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.

Parents
  • 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.

    The first fault earths one of the live conductors. The resulting system is then just like a TN-S system.

    I think I can show this as follows, in what is effectively an IT system

    Little or no risk of shock after first fault:

    There are THREE possibilities for a second fault:

    (a) Second fault on same live conductor at other equipment. There may be a risk of electric shock between simultaneously-accessible exposed-conductive-parts if load current is shared through protective conductors, and usually the RCD will operate in this case.

    (b) Second fault on the other live conductor on the same piece of equipment ... RCD won't necessarily operate; overcurrent protection should, however, be specified to operate under these conditions for protection against thermal effects, but there is likely low or no risk of shock at this stage unless any of the other exposed-conductive-parts were being touched or otherwise in fortuitous contact with Earth (in which case the RCD should be specified to operate).

    (c) Second fault on the other live conductor on a different piece of equipment ... could, however, lead to a shock risk if the protective devices were not present (or didn't operate), and either:

    • exposed-conductive-parts of equipment were simultaneously-accessible (voltage between equipment, but not between equipment and earth); or
    • any of the exposed-conductive-parts were being touched by someone else, or were in fortuitous contact with the ground or something else metallic connected to the ground (voltage between exposed-conductive-parts and Earth).

    So, cases (a) (b) and (c) for a second fault clearly shows the need for RCDs backed up by (or including) overcurrent protection.

  • Hi Gkenyon,

    Many thanks for a really great explanation of the scenarios and the summary indicating that probably the best approach is individual RCD'd and assurance that the the overcurrent projection device is correctly sized. for both cable protection and to clear the fault in an appropriate length of time

    Thanks

    Ian

Reply
  • Hi Gkenyon,

    Many thanks for a really great explanation of the scenarios and the summary indicating that probably the best approach is individual RCD'd and assurance that the the overcurrent projection device is correctly sized. for both cable protection and to clear the fault in an appropriate length of time

    Thanks

    Ian

Children
No Data