Installing a bathroom fan

[rollieb]

I am trying to wire a normal switch to power up a bathroom fan I have installed. Presently there is a dimmer switch for the lights. Normally I would not have a problem with wiring a dimmer but this one has only one 3 conductor cable running into the box, WH,BK,RED.

I always thought that to wire a light, you need 1 cable coming into the switch and another cable going to the light. I seem to have only 1 cable powering up the switch and the light still works. I`m confused on how this is possible. This poses a problem because I can`t seem to wire my new additional switch for the fan into the existing dimmer switch.

I hope I am making sense.

Rollie

 

[TicklyT]

That's the norm this side of the pond... Live and Neutral are present at the light fitting, and a single cable from the fitting to the switch only carries a live and switched live, as neutral is not reqiuired for switches or dimmers.

It is common here to connect the live of the switch drop to the live at the ceiling fitting, and use the second wire as switched live back to the light fitting, where it connects to the live of the lamp. Here this is normally fitted with a coloured sleeve to indicate it is a switched live, not a neutral. The third conductor is used as earth. The wiring regulations and cable construction we use does not permit the earth conductor to be used for any other purpose.

We would normally wire an extractor to the light fitting, not the switch, where live , switched live and neutral are present using a 3 core + earth cable through a 3 pole isolator (required for maintenance work on the fan). The live and neutral power the fan, and the switched live provides the 'start' condition to an over-run timer built into the fan.
The fan will then run whenever the light switch is turned on, and continue for the over-run period after the light is turned off without the need for an independent fan switch.

Of course, your wiring conventions, regulations and cables, as well as supply voltages will differ from ours in the UK.

This shows the UK wiring of a fan:-
http://www.diynot.com/wiki/electrics:lighting:fan