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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?

  • Seems like an an obvious answer, but is your microwave in a kitchen? – Edwin Mar 9 '15 at 1:33
  • @Edwin, what's the obvious answer? No sarcasm here, I'm genuinely curious. – alt Mar 9 '15 at 11:47
  • In a kitchen. Where's the microwave you're talking about? – Edwin Mar 9 '15 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 '15 at 16:35
  • Is the microwave cavity arcing? – ThreePhaseEel Sep 12 '15 at 19:17
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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 '16 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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