Chris wrote in news:5a8793cc-df83-4935-b543-
:

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.
....

Thanks again. I'll keep cooking with one hand in my pocket for now.

I would follow the advice of the others and contact the manufacturer
immediately, discontinue use!

Treat the oven like you would a live bomb!!! [Cut the circuit breaker,
unplug the oven, turn the circuit breaker back on]

Supply them with pictures [as best you can reconstruct] the exact
configuration of the oven, its contents, and the rack. Along with diagrams
and pictures of the burn marks.

My best guess as to something that might cause the arcing you observed is
highly unlikely:

1) the vapors from the heated butter [moist air and fat] producing a
conductive path inside the oven that happened to resonant at the magnetron
frequency.
2) The door and the metal rack also forming a resonant circuit.
3) capacitive coupling THROUGH the metal screen built into the door.
4) ionization of the air between the door and the rack, perhaps by a stray
cosmic ray that 'kicked off' the arc.
5) Your house just happened [for a few moments] to be at a point of
resonance in the 60 Hz power grid that covers the country.
Or a resonant, ionized path, within the area around the power transformer
that allowed a similar arc that allowed [intermittently] the 60 Hz HV AC to
jump from transformer to case to rack[there should be burn marks near the
transformer, also.

Question: where did the 'power' go after jumping to the metal rack? Any
'exit wounds'

None of the points seem probable but all seem barely possible.
Comment: YOU are lucky to be alive. Think about that arc jumping to YOU.

I used to fix radars for a living and I have seen some 'odd' things happen,
but nothing as odd as what you described.

--
bz

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

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