Failed EICR-is is justifiable to change RCD box(fuse box)

Discussion in 'Electrics UK' started by Iri, 31 May 2021.

  1. Licklieder

    Licklieder

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    If its one thing i've learnt by spending time reading the posts on this forum it's that whilst on the face of it domestic electrics are quite straight forward, the reality is that it's anything but - when you start digging into things, the rabbit hole goes deep - hence the 'am i missing something' Which EFLImpudence points out:


    I'm still getting my head arround the implications of TN-S vs TN-C-S, PSSC vs PEFC etc. need to do some more reading / learning. Which is never a bad thing.
     
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  3. JohnW2

    JohnW2

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    True up to a point, but it depends upon how pragmatic (and/or a believer in common sense) one is. A high proportion of the 'rabbit holes' that get discussed/debated (sometimes at great length) in places like this relate to minutiae or trivial theoretical issues and/or an excessive desire for very strict adherence with 'regulations' (even when they are silly!).

    The issue you refer to is a good example. Someone pointed out that the "Ze" and "PFC" figures recorded on the EICR were not compatible with one another. Whilst, in a general sense, that might be an indication that the person undertaking the EICR was not as careful as he/she should have been, provided that they are not 'wildly wrong' (which they aren't),the Ze and PFC figures are, per se, essentially irrelevant to the EICR.

    ... and don't forget that the EICR form offers the option of ascertaining the Ze and PFC 'by enquiry'. If one does 'enquire', the DNO will (if they give an answer at all) provide a worst-case figure for what Ze should be, without any knowledge of whether that figures is 'correct' (i.e. corresponds with what could be measured) for the actual installation concerned!
    ... and that is also the 'catch'. Unless/until one has a good understanding of such things, it's not possible to know when/how one can be 'pragmatic'/'sensible' (rather than obsessed with the minutiae of some 'official line').

    Kind Regards, John
     
  4. SimonH2

    SimonH2

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    AIUI from reading a document someone linked to here a while ago, it is now standard practice for the DNOs to only use CNE cabling for new installs and repairs. So in practice, a TN-S supply will only stay "pure" TN-S until the first network cable work is done anywhere on the circuit. Once that happens, there will be at least one bit of cable where the N & E are no longer separate. Over time, the networks (where TN-S was common) will end up a patchwork of separate and combined N&E !
     
  5. JohnW2

    JohnW2

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    That sounds credible, but it begs a question - if they repair' part of a TN-S cable with a bit of CNE cable, do they also 'PME' it?

    As I implied above, with the progressive appearance of this 'patchwork', is it not time that BS7671 stopped distinguishing between TN-S and TN-C-S (e.g. in relation to minimum main bonding conductor size), given that what appears to be TN-S when/where it arrives at an installation may have some of the characteristics of TN-C-S?

    Kind Regards, John
     
  6. SimonH2

    SimonH2

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    Yes, at the point where it changes to CNE, it is made PME with all the requirements that entails.
     
  7. JohnW2

    JohnW2

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    I'm not sure what you mean by 'all the requirements'.

    As I often remind folk, the late-lamented westie seriously disillusioned me as regards the meaning of the "M" of "PME" when he explained that "PMEing" an existing supply cable usually entailed just adding one earth at the far end of the run - so that, then, M=2, with one earth being at the tranny and the other at the far end!

    However, as I said, since such 'repairs' as you mentioned do go on, I think that BS7671 really should stop distinguishing between TN-C-S and TN-S.

    Kind Regards, John
     
  8. SimonH2

    SimonH2

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    Well the document I referred to went into great length about specific requirements for these mixed systems - which are significantly more than "just earthing the far end".
    The issue is that there will be customers relying on the characteristics of TN-S. So after inserting a section of CNE cable, they have to provide multiple earths.
     
  9. JohnW2

    JohnW2

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    Without knowing what this document was/is, I obviously cannot comment - but it is certainly the case that westie frequently told us that "just earthing the far end" was a common (probably 'the usual') way of "PMEing" the whole run of what had been a TN-S supply.
    I'm not really sure what "characteristics of TN-S" any customer could be relying upon. As I understand it, the reason for requiring PME (even if M=2 :) ) with TN-C-S (complete or 'partial') is to partially mitigate the 'risks' resulting from the fact that it is TN-C-S - in particularly, risks associated with faults in the CNE conductor.

    Kind Regards, John
     
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  11. SimonH2

    SimonH2

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    I think you've answered with one example ...
    With TN-S you don't have to worry about risks of your CPC (and hence everything connected to it) suddenly going to some dangerous and unpredictable voltage.

    But the main thing is that the DNO doesn't know what any customer may be assuming. So they can't make the supply any less safe than TN-S unless they have explicitly confirmed with each potentially affected customer that the changes are understood. So in practical terms, it's easier to just keep the system "safe".

    From memory, there are rules about having an earth electrode at the junction where multiple subscribers are starred off the feeder, and at certain points of a branch feeding multiple subscribers, and other stuff like that. it was definitely a lot more than just sticking an electrode at the far end of the run.
     
  12. JohnW2

    JohnW2

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    OK, fair enough. I personally would not call that "relying on the characteristics of TN-S" but, rather, something like "relying on the absence of characteristics of TN-C-S".
    Don't shoot the messenger :) To be fair to westie (who is obviously no longer able to defend his past statements), I think he only ever said that it was 'very common' to just add an electrode at the end of the run. That obviously doesn't mean 'always', so what he said is not incompatible with requirements in specific (I would imagine fairly unusual) situations such as you describe.

    More generally, I do wonder to what extent PME necessarily mitigates the possible consequences of a PEN fault on a TN-C-S supply, even if 'M' is appreciably greater than 2, since I somehow doubt that the PME electrodes have ultra-low impedances (I have a vague recollection of talk of 10, but may be wrong) - I would imagine that those impedances would be nothing like as low as the impedance of the PEN/CNE conductor (when intact).

    Kind Regards, John
     
  13. SimonH2

    SimonH2

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    Well I suppose there's a difference between having a protective earth that low enough in impedance to control the touch voltage to safe levels under fault conditions (and low enough to cause OCP to trip/blow); and one that's low enough in impedance to control the touch voltage under loss of PEN (which will hopefully be repaired before a separate fault occurs).
     
  14. JohnW2

    JohnW2

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    True - but the former of those situations is probably much more of an potential issue with TN-S (due to 'sheath failures'), yet there is no 'requirement' for PME to address that (very real) risk with TN-S.

    If they don't regard that risk (with any TN supply) to warrant PME, then the fact that they do 'require' PME with (only) TN-C-S presumably must mean that 'the reason' relates to the latter of the scenarios you mention (loss of PEN) - and, as I said, I'm far from convinced that the PME earths will necessarily always be able to 'adequately' address that issue.

    Kind Regards, John
     
  15. SimonH2

    SimonH2

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    I agree with pretty well all of that, except ...
    If a lot of the TN-S network originated when cables were lead sheathed, there would naturally be a distributed earth electrode - which is not the case with plastic sheathed cables. And also, losing the earth on a TN-S supply doesn't immediately impose a dangerous voltage on all your "earthed" items - it only removes the protection from the occurrence of a separate fault within the installation.

    So we circle back to ... putting rebar in all our new foundations and making our own local earths :whistle: Funny how things come around - once we had our own earth electrodes, then the DNOs went round PMEing stuff and disconnecting earth rods*, and now we're talking of adding them back in addition to the PME :rolleyes:

    * I recall a friend lived in a village near here when they PME'd the supply - back in the 90s IIRC. As part of that, they explicitly disconnected the earth rod and said local earths weren't allowed.
     
  16. JohnW2

    JohnW2

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    Sure - but, as you said, there were two aspects. Firstly, the provision of an adequate earth to facilitate ADS in the event of a fault within the installation. As I said, I would think that much more likely with TN-S than with TN-C-S, yet PME is 'required' only for the latter, which seems very illogical.

    The other issue is the 'dangerous voltage' (relative to true earth) that can arise on items 'earthed' within an installation in the event of loss of a fault in a PNE/CNE supply cable, specifically with TN-C-S. As I said, whilst 'PME' may (depending on where the 'ME's are located) may mitigate that problem - but, as I said, I am not convinced that it will (particularly if M=2, or not much more) necessarily reduce such voltages (relative to true earth) to below 'dangerous' levels. Of course, given a properly-constituted equipotential zone, that should not matter to people wholly within the building (i.e. within the equipotential zone).
    That was obviously ridiculous - if they believed that "local earths were not allowed" what did they propose doing about (other) extraneous-c-ps?

    As I understand it, a good few countries require local earth electrodes with TN-C-S - and, of course, we got perilously close to that in the UK back in 2018. As I see it, it is never going to do any harm (and, in fact, changes absolutely nothing if there are other extraneous-c-ps) but may, in some circumstances, do some good.

    Kind Regards, John
     
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