Failing incandescent bulb trips circuit breaker

[bob ward]

I've recently had a maybe 2 year old ordinary 60W household light bulb trip the fuse box circuit breaker when it failed.

Normally when an incandescent bulb fails there is a break in the filament and that's that, no overheating, no excess current draw.

This bulb had obviously got quite hot, the inside of the glass was discoloured, it was grey and smoky. To me this is most unusual, what has likely happened here?

[BobL]

Most incandescent bulb filaments break when they are switched of. The filament breaks because over time the metal slowly evaporates making it thinner and more brittle. On cooling the thinned out brittle filament cannot handle the tension from contraction and so it snaps. If it breaks when it is on and the gap is small it can sustain an arc like welder for a brief period. The arc is a plasma so instead of a few free electrons in the metal carrying a current now you have a ball of ionised metal gas and a many more freed electrons. This has a very low resistance so can carry hundreds or millions of amps (eg lightning) this will be more than enough to cause what you observed. Sometimes Thi happens when the globe is switched on because the filament is cold so has a lower initial resistance.