Why does one particular lamp in a multi-lamp fixture frequently burn out ?

Just a statistical anomaly. By the way, they often blow on turn on in any case as a cold (room temp.) filament has a very low resistance compared to its resistance when operating normally ... the start up current is about 5 times the operating current. An incandescent lamp that operates continuously gives many more hours of operation than one that is cycled several times per day.

(snip)

I would look for a mechanical resonance that is different for that socket, and that coincides with some source of vibration that shakes it at that frequency (e.g. walking across the room, air blower, nearby train passing, road traffic, etc.).

One possibility might be the light socket itself. Try changing that light socket.

Brian

--- My guess would be that it's because it's the lamp in the fixture which is closest to the feed.

That is: (view in Courier)

V1 V2 V3 V4 V5 ____ | | | | | MAINS>--O O--+-------+-------+-------+-------+ SWITCH | | | | | [LAMP1] [LAMP2] [LAMP3] [LAMP4] [LAMP5] | | | | | MAINS>--------+-------+-------+-------+-------+

Since all of the lamps are wired in parallel across a single-conductor bus, the resistance of the wire will cause the voltage drop across the bus to increase as it gets farther away from the switch, with the result being that LAMP1 will have a higher voltage across it than LAMP2 will, LAMP2 will have a higher voltage across it than LAMP3 will, and so on.

The lifetime of incandescent lamps is exquisitely sensitive to the voltage placed across them, (Someting like inveresely proportional to the third or ninth power of the voltage across them... I forget. Don Klipstein knows, though, so if he reads this he might want to clarify it) so even the tiny voltage gradient across the lamps might be doing it.

An interesting experiment, if you're so inclined, would be to not replace the burned-out bulb in order to see if the next failure is #2, then #3, then...

-- John Fields Professional Circuit Designer

the

What

Replace the socket. I live in a >25 year old home and when I have a fixture doing this (and I've had a few), changing it out cures the problem. I read this somewhere and, sure enough, it seems the guy was right. ;-)

I suspect rust or some kind of defect in the socket which causes arcing when the power is turned on

other environmental factors, does it get colder than other lamps...

Bye. Jasen

"Serious Machining" wrote in news: snipped-for-privacy@g14g2000cwa.googlegroups.com:

Is that particular bulb oriented differently? Believe it or not, there are specific designs for bulbs that are mounted horizontaly vs verticly.

Although incandescents usually blow on cold starts, in most incandescents cold starts do surprisingly little cumulative damage, even if the filament moves or makes an audible sound. The main thing that happens, at least in most incandescents, is that aging filaments become unable to survive a cold start a little before they become unable to survive continuous operation.

How incandescents usually blow out: An aging filament has a thin spot that runs excessively hot. This spot has temperature overshooting during a cold start, since it has less mass, higher resistance, dissipates more power, and resistance of tungsten increases with temperature. The thin spot reaches a higher temperature while the rest of the filament is still warming up than it runs at steadily. Protecting such a thin spot from the temperature overshoot only helps a little. Once a thin spot like that develops, the degree of this bad situation accelerates worse than exponentially - the lightbulb's hours are numbered once it becomes unable to survive a cold start.

Where avoiding cold starts are more likely to extend life: Sometimes in halogen bulbs, which have a way of having the filament form thin spots at the ends of the filament due to the halogen cycle attacking cooler tungsten. If there are thin spots that reach excessive temperature only during a cold start, then avoiding cold starts could make a major difference - not always, since something else could be about to go wrong with the filament.

- Don Klipstein ( snipped-for-privacy@misty.com)

I suspect possibly you are getting replacement bulbs of a bad brand or a bad production run or an anappropriate type for your situation. In that case, the other four sockets will start giving you high burnout rates once you start putting any bad replacements into them.

- Don Klipstein ( snipped-for-privacy@misty.com)