Wiring Matters Mythbuster articles.

Many thanks for the Mythbuster Articles. In Mythbuster #6 there is the following statement

" In itself, this is not a problem, as there is no risk of shock, but a second fault on another item of equipment or a distribution cable could present a shoc

See https://electrical.theiet.org/wiring-matters/years/2020/82-september-2020/mythbusters-6/#:~:text=All%20generators%20need%20to%20be%20earthed%20unless%20floating for Mythbusters #6

Please can you explain further, as my understanding is that if the two class 1 pieces of equipment have their exposed metal parts connected together via the earthing system then I cannot see how a shock risk would occur. If the fault to earth was L1 on both of them then not an issue as such, if one was L1  & one was L2 then fult current would flow and cause the fuse or MCB to trip.

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  • For those who (as I was) are not sure which mythbuster this alludes to , its series in Wiring Matters, and specifically, this one from September 2020

    The author James Eade is a heavyweight contributor to BS 7909, the BS that considers power at events, where things are far more pre-test and then plug and play, than they are for more conventional BS7671 wiring.

    As one who from time to time does things with outdoor electrics gensets of various sizes,  I am familiar with the issues, and like the photos in the article, have seem some nugatory earthing arrangements.

    As the article notes, the problem with generators, is that the meaning of 'earth connection' is a bit more nebulous than it is with mains, where in the UK at least it is always TN-something or TT.

    There are a number of odd fault cases where the thing you think is earthed to the generator earth, is dangerous to anyone standing on the real earth....

    Mike.

  • Hi Mike, 

    My apology I should have included a link to the IET article

    electrical.theiet.org/.../

  • no need to apologize, just took me a while to decode the question, and to save other folk that time I added the note.

    There is a very nasty fault mode not often properly explained that is especially an issue with vehicle and trailer mounted gensets, which of course have no electrode connection to earth terra-firma while on the move. The temptation is to park up, plug in a lead, and throw it out to supply lights or something on-site. Now if that lead snags on something (metal fence, or barbed wire seem to be favourites on camps, scaffold poles on events... ) the temptation for the time pressured rigger/ roadie is to pull, and then it reaches where it needs to, and all seems OK.   The risk is that this results in damage to the insulation and connection from a core in the cable to that fence or lighting scaffold, or whatever it was.
    Consider what happens if that was a phase core.....

    Everything works, but the 'live' end of the genset winding is at or near terra-firma 'true' earth potential and the end that is supposed to be neutral or earth is now flapping about at 230V RMS relative to the true earth. The vehicle or towed genest, being insulated by its tyres, is also at this voltage.
    This is fine, until someone standing on the ground tries to get back into the vehicle  or someone in the vehicle tries to get our, and ends up connected between a puddle and a door handle or similar  !
    There is then an argument for not having a neutral earth bond at the genset, or having one but also having an electrode and an RCD.

    Even with an electrode, it may be that the fault is better connected to true earth than the electrode, depending on the fault - temporary power to metal bodied sheds comes to mind and then the RCD is not just additional protection, it is absolutely essential.

    these situations are quite rare, but  worth sketching out in your head so you are aware and to know you have  covered all the cases.
    Mike

  • Hi Mike,

    Thanks for your input. particularly the scenarios

    I am particularly interested in electric shock protection in caravans being supplied by small portable generators or internally mounted inverters. The idea of having individual RCD's as shown in figure 5 of Mythbuster #6 article + correctly sized short circuit protection seems to cover most scenarios and relatively easy to retrofit

    Regards

    Ian

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  • Hi Mike,

    Thanks for your input. particularly the scenarios

    I am particularly interested in electric shock protection in caravans being supplied by small portable generators or internally mounted inverters. The idea of having individual RCD's as shown in figure 5 of Mythbuster #6 article + correctly sized short circuit protection seems to cover most scenarios and relatively easy to retrofit

    Regards

    Ian

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