Sharp microwave arcs *through* front door

[Chris]

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.

-Chris

 

[William Sommerwerck]

Are the door seals clean? Does the door close firmly and evenly?

 

[Chris]

Are the door seals clean? Does the door close firmly and evenly?

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?

 

[Eeyore]

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)

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

No surprise it arced.

Graham

 

[Eeyore]

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?

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

Graham

 

[Chris]

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

there.

No surprise it arced.

Graham

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
contained?

-Chris

 

[PeterD]

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?

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?

Do I have some kind of grounding
issue that I need to fix in before I repair this thing and start using
it again?

Myself, I'm not sure I'd *want* to try to fix it!

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.

Yes, check the door gasket?

 

[James Sweet]

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

No surprise it arced.

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.

 

[William Sommerwerck]

Chris wrote:

That isn't a suitable load for a microwave, is it? You need some water
in there. No surprise it arced.

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.

 

[William Sommerwerck]

On Dec 15, 6:09 pm, "William Sommerwerck" <[email protected]>
wrote:

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?

¿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.

 

[jakdedert]

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.

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
again.

jak

 

[Chris]

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.

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.

 

[Eeyore]

"Eeyore" wrote

Even an empty microwave should never arc to the outside,

I agree it shouldn't have arced 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.

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.

Graham

 

[Eeyore]

I don't think that's correct. Butter contains water and fat, both of which
absorb microwaves. I've melted butter without problems.

Maybe the quantity wasn't enough ?

Graham

 

[Chris]

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.

 

[William Sommerwerck]

Maybe the quantity wasn't enough?

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.

 

[William Sommerwerck]

Nah, it was 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.

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.

 

[Sam Goldwasser]

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.

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.

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header above is
ignored unless my full name AND either lasers or electronics is included in the
subject line. Or, you can contact me via the Feedback Form in the FAQs.

 

[Ron(UK)]

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.

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.

Ron(UK)

 

[Marra]

Are the door seals clean? Does the door close firmly and evenly?

Mains wont jump 2 inches !!!!!!