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Sharp microwave arcs *through* front door

Discussion in 'Electronic Repair' started by Chris, Dec 15, 2007.

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  1. Chris

    Chris Guest

    I've got a ~2 year old microwave that has shown no previous signs of
    trouble and is to all appearances in good working order. During
    normal operation (melting butter on low power) this morning we heard a
    familiar 60Hz buzz, and then sure enough it shot an arc out through
    the door to metal rack across a gap of some two inches. Following the
    burn marks back, the arc seems to have originated (or at least exited)
    beneath the chamber, where the inside of the door meets the body of
    the oven and roughly halfway across from the hinges. First the big
    question: whatever the failure was, shouldn't there have been a
    better path to ground available? Do I have some kind of grounding
    issue that I need to fix in before I repair this thing and start using
    it again? As for the cause of the arc, since it was able to arc
    across such a large gap I assume the failure involves the high voltage
    components, not the transformer or anything else seeing AC line power,
    so maybe the diode? The capacitor would fail short, so that can't be
    it, right? The local repair shop swore this was impossible, and I
    haven't been able to turn up any previous posts covering external
    arcs, except one from back in 2000 out the top of the oven, so
    hopefully this isn't redundant. Has anyone else encountered something
    like this or have an idea as to what might be going on? Thanks.

  2. Are the door seals clean? Does the door close firmly and evenly?
  3. Chris

    Chris Guest

    The door seals were passable, but not immaculate. The door itself
    seems to close perfectly normally. It is relatively new and lightly
    used, and there aren't any glaring issues like that. We did do some
    work on our house a while back and stirred up a lot of dust in the
    process, so contamination is a possibility. Supposing something like
    that was the cause of the arc, shouldn't it have still have followed
    some other path?
  4. Eeyore

    Eeyore Guest

    That isn't a suitable load for a microwave is it ? You need some water in

    No surprise it arced.

  5. Eeyore

    Eeyore Guest

    The cause of the arc was the absense of anything in the oven that could
    absorb the microwave energy effectively.

  6. Chris

    Chris Guest

    That isn't a suitable load for a microwave is it ? You need some water in
    I thought about that, but then I've done it a hundred times with no
    problem, and the micro wave has a build in 'melt butter' function,
    which seems to run it on low power for short bursts. That's what we
    were using at the time, so I dont think the load was the issue. Again
    though, even if that were the problem, shouldn't the arc have been

  7. PeterD

    PeterD Guest

    Yes, and you may want to check the outlet's ground wire just in case.
    But the sink may have been a lower impedance ground?
    Myself, I'm not sure I'd *want* to try to fix it!
    Yes, check the door gasket?
  8. James Sweet

    James Sweet Guest

    Even an empty microwave should never arc to the outside, I've never, ever
    seen that before, and I too have used microwaves to melt butter hundreds of
    times over the decades.
  9. I don't think that's correct. Butter contains water and fat, both of which
    absorb microwaves. I've melted butter without problems.

    Arcing usually occurs at a sharp metal edge. I once put a plastic jar of
    Adams peanut butter in my microwave to soften it up, and got serious arcing
    on the teensy bits of aluminum foil that had been left behind when I peeled
    off the inner seal.
  10. ¿Quien sabe?

    I would thoroughly clean everything, inside and out. Also check to see that
    there are no bits of foil, or anything metallic, in the oven, on or around
    the seals, etc.
  11. jakdedert

    jakdedert Guest

    I've had that happen recently. Same product (different brand)--some
    surprisingly SERIOUS fireworks immediately as I hit the button. If the
    lights had been off, it would have lit the entire kitchen from really
    minuscule bits of foil.

    I'm with the other poster, in that I'm not sure I'd ever trust that unit

  12. Chris

    Chris Guest

    Yeah, I'll double check. Also noticed a second burn mark closer to
    the hinges, but same vertical position. Just to clarify my
    description of the problem, I'm not talking about an arc like you'd
    see from a little metal in the microwave. I'm talking about a beefy
    pink arc about the diameter of a pencil.
  13. Eeyore

    Eeyore Guest

    I agree it shouldn't have arced to the *outside*.

    Ok. I have heard of other stories where people have done things with their
    microwave ovens the makers don't recommend and been puzzled why it broke after
    say the hundredth time though.

  14. Eeyore

    Eeyore Guest

    Maybe the quantity wasn't enough ?

  15. Chris

    Chris Guest

    Nah, it was a half a stick, well within the pre-programmed range, so
    that's not it. In any case, some bit of foil of just the right size
    or shape can lead to gradients high enough to get sparks, but this was
    not like this. This was some large fraction of the entire current
    output of the transformer, like the fireworks you see on youtube when
    people have pulled the guts out of the microwave and using it just to
    make an arc.
  16. I'm not sure. The "received knowledge" on such things is that if there's
    nothing in the oven to absorb the microwaves, the magnetron will eventually
    overheat (from the reflected unabsorbed energy). The lower the power
    setting, the less likely this is to happen.

    But overheating isn't arcing. Arcing requires a sharp metallic edge for the
    electric field to build to a point where the air breaks down.
  17. If a thorough cleaning and inspection doesn't resolve the issue, I'd get a
    new unit. The potential for damage or injury seems too great.
  18. This whole thing doesn't sound right. Regardless of whether the microwave
    oven is attached to Earth ground, the return path for the high voltage
    IS the chassis of the microwave oven. It would be almost impossible for
    that to be disrupted as the magnetron and HV transformer are bonded to
    the chassis. So, it's highly unlikely that the HV current (not the
    microwave energy) would want to jump from the oven to an external ground.
    Looking at the typical schematic, it's hard to come up with any sort of failure
    mode where such a potential difference would appear between the chassis
    and ground.

    I'm not saying something very peculiar didn't happen. Just that an
    explanation relating to the HV power doesn't make sense. I'd quicker go
    with some combination of load (butter), dirt, and other factors affecting
    the microwave distribution.

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  19. Ron(UK)

    Ron(UK) Guest

    It could be possible that some part of the door, possibly one of the
    hinges has lost it`s bonding to chassis earth and a potential is
    building up there then striking to the nearest earthed metal. it`s
    unlikely tho in my experience.

    There was a model of Phillips oven that could build up a charge on the
    thin aluminium cover over the lamp leading to quite spectacular arcing
    around the rivet which held it in place and acted as a swivel. A poor
    connection there was indicated by quite strong leakage of microwave
    radiation through the plastic menu strip on top of the oven.

  20. Marra

    Marra Guest

    Mains wont jump 2 inches !!!!!!
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