3-way light switching using SPST switches

The problem is the wiring, and the resistance of management to messing-up a recently decorated room by cutting chases and changing the cable ...

What I have done in the past is solder a 3C & E onto a twin & E cable and pulled up a floorboard above switch position and above light position and with Hellerine lubricant, used the old cable to pull through the new cable via the switch back plate and then through to the ceiling rose :-)

It does depend on how much of a faff it is to pull up carpet etc but it works well and is easier if it happens to be the loft or if you have stud and partition walls.

If you were prepared to rewire the switch as low voltage, I think you could do it without addtional electronics or changing the cabling:

1. Replace the lamp with a 12V version. eg convert a GU10 to MR16. Decide whether it's going to be AC or DC (halogens won't care but some LED lamps are fussy). Make sure the lamp is double insulated (no grounded metalwork).
2. Install a 12V PSU, either a transformer or DC PSU.
3. Take the existing twin and earth and wire it to the SPDT switch terminals, sleeving to make clear the earth is no longer earth. Any earthing of the backbox would have to come from the other cable in the switch.
4. Then wire as a regular 2-way lighting circuit.

It's rather icky to put LV in the same backbox as mains, but given the transformer isolation there shouldn't be fireworks if there's a single mains to LV short, and a double insulated lamp means that won't make the metalwork live.

I'm not sure if there's regs about LV and mains in the same fitting, but any time you have a transformer that's effectively what you have.

I wouldn't want to do this, but it could be done...

Theo

Well if you can't pull through new wires with the old, that only leaves the option of re-purposing the CPC and treating the circuit as a "lighting circuit without an earth", or going for some kind of signalling option (home brew or quinetic etc)

Thanks for the suggestion but mixing LV and mains is not good. Also, this is for a sort-of chandelier with 5 (or 7?) candle bulbs

Solid plastered walls :-( Some of the cables run in the original oval metal conduit but not these - actually, one might but the others don't.

So you need to do:

Yes, I suggested doing that earlier in the thread. A bit of a pain to make a box with 3 relays in it, but I might be able to steal a wire in one of the switches (currently 4 gang, but I can probably common 2 of the circuits) so that reduces it to 2 relays. From the POV of future maintainability I guess an adaptable box with a bit of DIN rail and a couple of rail-mounted relays is best, else just a couple of regular relays in a box.

Sometimes something smallish like a wago box with a trailing flex coming out of it can be tucked into a ceiling void above a rose etc. (and the rose covers the hole you make and repair in the ceiling to get the box through.

I have access above the light, so that's not a problem. My concern was what someone would think of it in the future if they find a home-brew box with 2 octal-based relays in it (I happen to have a pile of octal relays that have been waiting for a use since the late 60s!).

Well as long as it is done properly - enclosed, not just cocooned in insulating tape etc there there is not really a problem. Night be an interesting test for an electrician in the future trying to work out ho