The other day I flipped a light switch on. Immediately both the bulb burned out and the circuit breaker tripped. Is it possible that the failure of the light bulb (filament burning out) caused the circuit breaker to trip? Or is there a more probable explanation?

 

It depends -- what else was on the circuit the light switch controls? The circuit breaker tripped because of overcurrent. A filament simply burning out wouldn't cause that. Did you put a new bulb in and reset the breaker and it worked ok?

 

From what I've read, a light bulb draws the most current right when it's turned on and then increases its resistance as it warms up.

My hypothesis is that a weaken filament (one that's been used a lot) presents an even lower than normal resistance when turned on, and that was the cause of the overcurrent. The only tricky thing is that the filament has to sustain the overcurrent long enough to trip the breaker in addition to destroying itself.

Is this a possible scenario?

1. Turn on light.
2. Excess current begins to flow through weakened filament.
3. Circuit breaker begins to trip due to overcurrent.
4. Excess current continues to flow through filament destroying it.
5. Breaker completes tripping, disconnecting current.

 

This has happened to me a few times over the years, but mine is a floor standing lamp that plugs into the outlet. There are two separate bulbs, with individual twist switches in their sockets. Kind of a cheap thing, and use the cheapest bulbs. I never really investigated the why of it, but there was always a load pop from the bulb when switch on. I blamed the bulbs, but could be the cheap lamp, or the 'unique' electrical work of this 1946 build home, which I'm only slightly to blame,

True, the filament itself shouldn't... but if you ever busted open a bulb, for say using whatever was left for a firecracker igniter... You'd notice its wound like a tiny spring, and attached to two thicker wires which extend through the glass envelope. The little coil melts, a small bead forms a drips down between the two thicker wires. Might be enough of a short. Doesn't happen regularly , kind of rare, so figure for me, its the cheap Walmart brand bulbs. Recently switched to the compact fluorescent type out of curiosity (mainly for longevity, not Global Warming concerns), also kind of want to rip one apart and see the guts when it burns out...

 

Yes, it's fairly common.

The filament burned out causing an arc to strike between the electrodes. Arcs have a negitive resistance and if left alone will draw an unlimited current but the breaker tripped which cut off the current.

Old light bulbs could explode if the breaker wasn't fast acting enough to trip. New bulbs are fused internally so even if the breaker didn't trip the bulb's internal fuse will blow.

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Is 120VAC high enough to start an arc?

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Ron

 

Yes Ron, it is.

You can force that situation by slightly bouncing the hot lamp exactly at the moment when switching off. The red hot wire will break and leave a microscopic small gap between the two parts. (Visible only with 100X magnification)

When you switch on the (cold) lamp again the two ends of the broken filament will be close enough to try arc welding together. That's exactly the moment when the current increases drastically. The overall length of the filament is shortened by this process and the welding point is a heavy bead. Since the filament is still cold the resistance is low this is an additional aid for a nice short. It makes the lamp burn up internally and the CB trip.

I'm sure you can find movies about that at OSRAM or PHILIPS, more likely at OSRAM.

Have you noticed that filament type lamps always burn up when switching them on? Vibration caused by a passing truck is enough for the destructive work.

BTW and just for info: Some decades ago an undestructable filament was invented by an OSRAM engineer based on a material with equal resistance hot and cold. The plans for that rest in a safe. (Would have ruined the business!)

 

Hi Hero,

all very true but I have a complaint. As you know I'm German and still studying English. May be the pronunciation of the word is used in your part of the country. Looking up Webster's dictionary I couldn't find the word "negitive", but "negative".

Please don't sabotage my efforts anymore.

Regards

Hans

 

Yes my spelling is crap.

I always get negative and positive mixed up, one is spelt with an i and the other with an a which is quite silly.

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No problem if you consider that both words originate from Latin. The german language uses them as well positive = positiv and nagative = negativ.

To avoid misspelling use negatron and positron.

 

Thanks for that.

Another thing that will help strike the arc is that bulbs are filled with low pressure nitrogen which will have a lower breakdown voltage than normal air.

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While your observation is correct that bulbs are filled, they use an inert gas. I am not a chemistry student though!, I feel, Organ is generally used, if not vacuum.

Now, If the gas can cause arcing, all filament bulbs would have fused in the American context, by now.
It is a rare observation by PC88. I understand that switches used in American land do consist of zero crossing starting.

Perhaps, the concerned detector fails, and the bulb is momentarily ON at the time when Voltage is at peak. Then the cold filament would draw much higher current. the filament failure and the circuit breaker trip would have been simultaneous.

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Sarma.

 

Don't you mean Argon?

Argon is used for some bulbs but it's considered too expensive for cheap domestic bulbs. A vacuum is sometimes used (mainly or smaller bulbs) but the problem is the reduced pressure increases the evaporation rate of the filament.

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I'm not totally sure but I think my stick welder is putting out under 100V when I strike an arc with is (but lots of amps). Mind you, you have to get the strike right to start the arc (I like tapping, but others prefer striking the stick like a match).

As far as negative/positive goes, I just try to remember that "negative" derives from "negate" and that "positive" derives from "posit" (and "posative" just looks goofy).


Torben

 

throw a wrench across your car battery and tell me if it will arc or not