How can a tungsten filament bulb shorts or explode?

[thexeno]

Hi, I got an old classic tungsten bulb, not new and also not very old, when a friend turn it on and reported it "blinked" and burned. I thought ok, it burned, let's change it, it happens.

But then I thought: why a bulb blinks bright? Is it the arc the filament creates? Because I did not see it, but a non- electronics expert friend did, I can't describe how was the "flash".

Ok, but then when I change it I realize:

1. the local breaker tripped
2. the internals were all burned - check pictures: one internal wire completely evaporated, the screw got a hole, the little solder joint on the side of the screw got melted and burned. And the (not so weared) filament is broken on 3 points.

Seems a short circuit, but if so, how could that happens? What I miss?

[coppice]

1. the local breaker tripped

For decades I never saw a breaker trip when an incandescant lamp blew, but since about 2000 the breaker seems to trip with every bulb failure. Perhaps something has changed in bulb construction.

[IanB]

For decades I never saw a breaker trip when an incandescant lamp blew, but since about 2000 the breaker seems to trip with every bulb failure. Perhaps something has changed in bulb construction.

I think bulbs used to have a fusible link to protect the external circuit on failure. Maybe later bulbs cut this out to save money?

Ok, but then when I change it I realize:

1. the local breaker tripped
2. the internals were all burned - check pictures: one internal wire completely evaporated, the screw got a hole, the little solder joint on the side of the screw got melted and burned. And the (not so weared) filament is broken on 3 points.

Seems a short circuit, but if so, how could that happens? What I miss?

When a bulb fails the gap in the filament can cause an arc to form between internal parts of the bulb (evaporated metal forms a highly conductive plasma). This arc is effectively a short circuit and it causes a momentary current spike that can blow fuses or trip breakers. It was traditional for bulbs to have some internal protection to guard against this event.

[MosherIV]

My understanding of what happens when a filament lamp burns out is this:
the tungsten filament develops a weak spot
the weak spot is higher resistance
higher resistance causes hot spot, glows brighter
the hot spot reaches tungsten vapourizing temperature
tungsten vapourizes, creating higher gas preasure in bulb

If the bulb has been on for a while and the ends are hot, the solder may be softening, on verge of melting.
The increased gas preasure may be enough to push a hole in the solder.

[SparkyFX]

or it became leaky, drew oxygen and then simply burned up. The resulting mechanical movement might break the other parts of the filament as well.

[soldar]

My understanding of what happens when a filament lamp burns out is this:
the tungsten filament develops a weak spot
the weak spot is higher resistance
...

When the light bulb is turned on the overall resistance of the filament is very low so the weak spot heats up faster than the rest and vaporises. This is the reason so many filament light bulbs fail when turned on. If you use a dimmer or other current limiter to bring the light bulb gradually up to working temperature the light bulb will last much longer.

[mikeselectricstuff]

When a bulb fails the gap in the filament can cause an arc to form between internal parts of the bulb (evaporated metal forms a highly conductive plasma). This arc is effectively a short circuit and it causes a momentary current spike that can blow fuses or trip breakers. It was traditional for bulbs to have some internal protection to guard against this event.

Yes - that's exactly it - the arc burns back along the filament until you have a low-impedance arc across a shorter filament, or eventually the support wires if it lasts that long. This will usually trip the breaker first, but it is possible the heat can cause the glass to break. Before chnaging to LEDs, a blown bulb would regularly trip the 6A breaker in my house lighting circuit

[thexeno]

When a bulb fails the gap in the filament can cause an arc to form between internal parts of the bulb (evaporated metal forms a highly conductive plasma). This arc is effectively a short circuit and it causes a momentary current spike that can blow fuses or trip breakers. It was traditional for bulbs to have some internal protection to guard against this event.

Yes - that's exactly it - the arc burns back along the filament until you have a low-impedance arc across a shorter filament, or eventually the support wires if it lasts that long. This will usually trip the breaker first, but it is possible the heat can cause the glass to break. Before chnaging to LEDs, a blown bulb would regularly trip the 6A breaker in my house lighting circuit

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.

[Kilrah]

Once the filament ruptures it will flop around and increase the gap, then burn up itself as well.

[jogri]

When the light bulb is turned on the overall resistance of the filament is very low so the weak spot heats up faster than the rest and vaporises.

True for normal light bulbs, but not fur tungsten: Those bulbs actually repair their weakspots. It's called the van-Arkel-de Boer process. You basically have a small amount of iodine in your bulb that will react with free tungsten atoms that evaporated from the filament, forming WI4. This reaction (tungsten + iodine <-> WI4) is reversible, and the higher the temperature, the more WI4 will convert back to tungsten metal and iodine. And because you get the highest temperatures at the weakspots of the filament, the tungsten will deposit there and repair them.

This is why halogen lamps lasts so much longer than normal light bulbs.

[mikeselectricstuff]

When a bulb fails the gap in the filament can cause an arc to form between internal parts of the bulb (evaporated metal forms a highly conductive plasma). This arc is effectively a short circuit and it causes a momentary current spike that can blow fuses or trip breakers. It was traditional for bulbs to have some internal protection to guard against this event.

Yes - that's exactly it - the arc burns back along the filament until you have a low-impedance arc across a shorter filament, or eventually the support wires if it lasts that long. This will usually trip the breaker first, but it is possible the heat can cause the glass to break. Before chnaging to LEDs, a blown bulb would regularly trip the 6A breaker in my house lighting circuit

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.

Yes, either the arc expands along the filament, or the conductive plasma cloud hits the support wires

[thexeno]

When a bulb fails the gap in the filament can cause an arc to form between internal parts of the bulb (evaporated metal forms a highly conductive plasma). This arc is effectively a short circuit and it causes a momentary current spike that can blow fuses or trip breakers. It was traditional for bulbs to have some internal protection to guard against this event.

Yes - that's exactly it - the arc burns back along the filament until you have a low-impedance arc across a shorter filament, or eventually the support wires if it lasts that long. This will usually trip the breaker first, but it is possible the heat can cause the glass to break. Before chnaging to LEDs, a blown bulb would regularly trip the 6A breaker in my house lighting circuit

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.

Yes, either the arc expands along the filament, or the conductive plasma cloud hits the support wires

The fact that the filament is broken in 3 spots, 2 of them at the edges of the L and N lines, might facilitate such plasma expansion, as there are 3 plasma clouds and they can easily merge together.
So, looking for a proof of this theory, I was trying to find some slow-mo videos, but was not able in finding one showing the plasma and such reaction... Maybe could this be an idea for a Youtuber?

[coppice]

For decades I never saw a breaker trip when an incandescant lamp blew, but since about 2000 the breaker seems to trip with every bulb failure. Perhaps something has changed in bulb construction.

I think bulbs used to have a fusible link to protect the external circuit on failure. Maybe later bulbs cut this out to save money?

That could well be the reason. In the old days consumer units used wire fuses, so blowing the fuse was a PITA to fix. Now almost everyone has breakers, and flipping the breaker back on is so simple I guess the bulb makers are relaxed about popping it.

[Cyberdragon]

This must be 240v problem. Never had this happen or even heard of this over here in 120v land (and there are no bulb fuses here either). Normally, an AC arc is hard to maintain unless a substantial current was already flowing (hence breaker arc arrestors). However, due to the low pressure (usually of noble gasses), I can see a more severe arc forming.

[stevelup]

Here in the UK we have 240V and usually lighting circuits are protected by a 6A breaker. Having the breaker trip when an incandescent lamp fails is absolutely routine. I'd say it happens far more often than it doesn't.

That said, I don't have a single incandescent lamp left in my house now after finding the holy grail LED replacement.

And for a bit of thread drift for UK folks:- https://www.screwfix.com/p/luceco-dim2warm-bc-gls-led-light-bulb-810lm-10w/4275p

These are just fantastic. Easily as bright as a 60W lamp when on full. Lovely dimming curve right down to zero on any old dimmer, and colour temperature that reduces as you dim it. No buzzing. No flickering.

[jimdeane]

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

[Cyberdragon]

The 6A breakers combined with the higher voltage would be the cause of false trips then. Here we use 10A or usually 15A breakers for everything.

[schmitt trigger]

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.

[coppice]

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.

Bulbs mostly have a considerable amount of inert gas in them, so they are below atmospheric pressure when cool, and above atmospheric pressure when hot, allowing a very thin glass envelope to offer a reasonably robust product.

[Gyro]

For decades I never saw a breaker trip when an incandescant lamp blew, but since about 2000 the breaker seems to trip with every bulb failure. Perhaps something has changed in bulb construction.

I think bulbs used to have a fusible link to protect the external circuit on failure. Maybe later bulbs cut this out to save money?

Yes, in the good old days, at least in the UK, one leadout wire in the base had an in-line fuse. It was a small silica-filled glass tube integral with the lead (ie. no end caps).

Then, when manufacture moved to foreign distant shores, the inevitable happened. 

[Bud]

This must be 240v problem. Never had this happen or even heard of this over here in 120v land

It must be... Never seen that hapenning  in Canada (120V)

[IanB]

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.

[themadhippy]

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

we got em,mentioned  very briefly in the 17th edition of our wiring regs,introduced in the 18th (latest) edition and no doubt compulsory by the 19th

[floobydust]

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.

[rsjsouza]

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

Thanks Ian, I didn't know that. Interesting article, which also exposes the differences between a residential and an industrial grade outlet.
https://www.handymanhowto.com/electrical-outlets-side-wire-versus-back-wire/

[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.