Defective Bulb Tripping A Ckt. Breaker: How Possible ?


Hi,
Boy, this is a funny one.
Wife turned on a hall light switch last night, big flash by one of the hall lights, and the ckt breaker tripped.
Was about 2 days since putting in a new bulb there. Was the el-cheapo brand picked up at CVS, and made in China.
Replaced the bulb with a GE one, and so far everything seems O.K.
Question: I can't imagine how anything, like e.g. a broken filament perhaps, can short out the circuit. But, perhaps ?
Can anyone think of a bulb failure mechanism that might trip the breaker ?
Thanks, Bob
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Robert11 wrote: ...

...
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Actually, it's fairly common w/ incandescent bulbs, amd not only cheapies although they _may_ be more prone, I have insufficient data to confirm it...
A couple of mechanisms -- one, the "hot" element breaks and falls into the neutral base and poof! -- short circuit w/ a momentary high temperature which cause a brief vaporization. Another is that as the element is lit it is losing material until it thins to such a small cross sectional area the temperature rises until an ionization/vaporization flash is created as it shorts/arcs across to the base.
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Robert11 wrote:

The filament breaks and swings wildly. It can recontact the other side at a significantly shorter length and spot weld - brighter light, higher current.
The arc when the fillament breaks can ionize the gas in the bulb resulting in an arc discharge between the filament supports. I'm not aware this happens on 120V circuits, but I think it does on 230V circuits in Europe and fuses are built into the lamps.
-- bud--
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This second explanation is actually common at 120 volts. If a lightbulb blows at switch-on, there is often a noticeably bluish flash, with color temperature well above the melting point and even the boiling point of tungsten.
To mitigate these arcs:
1. Many lightbulbs have fusible links in one of their internal wires.
2. Gas-filled ones (nearly enough all 120V ones 60 watts or more, and some 25-55 watt ones) have a mixture of nitrogen and argon, as opposed to pure argon which would slow filament evaporation very slightly more and conduct very slightly less heat from the filament. Pure argon is very prone to forming arcs, and nitrogen reduces the tendency for arcs to form. Without the nitrogen, I suspect line voltage transients could even start arcs in a few lightbulbs!
- Don Klipstein ( snipped-for-privacy@misty.com)
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Robert11 wrote:

It's called a "tungsten arc" effect, where an arc occurs, conducted by vaporized tungsten between the broken filament ends and keeps melting back along the filament pieces until it is arcing directly between the filament connecting wires. Then that arc can present a low enough resistance to cause a fuse to pop or a breaker to snap.
When you get a "tungsten arc" it usually occurs just as you switch on the bulb and you may notice a brief brighter than normal flash of light, all over in a moment.
Some light bulbs are/were made with a "fuse link" built into one of the leads intended to blow under those arcing conditions to avoid your having to reset a breaker or replace a fuse.
Jeff
--
Jeffry Wisnia
(W1BSV + Brass Rat \'57 EE)
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