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Caterham



Member Since: 06 Nov 2008
Location: Birmingham
Posts: 6294

England 2011 Defender 110 Puma 2.4 XS CSW Stornoway Grey
any electricians want to comment
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?
Post #962655 23rd Aug 2022 5:25pm
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Co1



Member Since: 19 Aug 2018
Location: North Yorkshire
Posts: 3662

United Kingdom 2013 Defender 90 Puma 2.2 HT Loire Blue
I know what you mean, but you don’t need to worry about the fuse necessarily, you need to make sure that the 0.75mm cable can take the prospective fault current for the amount of time it takes for the circuit protection to activate. The PFC will be more than 6A, the 6A is the nominal current carrying capacity, what the cable needs to be able to carry is fault current for longer than it takes the fuse to blow.

Hopefully that makes sense?
Post #962659 23rd Aug 2022 5:48pm
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Caterham



Member Since: 06 Nov 2008
Location: Birmingham
Posts: 6294

England 2011 Defender 110 Puma 2.4 XS CSW Stornoway Grey
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?
Post #962679 23rd Aug 2022 7:20pm
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Co1



Member Since: 19 Aug 2018
Location: North Yorkshire
Posts: 3662

United Kingdom 2013 Defender 90 Puma 2.2 HT Loire Blue
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.
Post #962682 23rd Aug 2022 7:29pm
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Co1



Member Since: 19 Aug 2018
Location: North Yorkshire
Posts: 3662

United Kingdom 2013 Defender 90 Puma 2.2 HT Loire Blue
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!
Post #962767 24th Aug 2022 5:39pm
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Rashers



Member Since: 21 Jun 2015
Location: Norfolk
Posts: 3479

United Kingdom 2014 Defender 110 Puma 2.2 USW Corris Grey
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?
Post #962795 24th Aug 2022 8:35pm
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Caterham



Member Since: 06 Nov 2008
Location: Birmingham
Posts: 6294

England 2011 Defender 110 Puma 2.4 XS CSW Stornoway Grey
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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Post #962990 26th Aug 2022 4:02pm
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