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
... and at night!simple solution would be to ban the use of power tools in areas that require powered lighting
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?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.
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).Indeed. That's the point I was trying to make to mfarrow. Having lighting fail as a result of a fault on a sockets circuit because both share a common protective device is nothing to do with 'discrimination' in the normal electrical sense.
I don't have sleepless nights over it!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?
They may have the ability to, but only when manually put in that position.[ 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' ]
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.... 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.Indeed. That's the point I was trying to make to mfarrow. Having lighting fail as a result of a fault on a sockets circuit because both share a common protective device is nothing to do with 'discrimination' in the normal electrical sense.
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.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.
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).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.
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.I don't have sleepless nights over it!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?
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.EI guess it must just be me - but what does the above mean?...discrimination has been offered to the circuit, so in that way it is compliant.
avoid having the lighting circuit protected by an RCD which served any other circuit.
My terminology is purely taken as what BS7671 defines, maybe I will let you ponder and come to your own conclusions on that,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?
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?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.
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.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)
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'!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?
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....Or are we still referring to lack of electrical discrimination 'effectively' meaning both circuits are joined at the hip?
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'.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.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)
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.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'.
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