simple solution would be to ban the use of power tools in areas that require powered lighting 
now then where did I put that double pinion
now then where did I put that double pinion 
Basement CU
- Thread starter plreardon
- Start date
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... and at night!simple solution would be to ban the use of power tools in areas that require powered lighting
Seriously, though, do you not think it's a bit illogical that those who are worried about a fault in a sockets circuit (maybe supplying a power tool being used in a dark place) taking out the lighting (leaving the tool running for a few seconds in the dark) are not also worried about the possibility that the lighting circuit itself may develop a fault (or the lamp(s) dying!), leaving the user in the dark with the power tool continuing to run 'indefinitely'?
Kind Regards, John
I would be concerned if I was working in a workshop, where the means of isolation were by push button control, if lights failed. But if using hand held power tools other than 4" angle grinders, that tend to have the lock on switch, I would personlly feel that the potential hazards are minimal, as releasing your finger from the switch often prevents the tool from operating after a few seconds and those of us that are competent in using such tools should not be injured under these circumstances.
So, if you're happy with that, do I take it that you're not one of those who worry about a failure in the sockets circuit powering tools taking out the lighting - since that would, again, obvioulsy be a situation in which the tool would stop after a few seconds?
[ in passing, I would add that nearly all of my hand-held tools (drills, jigsaws, belt sanders, circular sanders, circular saws, routers, biscuit jointers, planers etc. etc.) can be 'locked on' ]
Kind Regards, John
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John, I'm afraid you're correct that I overlooked your other posts before, however I still stand by what I say, that in a room with no natural light it is much more desirable that discrimination is achieved, to reduce the risk of the lights going out on fault conditions. I agree that emergency lighting would be good in this situation, as I have in both windowless rooms in our house (utility and garage).
The question of two DNO supplies is arguably one to debate I think for new installs where it may be expected to be not much more expensive to ask for two cutouts, but for existing installs the cost of running a new service would run into the thousands in most cases.
When you're on the consumers side though the options for what to do with this single phase are a lot greater and a good deal cheaper. Spending a few extra quid on ensuring the lights and sockets are kept separate is IMHO money well spent and the difference in price can be marginal.
I know it might be slightly off-topic, but when it comes to RCDs I am definitely an advocate of all-RCBO boards, and I believe the marketing of "dual RCD" boards as the next best thing to this is wrong, as split load boards with a few RCBOs are still a much more satisfactory compromise as you can keep all your lighting and your computer-laden socket circuits out the way of the cooker/shower/kitchen sockets/boiler etc. It also ensures you have non-RCD ways for supplies to outbuildings etc. Dual RCD boards discriminate against the savvy electrician who wants local RCD protection and is forced to price for a separate fuse switch and is going to lose out to one who is happy to just slot in a £2 MCB on one of the RCDs.
The manufacturers would also sell a lot more RCBOs if they simply switched their marketing strategy from one of trying to sell the high integrity 'bronze' option only to selling the 'silver' split load and 'gold' all RCBO options at the same time, to allow the consumer to make an informed decision about their purchase.
I don't have sleepless nights over it!
They may have the ability to, but only when manually put in that position.
Most offer the quick release triggers, which if doing an risk assessment prior to starting work with these tools, the safest option would be the preferred choice.
And until it is regulated that a secondary or an alternative power source most be present in areas where power tools are used, I will do my best to accommodate and make provisions, when it is practical to do so. But it may not always be achievable or practical.
Maybe we're both nit-picking about definitions a bit. Even if, as you will have seen, I think that, in the domestic situation, the risk (of significant consequential harm) is very much over-played, I can't deny that it is desirable to have as much independence of circuits as possible - particular in the sense of making lighting as independent of everything else as possible. As you must be aware, my point has been that the issue is that of having a common protective device for two circuits - not discrimination (between two OPDs in series) in the usual electrical sense. Maybe you are using the word 'discrimination' in a more everyday sense, rather than the usual electrical one.
However, as I've been saying, there is not too much point in getting too concerned (or accusing someone of 'being on a different planet' for not being very concerned) over the possibility of a danger arising due to a fault on a sockets circuit taking out the lighting circuit unless one also addresses the situations which arises if a fault (even as simple as a dead lamp) arises on the lighting circuit itself or if there is a total failure of the power supply. As we've agreed, battery-powered emergency lighting is really the only solution to that. As you also point out, exactly the same potential problem arises if the lighting shares any protective device (e.g. an RCD) with any other circuit - so, as you say, all bets are off unless the lighting circuit is fed from an RCBO of its own.
Not only was westie's comment obviously tongue-in-cheek but, as he said, there would be little point in just having two separate services supplied from the same DNO main - about the only thing that would protect one against would be operation of a cutout fuse. As he said, two supplies would have to be fed from different HV sources to produce much chance that one supply would continue if the other failed. There may be some industrial situations in which that is a possibility, but I think local generators are the usual back-up against loss of a single DNO supply when continuity of supply is essential/crucial.
No argument there but, as I've said, that is not usually a matter of 'discrimination' in the usual electrical sense - it's about keeping the two circuits separate, with the minimum of common protective devices (cutout fuse is obviously unavoidable).
Kind Regards, John
As you will realise, nor do I (at least, in terms of normal domestic situations) - not the least because that is just one of several mechanisms whereby exactly the same 'hazardous situation' (lighting failure whilst power tools spin down or continue running) can arise. If one were were very concerned about that one mechanism, one would also have to address the possibilities of failure of the lighting, or lighting circuit, itself, total failure of the power supply, and avoid having the lighting circuit protected by an RCD which served any other circuit. Emeregency lighting is the obvious solution - and one which (together with lighting being fed from a different phase from everthing else in the cellar) I have implemented in the one place in my house (large cellar) which is very dark and in which many power tools are used.
Kind Regards, John
PBoD, I would still be grateful if you could explain what you meant by "offering discrimination to the circuit"- I really don't understand what you were getting at! Were you possibly referring to the 'token' (but virtually useless) degree of possible 'discrimination' which would exist between a 32A and 20A MCB, or between a 20A and 16A one?It was a genuine question - I truly don't understand what you mean. As I said, maybe it's just me, but I really don't understand what "offering discrimination to the circuit" means.WTF do you think it means, stop being an A.R.S.E
Kind Regards, John
Which if we go way back to my maiden reply on this thread, I believe I breezed over.
My terminology is purely taken as what BS7671 defines, maybe I will let you ponder and come to your own conclusions on that,
As useless as it maybe considered to be, it would be irresponsible not to introduce selective serial devices, when considering a set up such as the one in which the OP is requesting advice on.
I am talking in the electrical sense. Sorry but I'm getting confused by your wording above. A common protective device for two circuits would make those just one circuit, would it not? Or are we still referring to lack of electrical discrimination 'effectively' meaning both circuits are joined at the hip?
But it's a controllable risk, which for the sake of the OP I cannot understand why you are suggesting the circuit design employed doesn't need discrimination. It's just not the kind of thing one would wish to write down as a departure on the certificate where you have been responsible for both circuits.
In my line of work we take discrimination very seriously, and have changed CPDs and even cables in the past to ensure a compliant design. Maybe I am just being too picky and the rest of the world isn't like this?
If the protective device in question is an OPD, and if one is taking the BS7671 definition of 'a circuit' literally, then I suppose that you could say that it was 'just one circuit'. However, I don't think it's very helpful to take that view since it would mean, for example, that everything supplied by a sub-main would be 'one circuit', even if it split downstream into a number of 'final circuits' (per usual parlance) each with its own OPD. In any event, even the BS7671 definition does not apply to 'common RCDs', otherwise many UK domestic installations would consist of just one or two 'circuits'!
The "common protective device protecting two circuits (common sense definition of 'circuit')" I was referring to could be an RCD or, as per the situation we've been discussing, an OPD protecting a 'sub-main' (including one derived from a CU MCB) to feed a further CU (or collection of FCUs/switch-fuses etc.), which itself had OPDs for each 'final circuit' (common sense definition).
AsI'm sure you know, in the electrical sense (and per BS7671 definition) 'discrimination' applies specifically to protective devices (OPDs or RCDs) in series and refers to the ability of one to preferentially operate rather than the other(s) under certain conditions. With OPDs one attempts to achieve discrimination by having different Ins and/or device types. With RCDs, one attempts to achieve discrimination by having devices with different IΔn and/or time-delayed operation.
In the situation we've been discussing, the lack of (true) discrimination would be that between 32A MCB in main CU and secondary 20A MCB (for sockets) in the secondary one (or between 20A and 16A MCBs respectively for the second option protected). I would regard lack of satisfactory discrimation between those devices, per se, as merely an inconvenience, since it might result in the need to reset both devices after a fault or overload on the sockets circuit. The 'hazard' to which you refer arises as a secondary consequence of that poor discrimination if the lighting circuit is also protected by the upstream 'common' RCD.
If I understand correctly, the hazard which you concerns you is that lighting may be lost whilst a tool or machine is spinning down. If so, then, as I've said, there are several mechanisms by which that (or worse) can occur, only one of which seems to be concerning you. If your true concern were about the actual hazard (rather than ticking a box in relation to OPD discrimination) then you would presumably be just as worried about all the other possible mechanisms of the same (or worse) 'outcome'.
In the context of this thread (and all the similar ones), I'm not completely sure what you are suggesting should be done/advised. If there is to be a secondary CU feeding a 32A (or even 20A) sockets circuit, then one is not going to get anything approaching a good chance of discrimination (from an upstream OPD) if that upstream OPD is in a CU - since even a 50A MCB would not give anything approaching reliable discrimination against a 32A one. That seems to leave two main options. Firstly to 'do it properly' with a submain (of much fatter cable) protected by, say, and 80A fuse or, secondly, to run a the basement lighting from some separate supply (perhaps an existing lighting circuit) - although even that would ideally not be an RCD-protected circuit (unless it was an RCBO). As I've already said, given the OPs requirement is just for '2 lights and 2 sockets' (the latter perhaps for only occasional use, maybe never with any power tools in sight!), I wonder why a secondary CU (or the concerns we are discussing) is felt necessary at all. In any event, as I've also said, if 'the hazrad' we've been discussing is regarded as a concern, battery-powered emergency lighting is AFAICS the only practical way to cover all possible mechanisms whereby that hazard (or worse) could arise.
Kind Regards, John
I did not say that, and to me machines spinning down would be the least of my worries if I suddenly found myself in a pitch black room.
I have no problem mitigating all the mechanisms which may lead to what I consider to be the hazard here, which it lights going out in the dark, but some are easier to mitigate than others. The mechanisms I can think of, some of which we've discussed, are:
- Loss of DNO supply
- Loss of lighting circuit owing to a wiring fault (however caused)
- Loss of lighting circuit owing to a bulb failing on another part of the circuit
- Loss of lighting circuit owing to the light fitting/bulb failing in the basement
- Accidental manual operation of an MCB (kids/wife etc)
- Mal-discrimination of circuit protective devices, followed by a fault on another circuit
- Loose/poor connection
Clearly we have agreed the a reasonable mitigating measure for most of these (save a loose/poor connection on the switch wiring) would be emergency lighting, and I think we can all think of a mitigating measure for each one of them individually apart from the DNO supply over which we have no reasonably practicable mitigation measure.
With regards to discrimination therefore, which seems to be what you are suggesting cannot be reasonably achieved, I agree that sometimes it is difficult, but it can be done with reasonable due care, a PFCC reading, and a few calculations, most of which you can do in Excel, or even by hand.
To take the example someone gave of a 6A MCB which failed to discriminate with a 30A fuse, I can see where the standard MCB time/current curve would have it's limitations over a certain fault current, as the MCB has a finite time (0.01s on the standard graph) in order to trip. At a fault current of anything above circa 600A, this could easily take out both devices. Now, that sort of fault current may not be seen in a domestic situation but it can happen if close to substation with a good wind etc. In this case therefore a fuse may be more appropriate than an MCB, or even an MCCB in place of the submain device, the price of which can be comparable to fuse switches these days.
At the end of the day if you beleive that loss of local lighting will create a particular hazard then perhaps the easiest way is to mitigate it with one or more 8w non maintained bulkheads, although in domestic it would have to be pointed out to the client that the batteries do not last for ever (A full BS5266 compliant solution complete with log book, as fitted drawings, etc might be a bit over skill for a garden workshop )
As to discrimination, if you were for example doing a design and build contract on say a secondary school for health centre for the local authority then the spec would generally insist quite rightly on total discrimination between devices in final DBs and submain protection, which isn't actually that big a head ache to prove, there is at least one design package that will run through the design checking discrimination at each point, telling you at what fault levels it is acheived at each point etc, using manufacturer specific data for the family of breakers you have specified
)As to discrimination, if you were for example doing a design and build contract on say a secondary school for health centre for the local authority then the spec would generally insist quite rightly on total discrimination between devices in final DBs and submain protection, which isn't actually that big a head ache to prove, there is at least one design package that will run through the design checking discrimination at each point, telling you at what fault levels it is acheived at each point etc, using manufacturer specific data for the family of breakers you have specified
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