This unit is defective. The manufacturer should be contacted for a
replacement. Discontinue use at once. Fireworks inside are one thing.
The moment they reach outside, it is something entirely different!
Large amounts of microwave energy are leaking and could seriously burn
your eyes, nasal passages, lungs and internal organs before your skin.
Stop screwing around with this one.
The manufacturer SHOULD be willing to replace it for the sake of
liability alone.
On Dec 17, 8:32 am, Sam Goldwasser wrote:
Chris writes:
Thanks for all the replies. Its beginning to sound like I was right
to feel baffled. Its certainly not mains voltage, but before the arc
there was a buzz that sounded like mains frequency. Since you cant
see a flicker even at 60Hz (I'm in the US), I don't know the frequency
of the arc itself.
Ron: I had wondered if it was possible that the door had lost contact
(grease in the hinges?) with the chassis and was therefore able to
charge up. It still seems like it would have been easier to jump back
to the chassis than across the gap, but then I have two burn marks, so
maybe it went out into the door and then back to the chassis a ways
down for a while before hand?
Sam: I completely agree with both you and Ron that this seems far-
fetched. It seemed so unlikely that I started think I'd just made up
having seen the arc, and I wouldn't believe it if I didn't have the
burn marks staring me in the face. I also completely agree with your
reasoning for ruling out the HV as the explanation. Even so, I still
have some problems with the load explanation that I need to answer
before I trust my kitchen again. First, and most important, by the
same argument you made ruling out HV problems as the source, shouldn't
any spark created inside the oven by whatever mechanism be contained
by the chassis unless something has gone horribly wrong? On the other
hand, proceeding from the assumption that its impossible for the
entire chassis from floating up high enough to do this, is it even
possible for a load problem to cause a really large arc? In ballpark
figures, the oven is rated at ~1000 watts, the magnetron is some ~50%
efficient, say, so there can't be more than a than a few hundred watts
of microwaves bouncing around, right? Even if you made a tuned
resonator (one of these graphite and aluminum foil gizmos, for
example), and had no other load, is it possible to recover that energy
efficiently enough to something like this? At any rate, to be clear,
nobody thinks that in the absence of other problems I have reason to
doubt my house ground?
Did the fireworks happen *instantly* when you pushed START? Instantly
means in a fraction of a second.
Any effect would have 60 Hz in it since the microwaves are generated in
pulses that have 100 percent ripple at 60 Hz.
Could there be a 2 inch arc from 1000 W of microwaves? Sure.
And a half stick of butter or whatever is probably not a very good
absorber so some sort of peculiar resonant cavity effect is quite
possible. And, if there was dirt, grime, and goop 
in the area
of the door where the arcing took place, even more possible.
Beyond that, I'd say you have to try some more experiments. :-)
To be sure, check your outlet ground with a multimeter or outlet tester.
But plenty of microwave ovens are used without proper grounds despite the
warnings.
I can't imagine any problem with the house ground that wouldn't result
in other issues.
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