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| Caterham Member Since: 06 Nov 2008 Location: Birmingham Posts: 6298   
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hi there.
if you're a commercial sparky you'll be very familiar with the Schnieder Klik lighting distribution boxes with for example 6no 4 pole sockets for lighting. question being its common place for these to be on a 10A (and possibly 16A circuit - I think), however the premanufactured plug and lead uses (to the best of my knowledge) 0.75mm flex - which if I'm not mistaken is good for 6A. While that is more than sufficient to cover the load of the light fitting it's insufficient to be protected by the main fuse???? I seem to recall there was some kind of caveat in the wiring regulations that made this acceptable (ie up to 3 metre flexes) but can't find anything in the regulations. can anyone shed some light please? |
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23rd Aug 2022 5:25pm |
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| Caterham Member Since: 06 Nov 2008 Location: Birmingham Posts: 6298
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that does make sense but not sure how, where etc I would find the info for the cable.
wiring regs (so far as I know) only provide the current carrying capacity and not max current vs time and therefore to that end I presume its a fail and a larger flex is required? |
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23rd Aug 2022 7:20pm |
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| Co1 Member Since: 19 Aug 2018 Location: North Yorkshire Posts: 3674  
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You’d have to do the maths. It’s in the regs if you know where to look. If I get a bit of time tomorrow I’ll point you in the right direction. First thing you need to know is prospective fault current then work from there.
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23rd Aug 2022 7:29pm |
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| Co1 Member Since: 19 Aug 2018 Location: North Yorkshire Posts: 3674
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Sorry Caterham, didn’t get chance to get the regs out today. The sequence is to work out the Prospective short circuit current, then look up how long it takes the chosen fuse or breaker to break at that point (Appendix 4 from memory, might be wrong there). You then use the Adiabatic equation to either solve for time, or minimum cable size to prove whether the cable is up to the job. From memory again, the Adiabatic equation is in section 54 somewhere.
However, given that the pre manufactured pendants come with 0.75mm cable, you could likely safety assume that Schneider have done the maths for you! |
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24th Aug 2022 5:39pm |
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| Rashers Member Since: 21 Jun 2015 Location: Norfolk Posts: 3498 
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Ok. This is possibly a LED light issue, believe it or not!
LED lights, when they are switched on, actually have a very high in-rush current. It’s the same as you get with say a motor current when you first switch it on (a spike if you like). Many of the larger lighting manufacturers will give the inrush current or the maximum number of lights which can be supplied from a 6 Amp, 10 Amp MCB. Using a type C MCB will allow more lights per circuit as these circuit breakers will take a higher inrush current.. I nearly got caught out with this on a design I did a couple of years back. I was ignorant to the LED issue and it was more luck than judgement that it worked. I don’t think you will find anything in the IET Regs allowing say a 6 Amp klik unit to be supplied from a 16 Amp breaker. It would be up to the designer (and he or she needs to be signing the certificate of completion). The in rush is only for an incredibly short time and the continuous full load current of the circuit will never ever get anywhere close to 10 or even 16 Amps (thus why we use LED because they use less energy). The multiple socket unit should be wired in a cable which is protected by the circuit protective device. The flexes and klik sockets should never ever get anywhere near the 6 Amps they are rated at. It’s usually one klik socket per light. There is no need for a separate protective device (as in a 13 Amp Socket) as the light fittings shouldn’t over load the socket or the flex. Does that make sense, Caterham? |
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24th Aug 2022 8:35pm |
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| Caterham Member Since: 06 Nov 2008 Location: Birmingham Posts: 6298
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thanks guys that makes perfect sense.
I think the PSC and curves all relate to whether the circuit will trip in time to meet the requirements of the Wiring Regs (disconnection times) and no so much the cable? while I think it's perhaps a hyperthetical question / concern the way I see it is the flex between the multiple lighting distribution box and the luminaire is not protected by a suitable size fuse. ie 10A fuse covering the MLDB's but the 0.75mm flex are only rated for 6A while I think you're suggesting its ok (and in practical terms I completely agree) but from a Wiring Regs perspective I'm not sure how to back that up?  |
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26th Aug 2022 4:02pm |
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