Defective Bulb Tripping Ckt Breaker: How Possible ?

This often happens. It's because when the filament breaks, it falls across part of the wire nearer the mounting points, thus horting them out. That's why there's a bright flash, and the fuse/breaker goes because of the increased current through the much lower than normal resistance.

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rgds
LAurence

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On Tue, 31 Oct 2006 10:26:32 -0500, Robert11 Has Frothed:

Might have a problem with the base of the bulb or the socket.

--
Pierre Salinger Memorial Hook, Line & Sinker, June 2004

COOSN-266-06-25794

hall

I'm only going by memory and may have it a bit wrong. A tiny part of the filament wears to the point it creates a hot-spot which rapidly gets worse causing the first part of the intense flare. In that super-heated process a plasma state is formed within the bulb. In a plasma state the effective conductivity across the 2 filament suspension end points becomes orders of magnitude less than the original filament resistance so for very short duration , can pass tens or hundreds of amps. It is the same mechanism whereby a lightning discharge through air can pass millions of amps through a channel that is only about 1/2 inch in diameter which no metal could pass. The same discharge passing into the right soil/ conditions leaves a yards long but same 1/2 inch wide hollow tube punched through the ground, see keyword fulgurite.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Sure. There should be a fuse in one of the leads inside the bulb, but China ???

This all sounds nice BUT the two strands of the broken filament limits the current to ~ current to less than 1 Amp. !

More likely the Light-bulb in question was mounted Socket-Down and a portion of the broken filament dropped down and shorted across base conductors. The short length of filament's Resistance would be much less than the nominal ~ 200 ohms of the intact filament. The resulting arc would vaporize in the Argon Gas of the light-bulb and create a miniature MIG (Metal Inert Gas)Welding condition at which time, the Circuit Breaker would Open due to the excessive current draw. A photographic Flash-bulb would be a good analogue. Source Impedance and Current Limiting of Household wiring were engineered to safely harness electrical energy!

Yukio YANO

Have you recently changed from fuses to circuit breakers? A bulb blowing and tripping one isn't uncommon.

--
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    Dave Plowman        dave@davenoise.co.uk           London SW
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G'Day

This has happened to me. The bulb was a "Reflector Lamp Type R80" with an Edison Screw Base". It was used in a "down light" and they have a very short life (6 months or so). In the process of removing it the brass base and the glass globe separated and I had to get the base out of the socket (with power turned off of course.) with a pair of pliers.

Recently purchased a couple of bulbs from China and on the side of the box it says:-

For your added safety every globe has 2 built-in safety fuses.

Seems like the some manufacturers are aware of this problem

Lionel L Sharp

This is common, the filament breaks and an arc strikes in the fill gas between the ends, this quickly vaporizes what remains of the filament and will sometimes melt a good portion of the filament support wires before the fuse in the stem blows. If the fuse is poor quality or not present (as may be the case in a cheap import bulb) it can easily trip the circuit breaker. This arcing is what kills most dimmer switches and touch lamps, the triac is destroyed before the fuse can react.

This is one more thing I like about compact fluorescents, when they fail they don't blow with a bright flash and a pop which is always a startling experience when you flip on the only light in a dark room.

On Tue, 31 Oct 2006 20:58:00 +0000, James Sweet Has Frothed:

I saw flames shoot out of the side of the base of a screw in fluorescent.

--
Pierre Salinger Memorial Hook, Line & Sinker, June 2004

COOSN-266-06-25794

Right, they blow with a considerable BANG and a bad smell. The CFL in my wife's bedside lamp did that. Of course I had to open it up and see what happened. The first capacitor of the voltage-doubler ( a tiny

10uf cap ) had popped. Really popped. It spread black goo all over the insides of the thing.

Plasma, most likely, from the internal arc. Light bulbs are filled with argon gas which can conduct quite nicely once started.

Or the floppy wire ends had enough magnetic attraction to attract each other, which makes the field stronger, attracting each other some more...

Interesting, well I suppose it could happen, in 8 years of using them almost exclusively mine so far have all just failed to turn on, save for one that made a soft buzzing noise and let out a puff of stinky smoke.

On Tue, 31 Oct 2006 13:11:12 -0800, Ancient_Hacker Has Frothed:

Also could have caused a momentary neutron star.

--
Pierre Salinger Memorial Hook, Line & Sinker, June 2004

COOSN-266-06-25794

Happens more than a lot - depending on the circuit breaker.

Magnetic beakers trip faster than thermal ones.

Filament opens - the tungsten is in a partial pressure of nitrogen as a rule - not a vacuum just inert gas at low pressure. The filament separates and sparks an arc in the bulb. The ionized gas is a dead short.

Low pressure gas conducts electricity better than high pressure gas - until you start approaching a total vacuum. Ionization gauges are used to monitor vacuum pressures in lab instruments - put 200+ volts on it and it conducts until you get to near perfect vacuum.

That failure will trip a magnetic breaker and not phase a totally thermal breaker - event is over before the breaker heats.

Solid state relays are toast - if they drive an incandescent lamp that opens.

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The filament in the bulb vaporises at the point where it breaks, and produces a nice hot plasma and a dead short across the input. If you want to see such a plasma cloud, put a defunct bulb in the microwave oven(for a few seconds only).

That in turn can vaporize the connections to the base of the bulb. I have had lamps producing a pop sound close to an explosion that way, and sparks flying. On checking, the base of the lamp was looking burned around the attachment point of one of the wires. It works much better with 240 Volts :) :)

Yup.

If there's an arc, which it sounds like, that has negative resistance and the fault current can be very high.

Graham

You've got it about right. And in response to the other post, it's rarely a matter of the filament falling across the terminals.

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Nope, he's got it right. Once the arc forms, it may not stay between the broken ends. Because the arc is a negative resistance, increasing the current lowers its resistance. So, it can grow towards the ends of the filament in an instant and then it's nearly a short circuit.

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But they may blow with a puff of smoke and a acrid smell.... :)

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