There are 3 led downlights and an extractor fan wired with a bathroom fan isolator and separate light switch. The light switch also, when on, provides the live connection to the fan isolator switch, ensuring the fan is not left on permanently when the lights are switched off.(3rd wire provides timer circuit for fan to run on). Lights are full brightness when fan is off but dim when the fan runs. I am assuming this is due to the led lamps/drivers being starved of current. They are both fed from the same lighting circuit. Previous arrangement was a filament lamp in single fitting so I assume effects would be less.
I think you may have a wiring problem. From your description you circuit is acting like you have hooked your fan up in series with the lights. In a series circuit the lights will all stay the same brightness if they are all the same wattage and type. Once the fan comes on part of the voltage is dropping through the motor and the rest is distributed through the lights.
Take a close look on how the fan and the lights are wired up and if you have a meter verify that all equipment is wired in parallel and check for any crossover wiring.
I am only using the information you have given us. If the problem persists, you might edit your question and give us some photos or diagrams.
Seems like you're confusing two things: the timed fan control unit, and the isolation switch.
The isolation switch is, functionally, nothing special: it's just a triple switch. You could replace it with three 2-hole splices, and that's exactly what I recommend until you are done troubleshooting.
The timed fan controller has some electronics in it, and it is vulnerable to taking damage. What's more, some fan controllers may not "play well" with LEDs at all.
The idea is the fan will turn on with the light, but will continue to run for a limited time after the light is turned off. The fan controller gets always-hot and neutral, so it can pass that on to the fan.
You then run switched-hot to the light any way you please to, and the light's neutral returns via the fan controller. When the controller sees current flow on the light's neutral, it knows the light is on. This provides 3 opportunities for failure:
- If the fan controller's electronics have a problem
- If the fan controller's electronics and the LEDs don't play well together
- If the setup is wired contrary to the controller's wiring instructions