Extractor Fan Isolator Switch????

[JohnW2]

... and neither of us had said anything about anyone touching the fan

I would have thought ( or at least hoped ) that anyone talking technically about fans or any other electrical items in a bathroom would naturally consider the possibility of someone touching the fan.

Of course - but that's not the context of what you were writing about. You were talking about the possibility of condensation within the fan resulting in an RCD operating and therefore taking out the lighting circuit - that had nothing to do with people touching anything!

I haven't heard of metal ducting being affected by voltage via damp but I have seen some very wet fans, both from condensation and rain blown in.

I never cease to be amazed by how much water one often needs in an accessory (or whatever) before enough current flows for an RCD to operate. I imagine that's particularly true of condensation, since the water is then essentially 'deionised'. I would therefore be very surprised if the wetness one could get within a fan would ever be enough to create a low enough impedance connection to the ducting (which itself is unlikley to have a low impedance path to earth) to result in an RCD trip.

Should exposed ducting be bonded ? or should it be "earthed" to the CPC ?

Hmmm! If you did, it would merely increase the probability of the scenario you described - condensation within a fan taking out the lighting. The actual risk to life/limb by not having it is, I would have thought, close to zero.

Kind Regards, John