wiring for bathroom extractor fan with timer

[cookiemonster1]

Hi, I am having trouble with wiring diagrams for connecting an expel-air dx100t to my bathroom light using junction boxes. I realise that there are a lot of posts to do with connecting to the switch or light rose but mine are all junction box connections. The fan has a timer and I am using 3 core and earth. This is one of the last jobs to do before I have the electrics checked at first fix. Also can someone explain how these fans with timers work, do they come on when the light switches off ? Any help would be appreciated.

 

[securespark]

You may have a JB instead of a rose or other terminal arrangement, but the principle is exactly the same. You need (if you are triggering your fan from the bathroom light) a permanent live, switched live and neutral at the fan, via a TP switch.

AFAIK, the fans are triggered by the switched live and then the timers begin counting down once the bathroom light is switch off, the system being powered by the permanent live.