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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.

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    Opening the door should cut a safety interlock that cuts the power, something about cutting the power is causing a problem. It could be a defective switch that is shorting to ground, or an overly sensitive breaker that doesn't like the sudden power cut. Try the microwave on a different outlet.
    – Ariel
    Jan 19 at 22:33
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    You might want to stick to the cancel button, then. I recall that one of the anti-tampering measures is to basically blow a (non-servicable) fuse if the door switches operate out of sequence. Sounds like it might be trying to do that, but the breaker is saving it - so far.) The switch sequencing can be affected by age and wear on the parts that engage the switches. If you suceed in blowing the fuse rather than tripping the breaker, it's new oven time (from what I recall of what's been said about this 'feature')
    – Ecnerwal
    Jan 19 at 22:33
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    That's an illogical assumption and completely irrelevant. Every microwave oven is operated this way sometimes. I don't do that regularly, but if, say, my teenager realizes that her chicken nuggets are roasting she will. My elderly father uses his that way almost every time because he's not patient enough to estimate time well or use automatic modes. See also any gas station burrito counter unit. Again, irrelevant.
    – isherwood
    Jan 20 at 16:41
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    @RibaldEddie Microwaves are designed to be opened during operation. It should not cause any premature wear on any of the components. From the very manual in question : "You can stop the oven during a cycle by opening the door. The oven stops heating and the fan turns off. To restart cooking, close the door and Touch START. If you do not want to continue cooking, open the door and touch STOP."
    – J...
    Jan 20 at 16:56
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    He's not wrong, but does it matter? Open and closing an energized contact will have a shorter life span than one that's opened and closed w/o energy. But any MW that can't take that for 20y is junk. "I looked at one [contacter] the other day that was rated for 2 million cycles mechanically and 1 million cycles electrically." - Opened 20x a day, every day, for a 10y, is 73,000 cycles. So with w/e contactor they're talking about it would last 130y opposed to 260y. electriciantalk.com/threads/contactor-lifespan.12093
    – Mazura
    Jan 21 at 6:54

3 Answers 3

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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.

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    To add to this: Microwaves (or other things with doors) placed high up (upper cabinets or range-hood like this one) are more prone to door-hinges sagging over time. Because they are up high people tend to pull down on the door when opening it.
    – Tonny
    Jan 20 at 13:19
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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.

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    Out-of-warranty repair on (almost all) countertop microwaves is a total waste. On a typical built-in hood/microwave the service would be probably half the cost of a new machine where the switches cost relatively little. For a DIY it makes sense and for anyone who has to pay someone else, probably not. I dealt with a similar problem once (fire in microwave correctly burnt thermal fuses, preventing serious damage) and managed to diagnose, order parts and fix it before my Dad's girlfriend returned from vacation...
    – manassehkatz-Moving 2 Codidact
    Jan 20 at 17:25
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    Also, as you probably know but others might not, major appliances traditionally include a basic schematic, with key part #s, inside the case. So once you find the (sometimes cleverly hidden) screws to remove the front panel you may find useful information about what kind of switches you need to get. Or Google...
    – manassehkatz-Moving 2 Codidact
    Jan 20 at 17:28
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    Microwave oven door switches are a marvel of clever safety engineering, when working correctly they have one set of contacts that connects power to the magnetron power supply (That close ONLY when the door is fully closed), and a second set that short circuit the connection to the magnetron supply (That close when the door is less then fully closed). The second set deal with faults in the first set like say a welded set of contacts or such. They almost always fail safe, as your example has.
    – Dan Mills
    Jan 20 at 22:46
  • Wow. Now I'm at a loss as to what it could be. The only other thing I can think of is if something is getting pinched when you open the door, but that makes no sense - no wires should be running through the door or the hinge.
    – manassehkatz-Moving 2 Codidact
    Jan 24 at 19:45
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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.

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    Wow! As far as not just buying a new microwave: I have gone through only a few in the last 30 years (can't remember now if this is the 2nd or the 3rd) - the previous one had the turntable stop after several years, so not always the most even heating, but kept it going many more years until it truly quit. Current one (now several years old) had some crazy sparking after a few years and my first inclination was "OMG! Its unsafe/going to really have big problems" but it turned out to be just the mica waveguide cover gone bad which was a super-easy (and cheap) fix that I had never heard of before.
    – manassehkatz-Moving 2 Codidact
    Jan 26 at 19:05
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    Great diagnosis. Terrible design ... you could try telling the manufacturer that if they send you a new and better model you will be more inclined to forget about it than to look for a class action lawyer. Terrible handle/door design and terrible failsafe design if your circuit breaker is an unwilling part of it.
    – jay613
    Jan 26 at 19:54
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    They flex. Reminds me of the Young Sheldon episode where he was the only one to detect that his bread changed flavor and it finally turned out to be a corporation that changed the ingredients slightly to save money. Change of material in manufacturing. I wouldn't have thought of that. Thanks for sharing.
    – IdahoFamilyMan
    Jan 26 at 21:07
  • To delay the opening of the bottom of the door until the top has moved a little, you might add a magnetic door catch to the bottom of the door, mounted outside the door of course, and using the small steel plate that comes with the catch. If that's too ugly, you might find a spot near the bottom of the door where two rare earth magnets might hide in the clearance between the frame and door. My built-in has such a void that's about 1/2" tall, 1" wide and 1/2" deep with the door fully closed.
    – MTA
    Jan 26 at 21:25

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