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400 watt MH meltdown

Discussion in 'Lighting' started by JohnR66, May 12, 2005.

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  1. JohnR66

    JohnR66 Guest

    Please see the link. Half of the mogul base of the 400W MH bulb is gone! I
    thought we took a lightning hit, but if that was the case, I'd expect more
    explosive damage. This is constant heat/arc damage. Ballast is the
    autotransformer style with magnetic shunt. Has 120v, 208 and 277 supply taps
    (using the 120v tap). I'm getting 1.6 Ohms hot to neutral and about 4 ohms
    neutral to load tap so ballast looks fine. The 24uf 360volt capacitor in
    series with the lamp measures spot on.
    Apparently the insulator glass in the bulb base became hot and conducting.
    There is a big melted blob left in the base. The brass screw base on the
    bulb and in the base had melted spots.

    This outdoor wall pack looks clean inside (no bugs or water). No idea why
    this happened. Any ideas?

  2. If the lamp was not screwed firmly into the socket a lot of heat could
    have been generated between the center contact of the lamp and the

    Vic Roberts
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  3. I have seen a couple 400W metal halide lamps failed like this, although
    not quite this badly. I got a good look at one, and the glass between the
    base shell and the center contact of the base was melted.

    I am guessing that the center contact lost contact while the lamp was
    running, and an arc formed. The arc could have been at the edge of the
    center contact, where it would heat the glass insulator badly. And molten
    glass is a little conductive, and may also emit enough vapor of sodium
    compounds to make the arc more stable (it does not take much to do that).
    Although with about 3 amps of current involved, metal parts that the arc
    is on can emit enough vapor to make the arc more stable and able to be
    stretched to half an inch or more long.

    - Don Klipstein ()
  4. JohnR66

    JohnR66 Guest

    Thanks for the comments. As a side note. I got a new ceramic socket and
    bulb. The ballast is indeed working fine again.
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