SELV stands for separated extra low voltage which means 50 volt or less and no earth connection. So very likely 12 volt.
Google shows "The VA100SVXT12 is a 100mm (4") Axial Extractor Fan with a remote mounted Timer Transformer and built in Electronic shutters. With Features such as a Indication running Light and a IPX7 rated motor, making this fan suitable for Zone 1 applications. This Fan is complimented by a range of fixing accessories including wall or window mounting kits."
Some have built in timers, some have humidity sensors etc. The rules all relate to windows, if it opens you don't need a fan so no rules, if no window then normal way is for it to come on with the lights, a non opening window and you must be able to turn on fan without turning on the lights. There is nothing to say it should be triggered by lights, however where required it does stipulate run on timer. Hence why a load of different models to comply with Part F building regulations.
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