I use light bulbs (common incandescent) in the room and they
burn out periodically, ca. one bulb in 4 months burns out.
Very often, exactly at the moment when it happens, I see an arc (blueish light flash) in the switch, so the bulb always burns when being switched on. I don't see an arc anytime the bulb is normally switched, but only when the bulb burns out.
Question: does the bulb burn after the arc appears, or is it vice versa – when the filament burns it causes the arc in the switch?
My thought is that the bulb burns simply because of worn out filament, but
why is there almost always an arc in the switch at this moment?
Best Answer
When the lamps fails, it is often the case that the delicate filament collapes on itself, causing a short circuit. This causes a momentary peak of current. So much current that the short almost immediately blows itself open circuit again, due to a teeny explosion.
The lamp usually fails when switched on because the resistance of most materials, including the lamp filament, rises with temperature. When cold, it draws far more current than when hot- 10 or 15 times as much. So that is when the filament is under the most stress as it very rapidly heats up, so by far the most likely time for it to break. Additionally, it is because the AC wave is partway through rather than passing through zero, causing a sudden buildup of the electromagnetic field, causing a physical shock to affect the filament too.
Incandescent filament lamps last much longer if you never switch them off and on, for these reasons.
So to answer the question, the flash in the switch is caused by the failure.