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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  • It's always difficult to guess what was in the author's mind, but my guess is that, with small generators the available current can be quite limited - so even under dead short conditions there may not be sufficient current to cause an overcurrent device to open promptly. In such conditions the voltage from the generator would usually collapse and eventually the generator would shutdown or just stall. But the actual times, currents and voltages involved are rather vague -  if you had long cables between the faulty items (so a reasonably large impedance both in the overall fault loop and between the exposed-conductive-parts and a fairly beefy generator a decent voltage might persist between the parts for a reasonable length of time - at least compared with the requirements for shock protection - e.g. the voltage difference might exceed 50V and the overall time to shutdown may well exceed 0.4s for example. Of course as the shock voltage decreases permitted disconnection times could be allowed to increase - but again without some hard numbers it's not really possible to say by how much and therefore if the arrangement complies.

    In parts of the world where unearthed grid supplies are used, it common to have individual RCDs on every circuit - so in the double fault situation both faulty circuits are disconnected promptly without having to rely on large fault currents (note that doesn't work with a single common RCD upstream).

       - Andy.

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  • It's always difficult to guess what was in the author's mind, but my guess is that, with small generators the available current can be quite limited - so even under dead short conditions there may not be sufficient current to cause an overcurrent device to open promptly. In such conditions the voltage from the generator would usually collapse and eventually the generator would shutdown or just stall. But the actual times, currents and voltages involved are rather vague -  if you had long cables between the faulty items (so a reasonably large impedance both in the overall fault loop and between the exposed-conductive-parts and a fairly beefy generator a decent voltage might persist between the parts for a reasonable length of time - at least compared with the requirements for shock protection - e.g. the voltage difference might exceed 50V and the overall time to shutdown may well exceed 0.4s for example. Of course as the shock voltage decreases permitted disconnection times could be allowed to increase - but again without some hard numbers it's not really possible to say by how much and therefore if the arrangement complies.

    In parts of the world where unearthed grid supplies are used, it common to have individual RCDs on every circuit - so in the double fault situation both faulty circuits are disconnected promptly without having to rely on large fault currents (note that doesn't work with a single common RCD upstream).

       - Andy.

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