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I have a Frigidaire FGMV176NTB range hood microwave oven which was installed new about two years ago. It works fine, but recently started tripping the 15A standard breaker it's on (Siemens c. 1992) when we occasionally open the door to end cooking. This occurs every time or nearly so.

The only other loads on the circuit are a few LED lights. It operates normally in all other use cases, including when the timer ends cooking or the Cancel button is used.

Further test results...

  • The door is aligned with the frame as it was when new. I ran a test by lifting slightly and opening the door, and the breaker tripped immediately. Door sag/alignment does not seem to be my problem.
  • Power was connected to another kitchen circuit. A slight pull on the door handle tripped the GFCI outlet. The breaker also tripped on a third circuit without GFCI protection. A faulty breaker does not seem to be my problem.

Update again: And now the unit is completely dead. I'm ordering door switches and a primary fuse and will report back.

And again: I've replaced all the door interlock switches, which tested fine for continuity and function to begin with, and the primary fuse. The breaker still trips on opening the door during heating. Apparently that's not the problem.

isherwood
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4 Answers4

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Is the microwave door sagging a bit? This could be slightly changing the sequence of opening and closing of the microswitches in the door safety interlock, which the last time I looked are two normally open switches and one normally closed. As @ecnerwal suggests, switching out of sequence may create an intentional short circuit.

Try lifting the door slightly when you open it with the microwave running. If that prevents the breaker tripping, you may be able to repair, re-set or shim the door hinge to eliminate the sag.

MTA
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TL;DR Switch is Shorting on Door Open

Power was connected to another kitchen circuit. A slight pull on the door handle tripped the GFCI outlet.

That points to one of the safety switches in the door frame having a short. Everything is fine until you open it. As you open it, part of the current going through the switch shorts to frame == ground. That will trip a GFCI immediately and a regular breaker pretty quickly. But the switch is OK "in place" so everything operates normally as long as you don't open the door while current is flowing.

As I understand it, microwave ovens route the full "oven on" power (as opposed to control panel power) through these switches so that opening the door is guaranteed to turn it off. If the switch was less direct - e.g., a low-voltage control microswitch that the microcontroller in the control panel would monitor and then it would turn off the main oven power, if everything was working correctly then it would cut off in a tiny fraction of a second and everything would be fine. But if something went wrong - switch, wire or software (firmware) error, then the oven would keep producing microwaves with the door open. So a design relying on physically routing the actual power has fewer points of failure. This also explains why it usually happens but not 100% of the time: if you happen to open the door in a way that the switch doesn't manage to short, or shorts (without being connected to a fast-acting GFCI) but finishes cutting power properly fast enough, then the breaker does not trip.

If you turn off (timer or cancel) power first, then the door switch still shorts, but with no power running through it there is no harmful effect and no GFCI or breaker trip. Just like if you were (not recommended) to put a piece of metal in place of a light bulb but never turned on the light switch.

I do not think it is "switches out of sequence", though a detailed schematic would give a definitive answer. I don't think a design that would allow a short (as opposed to a disconnect) simply because two switches operated out of sequence would be considered OK in a consumer device. There likely are multiple switches (for safety) but I think one is shorting (insulation worn or wire pinched or similar) not an interaction between two switches.

manassehkatz-Moving 2 Codidact
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Well, it was the switches, but...

As I've mentioned elsewhere on this page, the switches tested fine originally. Still, for lack of a better idea, I replaced them. They're inexpensive, so why not eliminate that possibility? Didn't help.

... it's a matter of timing.

Some of the offhand wisdom on this page got me thinking about timing. It turns out that the primary and secondary switches aren't simply redundant. They're required to be opened in a particular sequence (top first) when the door opens. The latches and switch positions are designed to have that happen. That wasn't happening in my case, and a short circuit is the result.

Here's why.

Frigidaire is now owned by a conglomerate, like almost any other appliance company. They all use mostly the same parts manufactured and assembled on another continent from where I bought mine. Standards have incrementally declined, and now the doors on these things are flimsy plastic.

They flex.

And over time, particularly in the case of a range-hood unit where we tend to grab the lower part of the door handle, they twist enough that the lower switch opens first. Therein lies the problem.

Now what?

One YouTube solution has us putting tape on the lower door latch to delay its disengagement with the switch. I may resort to that, but I'll use aluminum foil tape for more precision and durability.

Another YouTube fix is to tilt the switch retainer bar in its mounting range to minutely shift timing. I don't think that'll be adequate in my case, and initial test results weren't encouraging.

For now, until I come up with a proper fix (or wrangle my retailer into taking this junk back), I'll use sticky notes to remind my family to reach higher when opening the door during cooking. By doing so the flex problem is avoided. We can stop making runs to the breaker panel in the basement. I can have my breakfast sandwich and eat it, too. All is right in the world*.

If you're wondering why I don't just buy a new microwave... I despise our modern throwaway culture. It's wasteful. It's harmful to the planet. It's not something I should have to do. So I won't, if I can help it.

* All is not right in the world, but we're trying.

isherwood
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Now I'm no microwizard but couldn't you switch the wires so the top is the bottom and the bottom is the top and fix this?

Scott
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