Wiring Matters Mythbuster articles.

Question

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.

mapj1 · Accepted Answer

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.

mapj1 · Answer

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

gkenyon · Answer

Ian Kidd said: 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 
 But not all scenarios, and not entirely adequately or satisfactorily for general use by 'ordinary persons'. The RCD will not automatically disconnect in all cases, until someone is actually getting a shock - particularly case (b) in my previous post, if there is fortuitous contact with the ground. Mike also explains this further. 
 The main problem is, that even if the circuits are confined to the caravan, if someone uses a socket-outlet to run a lead outside the caravan, and it becomes damaged so that a live conductor is in contact with the ground, the frame of the caravan becomes hazardous live. The RCD may not operate until someone touches the frame, in which case it may well be too late to save their life, particularly if someone is wet and barefooted. 
 The arrangement should only be used where there is control over the use of circuits, and the installation is under constant supervision by someone competent in such arrangements.

AJJewsbury · Answer

However, this type of system is currently not specified in any of national (e.g. BS 7671, AS/NZS 3000) or international (IEC 60364 series) wiring codes 
 
 BS 7671 does seem to mention that sort of system in section 717 (Mobile & Transportable units) - e.g. in Fig 717.1 (if choosing the "without" earth electrode option) and A722 even seems to suggest such a system for EVSE. 
 
 Bearing in mind that the vast majority of caravans fed by small portable generators and inverters have not got any effective electric shock protection in the event of the faults discussed. 
 
 Don't throw the baby out with the bathwater! Where the system is entirely contained within the caravan (I suspect likely to be the case with battery/inverter systems at least) - then the chances of a fatal shock are pretty minimal (there only being a short relatively low impedance between any two exposed-conductive-parts and very limited fault currents). With a conventional generator stood on the ground outside, or the supply being taken outside, there is a very small risk of a live conductor coming into contact with the general mass of the Earth - typically those conductors would already be insulated & sheathed - which is good enough to class as double or reinforced insulation in most circumstances. And the chances of a 2nd fault is already small - in normal systems we don't usually worry about a 2nd fault on ADS if the first fault is a break in the c.p.c. or a sticky RCD or double insulated systems getting damaged through to the conductors. In many of those circumstances the risks are on par with picking up a badly damaged flex with both hands with one hand touching a section of bare L and the other a section of bare N - and we have practically no defence against that sort of thing in any system. 
 - Andy.

AJJewsbury · Answer

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.

gkenyon · Answer

Ian Kidd said: 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.

gkenyon · Answer

Ian Kidd said: Thanks for your reply I do tend to agree looking from an ALARP prospective that there is a low probability of receiving a fatal shock 
 I don't think ALARP is applicable here, simply because we have a standard (BS 7671) that says separated systems should only be used for the supply of one piece of equipment only unless under supervision (which is not the case for most users of caravans)? See difference between 413 (especially Regulation 413.1.2) vs Section 418 (Regulation Group 418.3). 
 Or, put another way, 'ALARP' doesn't mean 'within the bounds of what I think I want to do or provide, this is the best I can do' ... the words 'reasonably practicable' in 'ALARP' include taking into account standards and best practice.

gkenyon · Answer

AJJewsbury said: BS 7671 does seem to mention that sort of system in section 717 (Mobile & Transportable units) - e.g. in Fig 717.1 (if choosing the "without" earth electrode option) 
 But not for caravans ... as I said, supervision or training to prevent the danger presented by running a lead outside the unit is extremely important. 
 AJJewsbury said: and A722 even seems to suggest such a system for EVSE. 
 One piece of equipment (vehicle) from one separated supply ... Regulation 722.413.1.2. 
 I would add, further, that what is presented in Figure A722 of BS 7671 is subtly different than pure separation, because of the connection of the "PC" (protective conductor) to a line conductor on the separated side. This would certainly be quite serious if it were not for the RCD immediately after the "N-PC" connection. This arrangement is like the 'Floating Generator' of Figure 4 of the referenced IET article, rather than 'electrical separation' as per Figure 5 of the article, the latter of which can only be used under supervision per Section 418 of BS 7671. 
 AJJewsbury said: Don't throw the baby out with the bathwater! Where the system is entirely contained within the caravan (I suspect likely to be the case with battery/inverter systems at least) - then the chances of a fatal shock are pretty minimal (there only being a short relatively low impedance between any two exposed-conductive-parts and very limited fault currents). 
 But the very real danger of someone running a lead out of the caravan if socket-outlets are provided is a wholly different matter, and not one that BS 7671 would readily align with at this juncture.

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