How to make this switching action with standard parts?

[dom]

The problem - 1 bathroom and 2 en-suites.

There is one central fan extractor, serving all 3 - all ducted in.

Neither en-suite has an exterior window, so these rooms require the fan to have a 15 minute run-on.

(I'm planning to use a faceplate switch outside the bathroom/en-suite door, rather than a pull-cord)

The obvious way to wire it is for the light switch to each room to be dual-pole, and the second contact from each light switch is commoned together to the run-on timer (I don't mind if the bathroom goes through the run-on timer, but it could probably just connect direct to the fan).

The only dual-pole switch I can find are in the grid-switch range, and rated at 20A (I'm using MK logic plus range elsewhere).

So I *can* do it.

But is there an easier way using standard switch/faceplate parts?

(and it would be nice if the faceplate switch had a switched indicator neon so you know the light is switched on inside the bathroom/en- suite)

Also - as this is a slightly unusual configuration, and the 3 rooms are 2 downstairs and widely spaced, and 1 upstairs - I'm thinking of putting it all on its own 6A RCBO marked "wc's & fan".

There will of course be a fan isolator switch next to the fan (which is built into a studwork cavity in the bathroom wall)

[Andy Dingley]

Double pole switches

"Crossover" switches (used for 3-way switching) and some perverse and non-obvious wiring to confuse the next poor guy who tries to isolate it

[Ron Lowe]

It's essentially a logical 'or' function.

Dual-pole switches would be the simplest way, with the second pole forming a wired-or function to the run-on timer, exactly as you envisage.

Obviously, you could not use a single-pole switch in a wired-or config:

- that would make the lights in the other bathrooms come on whenever one was switched on.

The only other possibility with single-pole switches would be a bank of relays. Each bathroom light switch would energise both the local light, and a relay coil. The relay poles would then all have a common L feed, and a common wired-or output to the run-on timer.

You also need to consider the implications if there are RCDs involved, and the various light circuits are on different RCDs ( as they are spaced around the house). Your proposed solution would have switched Lives from possibly various circuits all energising a fan timer on possibly a different circuit, with a different Neutral. Now, the input impedance to start the timer may be high enough to not cause an imballance, but it will likely add to any existing residual imballance.

The relay-based solution, although complex, would isolate the different circuits. Each relay coil would be energised with a L and N from the local circuit, and could switch a L from a seperate circuit independently.

Ho Hum.

[RubberBiker]

Yes, but standard parts would be neater.

Only available as gridswitches in 20A.

I couldn't think of way of doing it with an intermediate switch. I suspect internally they're made from dual-pole changeover, but on the MK logic plus range you don't get access to separate the two poles - they're commoned together internally.

[Mike Clarke]

MK do a Logic Plus 20A DP switch . It's a bit more chunky than the 10A light switches but is a similar style.

[R]

One 2 Wire PIR Activated Timer in each room connected to the fan unit with diode to prevent feedback. Search Screwfix for "2 Wire PIR Activated Timer"

[RubberBiker]

I had a good look a that, - problem is AC power can't be combined with diode logic the way DC can - without half-wave rectifying it.

(If it could, problem solved - I'd combine the two switched-live wires into a single timer, then no PIR would be needed).

[RubberBiker]

That's one to be considered along with the MK gridswitch version.

Both give me the switching action I need, gridswitch has the advantage of being centred on the plate (so looks more like a light switch), the one you link to has the advantage of a better visual match with the rest of the MK logic plus range.

Does anyone know if the the 20A switch has a somewhat heavier mechanical action than normal 10A light switches?

(IME they usually do)

[RubberBiker]

My thinking too.

Relay logic is last resort - fine for appliance internals, but I feel it's a bit of a contraption to build into fixed wiring.

Not a problem - the bathroom lighting, 2 en-suites lighting and the fan will all run from a single, dedicated, RCBO.

[Bob Minchin dangling via a don]

Low current mains relays will do your isolation with relative ease Bob

[RubberBiker]

Possible, but not ideal. Sure I can make up a box with a couple of relays in it, but it's "unconventional" for fixed wiring - I'm looking for a "standard" solution.

[Dave Plowman (News)]

Yes. If you want a normal light switch operation you're into using relays.

[Dave Plowman (News)]

Is this for yourself or a job?

5 amp double pole light switches don't exist because there's no call for them. ;-)

[RubberBiker]

I'm calling for them!

I suspect the extractor manufacturer (vent-axia) would suggest 2 timing units that can have the outputs commoned. (so they probably contain the relay logic). The manufacturer's instructions offer no guidance, even though the unit is specifically designed as a multi- room extract.

But if it can be done with wired logic, so much the better.

I was hoping some old lag would come up with a cunning "secret of the trade" type solution - or that some models of intermediate switches can be adapted.

[Dave Plowman (News)]

Intermediates tend to be more clunky than SPST or SPDT. They are IIRC a DPDT with the commons connected but not accessible.

[Mike Barnes]

I think it can't.

In a completely different application here, where a logical "OR" was required, I came to that conclusion. I fitted a relay and it solved the problem perfectly. If the house is quiet I can hear the "clonk" as it switches off (not on, strangely), but other than that you wouldn't know it was there.

Another solution would be X10 power-line logic, which might save you some wiring as you say the rooms are widely spaced, but that would be overkill I suspect.

[RubberBiker]

Yes - have just taken one apart to check. It would actually be possible to snip off two of the internal contacts to make double-pole, one-way - but "customising" a switch internals would be a desperation solution.

Seems like it's use a 20A double-pole surface switch (and accept the clunkiness), or use a double-pole pull-cord (e.g.

[RubberBiker]

"On" is damped by the spring, "off" is the spring crashing the contacts back.