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

Discussion in 'Electronic Basics' started by Serious Machining, Jan 21, 2006.

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  1. 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
  2. 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.
  3. (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.).
  4. Brian

    Brian Guest

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

  5. John Fields

    John Fields Guest

    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]
    | | | | |

    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...
  6. 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. ;-)
  7. I suspect rust or some kind of defect in the socket which causes arcing when
    the power is turned on
  8. Jasen Betts

    Jasen Betts Guest

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

  9. Is that particular bulb oriented differently? Believe it or not,
    there are specific designs for bulbs that are mounted horizontaly
    vs verticly.
  10. 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 ()
  11. 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 ()
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