How can a tungsten filament bulb shorts or explode?

[james_s]

But the tungsten get interrupted on one short spot: I would explain this if the arc somehow expands to bypass the whole filament. Or is that what are you saying? If so, I don't get how the plasma, once generated, can bypass the whole filament, rather than only the spot of missing tungsten.

It works the same as those little UV lamps with a V shaped tungsten filament. Once the arc strikes, it has a much lower impedance than the filament so it shunts by the filament and stretches between the lead wires. Most medium and larger sized incandescent lamps are not vacuum filled but have an argon gas fill, this makes it fairly easy for an arc to form once the filament breaks.

Putting a gas filled incandescent lamp in a microwave oven is quite spectacular, but don't do it for more than a few seconds or the pressure will cause the envelope to rupture.

[james_s]

What if it's tripping an arc-fault breaker?

You will not find arc-fault breakers in the UK.

The cynic in me says that AFCIs were pushed into the US electrical code to improve the profits of the people who benefit from them: (a) installers who use "stab in" terminals on receptacles to save installation time, (b) people who make and sell AFCI units.

Stab in terminals are inherently dangerous and I'm almost certain they would not be allowed in the UK.

I believe AFCIs are a band-aid over those horrid backstab terminals, I personally have encountered three near-fires caused by those, one in my own house so I suspect they are very common. The electricians love them because an apprentice or two can outfit a whole house in a few hours but for the end user they are terrible. I refuse to use anything but spec grade (commercial) receptacles in work I do. I've kept my original 1979 service panel mostly because if I upgrade it now I will have to outfit almost every circuit with expensive and annoying AFCIs.

[james_s]

What is the bulb wattage?

I read somewhere that lamps below 40W had full vacuum, and above that they had a small amount of inert gas, to aid in the self healing.

The gas fill reduces the rate of evaporation of tungsten from the filament, that's basic physics, evaporation occurs faster at lower pressures. Unfortunately the gas fill causes convective losses, cooling the filament which reduces efficiency and this is most pronounced with longer, thinner filaments required for low wattage lamps so smaller bulbs tend to be vacuum filled. This is especially true with higher voltage, 240V incandescent lamps are considerably less efficient than 120V incandescent lamps which themselves are less efficient than 12V lamps.

[MrMobodies]

I had an old 60w candle light and lasted for many years. It wore out and stopped working one day so I put in another one, it was branded "Polaroid" and was also 60w.

I smelt something burning and I traced it to the light and it was boiling hot to touch.
Took out another "Polaroid" 60w light and it did the same thing.

I measured over 200c on the non contact temperature sensor and it heated up the light fitting to about 100c.

I found it unusual as the previous no-name one never got that hot.

[james_s]

It may have been a gas filled vs vacuum thing. I'd expect the surface of a gas filled incandescent to run hotter although any incandescent gets quite hot.

[VK3DRB]

I have seen a CB trip with an incandescent globe filament failing, twice over the last 40 years. The cause may be simple... as the filament fails and dislodges, shorter remnants of it shorts out the active and neutral wires elsewhere, causing excessive current and blowing the CB.

Incandescent light bulbs are illegal in Australia and soon halogen light bulbs will be banned... https://en.wikipedia.org/wiki/Phase-out_of_incandescent_light_bulbs. From September 2020, those with the Vision Mantis here might have a problem getting their globes. Fortunately, Australian government policing of imports is very weak.... just buy them on eBay. However, I finally found a good LED alternative for the Mantis - less heat, less power, and you can disconnect that noisy fan.

[thexeno]

Mains wiring and a light bulb both have inductance, so a lamp failure generates a HV transient.

OP's light bulb arced with enough fault current to melt the socket cap and trip the breaker, yet do no damage (melt) the thin wires and filament inside the bulb. You can see the arc was instead outside the envelope- between the socket cap and center pole wire.

I have seen many incandescent lamps burn out and take out the SCR or triac in dimmers. But with 120VAC mains, no breakers tripping.

I don't think it was outside such arc: one of the two thin wires in the bulb is evaporated, showing the short circuit current was happening inside. The fact that the cap has a hole, I believe the current vaporized the metal, as they seems to be very cheap nowadays.