Sure, but that will only happen if one has 10A worth of load (extremely unusual in domestic lighting circuits these days, I would think), regardless of the rating of the OPD - and, as has already been pointed out, the differences between a 10A-rated and 6A-rated accessory is irrelevant in relation to fault currents.
But I don't think the regulations allow for "it's unlikely". True, in this day and age of plentiful sockets we aren't likely to see people plugging the iron (or whatever) into a light socket, but in principal there is nothing to stop someone overloading a circuit. Besides there are faults that can pull large currents, but not the large currents that trip the protective device quickly.
I think you've let your thinking slide off at a bit of a tangent. This part of the discussion is not about the MCB but, rather, the switch - and whether there is a sensible reason for requiring a 10A rated switch to control a single lampholder or 'ordinary' light fitting, even if the circuit as a whole has a 10A OPD (It started with Bernard's comment "A system that allows 10 amps to be switched by a switch rated at 6 amps will have a high risk of the 6 amp switch's life time being short before arc eroded contacts start to become hot.").
Aren't we close to one of those 'common misconceptions' again? The transient high 'fault' current (hence, one imagines, probably the loudness of the ping) will be determined by Ohm's Law, regardless of In of the MCB. A 6A MCB might operate fractionally quicker, but I suspect that the pinging and dying is all over very quickly, anyway!
If the circuit is overloaded, then a 10A OPD may allow the switch to continue carrying and switching a fault current that is in excess of the switch rating, while a 6A OPD would trip. I don't think that's going off at a tangent. Where the fault current is higher than that required to trip the 10A OPD (but not into the "many times rating" realms mentioned), it's trip time and let through energy will be higher than that through an equivalent 6A OPD. That is after all one of the reasons we have different ratings to choose from, and why the OPD selection may change depending on factors other than the nominal current carrying capacity of the cable and fittings.
Generically they do - see 433.1.5 for example.
It needn't be a fault current, just one in excess of 10A. A B10 will allow over twice the rating of the switch to flow pretty much indefinitely.
We often get people asking if they can put a socket on a lighting circuit. And I'll bet there are plenty who do without asking.
Fair enough .....
That's obviously true in a literal sense. However, I think that these postulated 'high but not incredibly high' fault currents are probably extremely rare; it's very difficult to think of what sort of fault would result in a lamp or light fitting normally carrying say, 0.1A - 0.5A to draw, say, 9A. Indeed, BS7671 doesn't even consider faults other than 'bolted' (negligible impedence) ones, presumably not the least because it would be very difficult to achieve the required disconnection times for such faults (which are more like 'overloads').
Well I've quoted one already - though I have no idea what the actual current was. Don't forget that it's not necessarily one fitting that has to take (eg) 9A - there may well be other loads on the circuit as well which you can subtract from the required current.
It clearly wasn't blowing the fuse (of whatever rating of wire had been fitted), but was certainly looking to set the place on fire.The chokes of standard fluorescent fittings often get 'too hot to touch' in normal operation, so I suspect that (unless it burst into flames) the example you quoted did not necessarily involve a lot more current than 'normal' (you did admit that the fuse didn't blow).
Yes but, again, don't forget that I was talking about rating of the switch (unlikely to be controlling more than a small number of fairly small loads, particularly in a domestic setting), in a lighting circuit. I still think it would take an increadibly unusual and extremely rare situation for such a switch finding itself carrying enough 'fault' current to trip a 6A MCB but not the 'very high fault current' that would trip almost any MCB.
But now you're talking about overload, not fault. As we've discussed before, there is a limit to how far a designer can (or, I would say, should) go in trying to antipicipate all the crazy things people might do in the future. I would say that installing a 2A socket was a 'reasonable' (albeit not guaranteed) way of minimising the changes of someone connecting a high load controlled by a 'lighting' switch. In any event, even the 10A switch would be 'overloaded' by a 13A load that some idiot decided to connect tyo a 2A plug! In BAS's example, I doubt very much that many people put switches (of any sort, unless FCUs) in spurs to 13A sockets off lighting circuits.BAS has also come up with a potential "fault" load source. I've seen houses where there are sockets for floor standing lights which are switched at a wall switch. I'm sure you can imagine a few people who might not understand the limitations of what is OK to wire into an unfused 2A plug
I think that a good few of us can identify with that! In my teens, I experienced a good few shocks (some bordering on above-LV, per today's definition), any of which (I realise in retrospect) could have killed me - but didn't! I then rapidly developed much more respect for electricity, and can recall remarkably few shocks, and certainly no 'serious' ones, in the last 40 years (looking for some wood to touch!).When I look back, with what I know about electricity now, I sometimes wonder how I'm still alive. Two core extension with a croc slip to put on the nearest water pipe anyone ?
The maximum CCC of 1mm² cable is actually 17A ('free air' or tray, Method E), but is 15A when 'clipped direct'
Indeed, but as you've seen, there are people prepared to argue about the very unlikely eventually of a fault resulting in a high (but not very high) current.
Assuming you're talking about BS1363 sockets, that's a bit different, since there will be a maximumum of 13A fuse in any plug plugged into such a socket.
It's fine, unless you have installation Method 103 (totally buried in insulation).
See above.
See above.
Do you mean a 15A fuse or a 16A MCB? That would only be acceptable for 'clipped direct' or 'free air', but I personally haven't seen any lighting circuit, with any cable size, which has a 15/16A OPD.I regularly see 15A breakers on 1 milli lighting circuits!
Well, yes, it happens - but, as I said, rarely in a domestic setting. 1200W is obviously quite close to the limit of a 6A switch, but it's a long time since I've seen/used anything other than a 10A one.
We seem to be back to that 'common misconception' again. As you know, a fuse/MCB does not 'limit fault currents', which are determined solely by Ohm's Law - all they do is limit the duration of that current. So, if we're really talking about 'faults' (per BS7671 concept), rather than 'overloads', the rating if the OPD is pretty irrelevant to the ability of a switch to 'cope in a safe way'. As has been said, with a Zs of 0.5Ω you'll get a fault current of about 460A flowing through the switch regardless of the rating of the OPD, and such a current will result in operation of a 50A Type B MCB essentially as quickly as a 6A one. I'm therefore not convionced that the theoretical argument has much practical relevance.
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