What could cause this

[WestyNottm]

The double pole ceiling switch is really just an electrical isolator for the shower for safety reasons. The contacts of the switch don't actually switch the full current to the shower - that is done internally to the heater when the water flow is established. Therefore, a switch that has gone like that is almost always a loose connecting screw, probably not tightened correctly when it was fitted and the connection stressed by the heavy gauge wire when it was screwed back into position.

 

The heavy current flow when the shower was operating would have resulted in arcing around this area with subsequent carbonising and eventual break down.

 

In spite of the looks of it, and the smell, a fire is a very rare occurrence as the material of the switch is fire retardant and self extinguishing, as is the plasterboard ceiling to which it is fastened.

[Man On The Clapham Omnibus]

I spent many a happy hour in the Materials Lab at Hoover's Perivale factory, when I was an engineering apprentice studying polymer technology, doing tracking resistance tests on thermosetting moulding materials for motors and switches. The current carrying parts will probably be housed in phenol formaldehyde and the outer housing in urea formaldehyde. It's the urea bit that causes the fishy smell when it gets overheated. The phenolic will not catch fire but it could contribute to a fire if one were established by other means. I have found that the neutral terminal in 13 amp plugs works loose rather than the live one because, I assume, the neutral one is directly on the pin whereas the live one is isolated from the jiggling movement of insertion and removal by being on the fuse carrier. I have to say that I am sceptical about the vibration of the 50Hz current having anything to do with it, or indeed if there is any vibration except in iron laminations in transformers and inductors. But.....