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.
Lighting – LED downlights working in conjunction with bathroom fan
bathroomlighting
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Best Answer
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.
Good luck.