A normal tungsten 60W Philips bulb went blue when it died. Ever seen that before?
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8 years ago
Dead blue bulb
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8 years ago
Its a flash deposit of tungsten metal on the inside.
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8 years ago
I had a bulb go kind of dark blue black many moons ago in the 70s. It was one of these photoflood ones for cine use. It was Osram. Must be either the gas or the filament material I suppose. Brian
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8 years ago
Why doesn't that normally happen? Mind you, the bulb was horizontal if that makes a difference.
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8 years ago
This was a very old bulb if that makes a difference.
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8 years ago
Depends how much it arcs and sparks when the filament fails. For an instant you get a blob of molten metal that is vapourised by a sudden surge in current as the lamp blows. This can create enough earth leakage to take out sensitive ELCBs - especially with spotlights.
Most of the time the filament just snaps apart harmlessly...
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8 years ago
Earth leakage to where? The bulb is only connected to live and neutral inside.
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8 years ago
Because you don't usually get a proper arc, the filament just breaks with no real arc.
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8 years ago
Describe in great detail why this one did.
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8 years ago
Not possible to be sure of the fine detail but presumably the filament didn't just fall down into the bulb as it failed and you got an arc for a while which produced that flash deposit of metal film on the inside of the bulb instead of just the usual broken filament.
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8 years ago
Ah, I remember a key piece of information. I knocked the bulb when it blew. Perhaps preventing the filament from falling properly. I had just fitted the bulb into the fitting and was replacing the cover.
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8 years ago
Speculating a bit, I can envisage two modes of failure. In the first, the filament just breaks due to having become brittle over time. There would be a brief arc while the two ends of the broken filament separated, magnitude and duration of the arc possibly depending on the point in the AC cycle that the filament breaks.
In the second, the filament becomes thin over time, one spot thinning preferentially. The resistance at this spot will increase and it will get hotter, resulting in more thinning. Eventually it gets so hot that it flash vaporises and quite a lot of filament is evaporated, to be re-deposited onto the inside of the bulb. I would guess that it's the second mode of failure that caused the effect you have.
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8 years ago
I think its more likely to be a mechanical effect. When the break occurs with the part of the filament that has just broken can fall, the arc doesn't last for long at all, because it falls. When it doesn't fall so quickly, you get an arc for longer and so get more metal film on the inside of the bulb.
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8 years ago
In message , Jim Thomas writes
I've not been following this but I am reminded of the appearance of aged vacuum valves and wonder if there is a link.
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8 years ago
Isn't part of that the silvery blue effect you see from the remains of the getter used during manufacture?
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8 years ago
Pass:-)
One other point.. If a filament bulb is mounted upright there is a chance that a broken off section will short across the supply connectors.
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8 years ago
I agree on the Getter with valves.
With the bulb, it is possible that as th ebulb was horizontal and being handled, the filament break could have been swirling around - and rather than falling away, continued to maintain and arc.
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8 years ago
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Agreed! Quite spectacular sometimes. I had one go a couple of days ago.
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8 years ago
Unlikely. Those normally do all look like that. You don't usually see incandescent bulbs fail like that.
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8 years ago
To the bayonet cap which should be earthed via the holder.
Was this a clear or pearl light-bulb?
I have often seen this with low voltage MES bulbs when run continuously as say dial lamps in old radios, and festoon bulbs in bell pushes. The bulbs can still be working and show signs of tungsten deposits.