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I had a weird electrical problem and found the fix, but I'd love to understand the mechanism by which this happened. Disclaimer: I'm not an electrician, buyer beware, etc.

The symptom I had was 3 separate Siemens AFCI breakers tripping, seeming randomly. I spent a lot of time puzzling over this, and eventually noticed that something weird was happening with the 240v heater (smart line voltage thermostat) that was also fed from the same sub-panel. For context: all these breakers are in a sub-panel fed from the main panel. The sub-panel and main panel are standard two leg 240v where you get 120v to ground from each, and 240v between the legs.

Checking the voltage at the heater, I observed 11 volts across the hot legs, where I would expect 240v. (but 120v from each leg to ground!) I chased the 11v up the chain, and discovered that the breaker that feeds the sub-panel was somehow "half" on, such that (I think) it was only electrifying one leg for the entire sub-panel. That in turn caused a 240v wall heater that was fed from the sub-panel to go haywire -- it would start up to deliver heat, die, and repeat.

Other 120v circuits in the sub-panel were not affected. Amusingly, there's also a dryer fed from the sub-panel and it spun happily but didn't dry clothes. (I didn't notice the wet clothes until after all this troubleshooting) Apparently the motor runs off a 120v leg but the heating coil is 240v. (not 11v!)

Anyway, I pushed the sub-panel feeder breaker to "fully on", and everything was fixed. And I lol'd. But it would be interesting to better understand the mechanism by which this all happened. I have two theories that I put in answers.

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    Should break this up into a question and then you can answer your question on how you fixed it, in the answer box below. People look for answers for their problems. Questions won't be looked at by themselves without answers.
    – crip659
    Oct 16, 2023 at 11:04
  • edited to separate my two answers from the question in response to crip659's comment
    – FixStuff
    Oct 16, 2023 at 15:39
  • You could test "theory 2" (AFCIs sensitive to voltage fluctuations) by disconnecting one leg from the subpanel and waiting a while for routine usage to occur. Only if you know how to properly torque it back. If the AFCIs trip, this lends weight to this theory and eliminates the other one (arcing in faulted or faulty breaker). But if the fault is now cleared, there's not much incentive to better understand AFCI behavior in situations where there is a broader and more serious fault.
    – jay613
    Oct 16, 2023 at 16:17

3 Answers 3

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AFCI breakers are designed to trip when they sense arcing on the wires. Arcing generates a lot of extra high frequency electrical signals on the wires that are arcing. The AFCI listens for these signals and trips.

Another theory: the feeder breaker that was "half" on was almost connected and allowing arcing.

AFCIs can sense arcing further up the supply, too. So they may have heard the arcing in the funky breaker and tripped.

The feeder breaker is supposed to be a handle-tied common-trip breaker, and it isn't supposed to be "half" on. Maybe that breaker is bad and needs to be replaced.

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  • The feeder breaker was (is) handle tied, totally agree it shouldn't be capable of being half on. If it happens again, I'll definitely replace the breaker.
    – FixStuff
    Oct 16, 2023 at 16:14
  • That's interesting that AFCIs can detect arcs higher up, but wouldn't that roughly equally effect all the AFCIs in the sub-panel? Edit: Oh I see, maybe it would only affect the AFCIs on the loose leg.
    – FixStuff
    Oct 16, 2023 at 16:17
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OP theory 1: the haywire 240v heater running on 120v created some sort of surge/emp in the heater feeder wire that in turn "infected" other (truly 120v) wires that were bundled in the wall with heater feeder wire, causing their AFCIs to trip.

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OP theory 2: the 120v circuits whose AFCIs were tripping were on the 120v leg that wasn't getting power. (but was accidentally getting power somehow, maybe through some 240v appliance) Depending on load, the resistance of the accidental backup path would be too much, and cause an overload condition from the pov of the AFCI breaker.

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