Creative light switch wiring

[freddo]

But what about my single-core cables? How do both line and neutral get to all the switches?

Here is my attempt:

I've only seen sheath return wiring once the circuit was mostly gone, just a few feet of 1 core imperial MICC feeding a 5A round pin socket. A neat system, shame it can't be used in most installations.

 

[Lucien Nunes]

Here is my attempt:

That is exactly what I had in mind. I try to encourage people to look at and think about some of these obsolete ideas to better understand why we do what we do, the way we do it. Earthed-concentric is neat and can be safe if the workmanship is sound, but cannot realistically be RCD-protected and is therefore obsolete for the wiring of buildings although standard for DNO work.

Small MI is great for concealment and single-core obviously makes the ideal use of its overall cross-section. There was also a reduced-diameter twin core 250V grade; a short-lived idea to make MI more convenient and economical for domestic work, only available in a few sizes. I'm trying to remember the smallest cable, possibly 0.187 ins. diameter for 2x 0.0015 sq. ins. conductor. If anybody has 'The Electricians Mate' to hand, please could you check that as I'm not near mine. Also what was the smallest overall diameter of single-core 440V cable, such as could have been used for an EC circuit? (there was no single-core 250V).

failed after it started flashing over and would sometimes trip the MCB when the light was switched off. The switch soon failed after the tripping started

Arcing L-N is another hazard with the Carter system and indeed anything that switches between supply poles under load (e.g. motor reversing.) OK with long-break tumbler switches and contactors but more likely with micro-break light switches.

 

[freddo]

Small MI is great for concealment and single-core obviously makes the ideal use of its overall cross-section. There was also a reduced-diameter twin core 250V grade; a short-lived idea to make MI more convenient and economical for domestic work, only available in a few sizes. I'm trying to remember the smallest cable, possibly 0.187 ins. diameter for 2x 0.0015 sq. ins. conductor. If anybody has 'The Electricians Mate' to hand, please could you check that as I'm not near mine.

So close! 0.182 ins. (4.6mm) Tiny considering its 16A rating.

Also what was the smallest overall diameter of single-core 440V cable, such as could have been used for an EC circuit? (there was no single-core 250V).

0.157 ins (just under 4mm) diameter for 0.0015 square inch cable, rated 20A when exposed to touch and used for concentric wiring.

 

[Lucien Nunes]

Thanks @freddo.

For the 'younger reader', the sizes of imperial TRS and VIR singles were given as stranding and diameter, e.g. 1/.044 (one strand of 0.044 ins diameter) which is almost exactly 1.0 sq. mm. But MI is always solid-core and the size was given as CSA, with the equivalent to 1/.044 being 0.0015 sq. ins. Then as now, this was the smallest cable in general wiring use.

OTOH the matching of cable, glands and seals was done by overall diameter, not by number and size of cores as it is today (e.g. 2L1 cable requires RGM2L1 glands). Instead you would order glands with a number indicating the cable diameter in thousandths of an inch. So for the 2x 0.0015 250V cable mentioned above, the diameter was 0.182" requiring a gland size 182. You will find this 3-digit number on the cap of all imperial MI glands.

Going back to the Stannos cable I referred to earlier, which was an early metal-sheathed cable designed with EC applications in mind, I think it was around in the era of cable sizing in SWG, so the nearest core size was 1/18 (one strand of 18SWG) which is slightly larger.