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

[Serious Machining]

I have a five lamp ceiling light fixture in which one particular lamp
frequently burns out while the other four lamps last for over a year's
time. The bulbs are 60 watt incandescent globe. All the lamp sockets
are wired together along with the switched 'hot' wire. Similar for the
neutral wires. Everything appears tight - bulb to socket, socket to
fixture, wires connected to socket, joining of the five lamp wires to
house wiring, house wiring to/from wall switch. I have even exchanged
bulbs among the five as well as installing new bulbs in the offending
lamp - and nothing seems to resolve the problem. If I leave the blown
bulb in the lamp, the other four continue to work just fine and last a
long time. The problem lamp generally blows when the fixture is
switched 'on'. The socket for this lamp does not get any hotter than
the others and I have never detected any arcing or smelled ozone. What
could make this lamp most susceptible to the jolt of the inrushing
voltage & current when the switch is flipped 'on' ?
- Dennis Anderson

 

[Charles Schuler]

I have a five lamp ceiling light fixture in which one particular lamp
frequently burns out while the other four lamps last for over a year's
time. The bulbs are 60 watt incandescent globe. All the lamp sockets
are wired together along with the switched 'hot' wire. Similar for the
neutral wires. Everything appears tight - bulb to socket, socket to
fixture, wires connected to socket, joining of the five lamp wires to
house wiring, house wiring to/from wall switch. I have even exchanged
bulbs among the five as well as installing new bulbs in the offending
lamp - and nothing seems to resolve the problem. If I leave the blown
bulb in the lamp, the other four continue to work just fine and last a
long time. The problem lamp generally blows when the fixture is
switched 'on'. The socket for this lamp does not get any hotter than
the others and I have never detected any arcing or smelled ozone. What
could make this lamp most susceptible to the jolt of the inrushing
voltage & current when the switch is flipped 'on' ?

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.

 

[John Popelish]

I have a five lamp ceiling light fixture in which one particular lamp
frequently burns out while the other four lamps last for over a year's
time.

(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.).

 

[Brian]

I have a five lamp ceiling light fixture in which one particular lamp
frequently burns out while the other four lamps last for over a year's
time. > - Dennis Anderson

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

Brian

 

[John Fields]

I have a five lamp ceiling light fixture in which one particular lamp
frequently burns out while the other four lamps last for over a year's
time. The bulbs are 60 watt incandescent globe. All the lamp sockets
are wired together along with the switched 'hot' wire. Similar for the
neutral wires. Everything appears tight - bulb to socket, socket to
fixture, wires connected to socket, joining of the five lamp wires to
house wiring, house wiring to/from wall switch. I have even exchanged
bulbs among the five as well as installing new bulbs in the offending
lamp - and nothing seems to resolve the problem. If I leave the blown
bulb in the lamp, the other four continue to work just fine and last a
long time. The problem lamp generally blows when the fixture is
switched 'on'. The socket for this lamp does not get any hotter than
the others and I have never detected any arcing or smelled ozone. What
could make this lamp most susceptible to the jolt of the inrushing
voltage & current when the switch is flipped 'on' ?

---
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...

 

[Anthony Fremont]

I have a five lamp ceiling light fixture in which one particular lamp
frequently burns out while the other four lamps last for over a year's
time. The bulbs are 60 watt incandescent globe. All the lamp sockets
are wired together along with the switched 'hot' wire. Similar for the
neutral wires. Everything appears tight - bulb to socket, socket to
fixture, wires connected to socket, joining of the five lamp wires to
house wiring, house wiring to/from wall switch. I have even exchanged
bulbs among the five as well as installing new bulbs in the offending
lamp - and nothing seems to resolve the problem. If I leave the blown
bulb in the lamp, the other four continue to work just fine and last a
long time. The problem lamp generally blows when the fixture is
switched 'on'. The socket for this lamp does not get any hotter than
the others and I have never detected any arcing or smelled ozone. What
could make this lamp most susceptible to the jolt of the inrushing
voltage & current when the switch is flipped 'on' ?

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. ;-)

 

[Mike Mainville]

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

 

[Jasen Betts]

switched 'on'. The socket for this lamp does not get any hotter than
the others and I have never detected any arcing or smelled ozone. What
could make this lamp most susceptible to the jolt of the inrushing
voltage & current when the switch is flipped 'on' ?

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

Bye.
Jasen

 

[Gordon reeder]

I have a five lamp ceiling light fixture in which one particular lamp
frequently burns out while the other four lamps last for over a year's
time. The bulbs are 60 watt incandescent globe. All the lamp sockets
are wired together along with the switched 'hot' wire. Similar for the
neutral wires. Everything appears tight - bulb to socket, socket to
fixture, wires connected to socket, joining of the five lamp wires to
house wiring, house wiring to/from wall switch. I have even exchanged
bulbs among the five as well as installing new bulbs in the offending
lamp - and nothing seems to resolve the problem. If I leave the blown
bulb in the lamp, the other four continue to work just fine and last a
long time. The problem lamp generally blows when the fixture is
switched 'on'. The socket for this lamp does not get any hotter than
the others and I have never detected any arcing or smelled ozone. What
could make this lamp most susceptible to the jolt of the inrushing
voltage & current when the switch is flipped 'on' ?
- Dennis Anderson

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

 

[Don Klipstein]

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.

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 ([email protected])

 

[Don Klipstein]

I have a five lamp ceiling light fixture in which one particular lamp
frequently burns out while the other four lamps last for over a year's
time. The bulbs are 60 watt incandescent globe.

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 ([email protected])