round here, SSE in Hampshire and Wiltshire it certainly is and is labelled as such on the main intake as either PNB TNS or PNB TNCS

    Apologies for the confusion. Question was rhetorical really, I was intending to point out that PNB certainly does require special consideration due to the dangers I went on to communicate:

    gkenyon said:
    Capacitances and fault-current flows in broken neutral (between source and earth) situations in three-phase systems do lead to huge safety problems

    If the neutral breaks in a single-phase part of the network, the situation will be obvious because of voltage collapse. This is not necessarily the case if the neutral break occurs in a three-phase part of the network, and in PNB systems can lead to all sorts of problems (as it can in PME systems).

  • 0 in reply to Chris Pearson

    Definitely not TN-S, nor TT.
    might be the 'almost PME' flavour of PNB, where there is one electrode and at each house N is linked over in the service head to  create  the house CPC. 
    Might also just be real PME if there are electrode wires running down the poles at more than one location. Commonly there is one at the first post that is not shared with the HV, as pole transformers are  quite often 'hot' sites, so HV and LV earths are spaced apart, and then maybe one or more nearer the load end of the LV runs.  

    It has to be said that hiking about with a pair of binoculars will reveal a lot of rural installations that are not quite as simple and clear-cut as the text book illustrations - for example neighbouring houses that are a mix of  TT and TN-something sharing the same  LV feed and so on.  It all works in practice of course but the theory hits only in a few places where it touches.
    Mike.

  • 0 in reply to mapj1
    mapj1 said:
    Might also just be real PME if there are electrode wires running down the poles at more than one location

    Nope. All poles accessible, and no cable running down any of them.

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