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Posted by Robert11 on October 31, 2006, 10:24 am
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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Posted by dpb on October 31, 2006, 10:49 am
Robert11 wrote:
...
> Wife turned on a hall light switch last night, big flash by one of the hall
> lights, and the ckt breaker tripped.
...
> Question: I can't imagine how anything, like e.g. a broken filament
> perhaps, can short out the circuit.
...
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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Posted by Bud-- on October 31, 2006, 12:07 pm
Robert11 wrote:
> 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
>
>
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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Posted by Don Klipstein on November 5, 2006, 2:13 pm
>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.
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 (don@misty.com)
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Posted by Jeff Wisnia on October 31, 2006, 12:14 pm
Robert11 wrote:
> 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
>
>
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)
The speed of light is 1.8*10^12 furlongs per fortnight.
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