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Neutral to earth on genny

lyledunn
Can I ask your opinion in relation to neutral /earth on this rather untidy changeover arrangement located just downstream of the main isolator? There are 2no 3-pole contractors with mains in bottom left and generator in bottom right. On the stud adjacent they have connected gen neutral, DNO neutral and installation earth. The arrangement means a self-imposed conversion to an unauthorised TNCS. I can’t be sure if the existing supply is already TNCS.
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    5460ce7e7d52d5f8fd148fdb2ec45a45-original-732f1b70-67bb-4ba0-a1e8-84856da12393.jpg
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    Hmm It would be OK in Australia, where MEN is permitted, but not in the UK.


    To be clear I have understood, the contactors are connecting the load to either the genset or the supply  but never both at once - its not a parallel running phase synchronous unit ?

    Ideally there should be a contactor pole in the neutrals as well - clearly the genset NE side needs to be solidly bonded, but really the DNO N E should not be.

    If you just remove the link, so the genset needs the DNO link to earth it;s star point then there is a class of DNO side faults that upset operation on genset.

    If the supply is already TNx derived, and the earthing is at least as chunky as the neutral then it will not do much harm in the short term though it violates ESQR , and it may inject diverted neutral current into the earthing network. If the supply is TT, then you may end up acting as the electrode for the substation transformer, and the preffered path for other peoples fault currents, which is not good. And it confuses any RCD on the supply side of the changeover, but I imagine there isn't one.

    Mike.

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    Thanks Mike, my thoughts entirely. It’s just a standby unit being subject to EICR. I don’t want my report ending up in Mr Stone’s portfolio of bad boys!
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    Just do not ask me  if it is C1, C2 C3 etc - it is not meeting current UK legislation, though it probably is not that dangerous .

    Mike
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    My little black book is going to have to get bigger soon! I am not lining up Lyle though, he is not doing anything I wouldn't as far as the things he reports here. It is not everyone BTW, it is the bad few. I spoke to an extremely competent NICEIC contractor's Technical Director this morning on a fairly tricky problem, to verify his calculations. He was certainly not the typical QS. I do not wish to appear to be the thought police, simply "concerned" of the UK.


    This change-over system needs the neutral pole in my view, the E-N bond needs to be in each of the supplies, be it generator or mains. It's probably a C2 as it could be dangerous with the right circumstances, what are you going to write Lyle?
  • Former Community Member
    0 Former Community Member
    lyledunn:

    . I can’t be sure if the existing supply is already TNCS.


    Hi Lyle, if the main earthing arrangement is a code FI, is there any point in continuing the rest of an EICR? What limitations have been agreed prior to carrying out the inspection?


    Any other pics of the intake, generator etc may help?


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    I would agree with a C2, although as a DNO engineer, I would be tempted with a Danger Notice. The two contactors should be 4-pole, and the earth should be tied to its generator neutral on the “generator” side of its contactor. 


    Regards,


    Alan.
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    Shows you how subjective these EICRs are. Here we have one top man on this forum saying “it’s probably not that dangerous” and another who is prepared to serve a Danger Notice. 

    perhaps Alan would be kind enough to expand on the dangers as he sees them?

    David, WB, I cannot give a C2 or FI as the report must have a satisfactory designation so such things must be rectified before the issue of the report. I have a list of other things that fall into the C2 category for this particular installation. However, it seems to me that any given code should be accompanied by a clear indication of the reasons the call has been made along with a reference to at least the most pertinent regulation that has been breached.
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    Does the installation have its own earth electrode suitable for the generator when the supply is lost?
    • If it does have a suitable earth electrode, then it depends on whether the installation is PME as to whether danger is immediately possible
      • If it's already PME, and main bonding is suitably sized for that, then immediate danger is unlikely, It's a breach of legislation (if it's an LV supply under ESQCR and that should be noted) so there's a choice of C3 or FI - I'd go for the FI and reason being breach of legislation.

    • If it's not already PME, or bonding is not suitable, then the installation and service cable are susceptible to damaging  diverted neutral currents at any time - Code C2, or C1 if signs of overheating of any protective bonding conductors already. Again note possible breach of legislation - if bonding not suitable sized for PME, appropriate ref to Chapter 54.

       


    • It if does not have a suitable earth electrode, then I'd be tempted to go for C1, citing no suitable means of earthing for an intended mode of operation (mains loss could remove the link between N and/or supply means of earthing and Earth), ref Regulation 551.4.3.2.1. And again note possible breach of legislation.

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    lyledunn:


    David, WB, I cannot give a C2 or FI as the report must have a satisfactory designation so such things must be rectified before the issue of the report.


    I think it needs to be rectified - it's especially important if there's no earth electrode on the consumer side, but diverted neutral currents is a distinct possibility unless the premises is some distance from other installations and there are no shared extraneous-conductive-parts.


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