UV light + electric current = boom?

[Guineafowl]

Sigh... A single good experiment can massacre any amount of ideas
Thank you for following this up.

I know that some old potentiometers can produce fine graphite (or something like graphite) dust. Maybe that, on a surface? That equipment seemed to be quite old...
But it's just some afterthought. I agree with Jim Hardy that it's already hopeless without dissecting that box.

I’ve been watching this thread on and off and wondering why the box hasn’t been post-mortemed, with pictures. Lots of fruitful hypothesising and experimentation going on, but surely the answer lies within?! It’s a light switch, not Schroedinger’s box.

 

[jim hardy]

It’s a light switch, not Schroedinger’s box.

Laugh out loud on that one Guineafowl ! Dead cat in there?

Boring Anecdote
My old Dodge van refused to start one morning. Start relay on the firewall sounded weak.
I removed the starter relay, pried off the top and found a huge red ant squashed between the contacts.
Those big ants had a nest in the palm tree adjacent my driveway. I wondered what on Earth made that silly ant crawl up under the hood and into that relay.
Well ! Next morning same symptom exactly .
I figured no way could two ants be so crazy but sure enough i pulled another squashed ant out of the same contact.
Why they preferred the starter relay over the horn relay immediately adjacent only those ants knew.
Anyhow, at this rate in a few more days the little tabs on the cover that you must bend to get the top off would fatigue and break... so i decided to make the relay less palatable to them.
Sprayed the inside of the lid with Black Flag home insecticide and reinstalled. No more ant incursions..
old jim

.. .

 

[the_emi_guy]

I’ve been watching this thread on and off and wondering why the box hasn’t been post-mortemed, with pictures. Lots of fruitful hypothesising and experimentation going on, but surely the answer lies within?! It’s a light switch, not Schroedinger’s box.

If the OP is uncomfortable opening the box, an electrician should be called. I know if I had sparks flying out of my dimmers for unknown reasons I would not delay in fixing it or getting it fixed. I would be afraid that I would come home from work and find my house burned down.