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I live in a recently redone home and I'm having a very odd electrical issue. My microwave is intermittently tripping a combination arc fault breaker only when it is mostly empty, e.g. one slice of pizza. When the microwave is fuller, it works fine. The breaker does not seem to be the problem: I've moved the microwave to a different circuit and had the same results. I have a new Panasonic Inverter microwave; my old microwave exhibited this same problem. No other appliance in this home has caused any trips.

Breakers involved are Siemens QAF2 CAFCI.

Any ideas?

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  • Seems like an an obvious answer, but is your microwave in a kitchen?
    – Edwin
    Mar 9, 2015 at 1:33
  • @Edwin, what's the obvious answer? No sarcasm here, I'm genuinely curious.
    – alt
    Mar 9, 2015 at 11:47
  • In a kitchen. Where's the microwave you're talking about?
    – Edwin
    Mar 9, 2015 at 20:15
  • Yeah, it's in a kitchen. No other kitchen appliances are on that circuit though - the stove and refrigerator are on separate dedicated circuits.
    – Francis
    Mar 12, 2015 at 16:35
  • Is the microwave cavity arcing?
    – ThreePhaseEel
    Sep 12, 2015 at 19:17

5 Answers 5

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Most microwave ovens will generate arcs and sparks inside their cavity if they are run with an insufficient load, or no load whatsoever. The high-frequency content from these arcs is being coupled (likely capacitively) through the HV transformer section of your microwave onto the AC line, where the AFCI can then see it and proceed to freak out, thinking the arcing is actually an AC mains arc, not a microwave cavity arc.

Suggestion: throw a coffee mug of water in with the slice of pizza. This does the same thing as Keshlam's sacrificial bread slice -- i.e. provides more load to absorb the microwaves instead of letting them bounce around the cavity until they arc, just without having to waste a slice of bread.

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    Exactly. The breaker trips because the oven is getting damaged by arcing. This is correct behavior. Running microwave underloaded is incorrect behavior.
    – Agent_L
    Feb 10, 2016 at 17:22
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Some older microwave ovens really need at least some minimal load to stabilize them. Mine makes very unhappy noises if I accidentally hit start with nothing in it. I don't know the circuit well enough to venture a guess about why, but I've learned not to try anything smaller than a slice of bread. If I had to, I'd add a sacrificial slice just to make sure.

It's possible that yours has a similar limitation.

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Seem like the afci breaker you have right now affected by the microwave frequency ,try to replace the afci breaker.

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Had exactly this problem - same microwave, same breaker. The eventual cure was to replace the Siemens breaker with this Eaton one: BRCAF120

Did the same on another circuit which tripped each time I turned the kettle on, and that issue was resolved too.

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  • You shouldn't use a mismatched breaker for your type of panel; however, you should be able to get a workable version of the same guts by ordering a CLCAF120 instead
    – ThreePhaseEel
    May 23, 2020 at 23:32
  • @ThreePhaseEel Can you help me answer a similar question diy.stackexchange.com/questions/215982/…
    – Arian F
    Feb 7, 2021 at 21:18
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I had the same problem; one of these solved it

https://www.tripplite.com/isobar-2-outlet-surge-protector-direct-plug-in-1410-joules-3-leds-black-housing-5-in-height~ULTRABLOK

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  • Hello, and welcome to Home Improvement. Whilst this may theoretically answer the question, it would be preferable to include the essential parts of the answer here, and provide the link for reference.
    – Daniel Griscom
    May 24, 2020 at 0:22

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