Not so much electronics, more electrics

[hooch]

I have two halogen bulbs on the same lighting circuit (240V - UK)
controlled by the same two switches, i.e, either switch will turn on
or turn off both lights. Recently one of the two bulbs died (open
circuit, as expected), causing the fuse / circuit breaker to trip.
Can anyone offer an explanation?

 

[JANA]

All incandescent light bulbs use a tungsten filament. When the filament
fails, this is because it eventually melted away to be an opened circuit.
Over the lifetime of a light bulb, as the filament is heated to near white
hot temperatures to generate light, it starts to slowly break down.

When the filament in a lamp burns out, it sometimes has a bit of a meltdown.
Sometimes pieces of it can short out the metal support contacts feeding
itself. If there is a short, it can cause a fuse to blow, or a circuit
breaker to go to the open mode.

--

JANA
_____


I have two halogen bulbs on the same lighting circuit (240V - UK)
controlled by the same two switches, i.e, either switch will turn on
or turn off both lights. Recently one of the two bulbs died (open
circuit, as expected), causing the fuse / circuit breaker to trip.
Can anyone offer an explanation?

 

[hooch]

All incandescent light bulbs use a tungsten filament. When the filament
fails, this is because it eventually melted away to be an opened circuit.
Over the lifetime of a light bulb, as the filament is heated to near white
hot temperatures to generate light, it starts to slowly break down.

When the filament in a lamp burns out, it sometimes has a bit of a meltdown.
Sometimes pieces of it can short out the metal support contacts feeding
itself. If there is a short, it can cause a fuse to blow, or a circuit
breaker to go to the open mode.

Ah, thanks. That's OK, then. I thought it may have been something to
do with the wiring.

 

[clifto]

I have two halogen bulbs on the same lighting circuit (240V - UK)
controlled by the same two switches, i.e, either switch will turn on
or turn off both lights. Recently one of the two bulbs died (open
circuit, as expected), causing the fuse / circuit breaker to trip.
Can anyone offer an explanation?

I have read that filament-type bulbs will sometimes arc when the filament
opens, and that the arc can draw a whole lot more current than one would
expect.

 

[Dave Plowman (News)]

I have two halogen bulbs on the same lighting circuit (240V - UK)
controlled by the same two switches, i.e, either switch will turn on
or turn off both lights. Recently one of the two bulbs died (open
circuit, as expected), causing the fuse / circuit breaker to trip.
Can anyone offer an explanation?

Pretty common. There are many theories why, but bulbs often short out on
failure. Many incorporate a fuse to try and prevent it taking out the
circuit. In the UK 'C' type circuit breakers are available which stand an
overload for longer before tripping. Might be useful if the light in
question is in a hallway where others will give at least some light if it
blows.

 

[Sam Goldwasser]

All incandescent light bulbs use a tungsten filament. When the filament
fails, this is because it eventually melted away to be an opened circuit.
Over the lifetime of a light bulb, as the filament is heated to near white
hot temperatures to generate light, it starts to slowly break down.

When the filament in a lamp burns out, it sometimes has a bit of a meltdown.
Sometimes pieces of it can short out the metal support contacts feeding
itself. If there is a short, it can cause a fuse to blow, or a circuit
breaker to go to the open mode.

Geez, this again???

While the scenario of the bits of the filament shorting is possible with
some lamps (usually with long thin filaments, though unlikely with the
short filaments of halogen lamps), the more likely cause is the arc
resulting when the filament opens. This results in the arc moving
towards the filament supports, with a lower resistance than the filament
had originally, leading to a high current.

See: http://members.misty.com/don/bulb1.html#wbs

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header above is
ignored unless my full name AND either lasers or electronics is included in the
subject line. Or, you can contact me via the Feedback Form in the FAQs.

 

[clifto]

While the scenario of the bits of the filament shorting is possible with
some lamps (usually with long thin filaments, though unlikely with the
short filaments of halogen lamps), the more likely cause is the arc
resulting when the filament opens. This results in the arc moving
towards the filament supports, with a lower resistance than the filament
had originally, leading to a high current.

What I never figured out is how the arc is maintained for more than 1/120
of a second when there's no air around the arc to ionize.

 

[James Sweet]

What I never figured out is how the arc is maintained for more than 1/120
of a second when there's no air around the arc to ionize.

There's argon or other inert gas in there, it ionizes just like the arc in a
discharge lamp.

 

[JeffM]

What I never figured out is how the arc is maintained for more than 1/120
of a second when there's no air around the arc to ionize.

Tiny bulbs are often evacuated--but larger bulbs are filled with gas.
The gas pressure helps prevent the filament from evaporating
(which would not only thin it prematurely
but would lead to metal deposition on the inside of the glass,
dimming the light output).

Look at the light bulb aisle next time you go out.
Standard bulbs are filled with Argon.
"Super" bulbs are filled with Krypton or Xenon.

 

[Sam Goldwasser]

What I never figured out is how the arc is maintained for more than 1/120
of a second when there's no air around the arc to ionize.

There's an inert gas.

Only the smallest incandescent lamps have a vacuum inside.

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header above is
ignored unless my full name AND either lasers or electronics is included in the
subject line. Or, you can contact me via the Feedback Form in the FAQs.

 

[hooch]

There's an inert gas.

Only the smallest incandescent lamps have a vacuum inside.

As, I think, do TV picture tubes. I like your TV Repair FAQ, by the
way.

 

[James Sweet]

As, I think, do TV picture tubes. I like your TV Repair FAQ, by the
way.

And any other vacuum tube, but this discussion was specific to lightbulbs.