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I'm trying to diagnose an issue that has cropped up in the last few days regarding a nuisance tripping breaker.

For background, I've lived in this apartment for 6 months now, with no notable changes to what's plugged in where that I can think of. This is a 2016 built building.

However, over the past three days I've had my breaker trip once every evening, usually when turning on a light switch on a completely different breaker. Here's a pic of the breaker. The manual I pulled up for this series of breakers suggests that an LED can indicate the last fault cause, but there's no illumination from the box, and instead I always get this orange trip flag:

Breaker box with a gray 20A GE breaker in the tripped position, with an orange? trip flag

Now, we have a couple of clues here. This appears to be an AFCI, or maybe DFCI, breaker. The fact that this is tripping when a light switch is flipped on another breaker makes me think the neutrals are connected to each other somewhere. There's a lot of interesting stuff on the circuit that trips, like a server rack with a UPS and some LED lights. However, none of the equipment there has changed recently, and if I pull my UPS logs, I'm drawing about 400W at full load, or about 3A. There's other things not going through the UPS, like some monitors and laptop chargers, but this is a 20A breaker and should have plenty of overhead to draw multiple times that and still sit under the 16A-at-load rule of thumb.

At first, I was content to blame the fact that I have an awful CFL + dimmer switch setup installed in the living room (not by my choice), and the fact I had been using the dimmer recently. My reading into AFCI suggests that CFLs, dimmers, and crossed neutrals can have an effect like this.

However, today the breaker was tripped with these lights on max, which is a condition I've had them in for the last several months.

I'm scratching my head at this one, and since we're in shelter-in-place out in California, I'm puzzling over the next steps before I call my landlord.

Some questions overall: 1. Any guesses about why this would happen when turning on lights in the other room, on a different circuit? Is my AFCI + combined neutrals theory correct? 2. Any guesses about why this would happen now, but wouldn't have for the last 6 months? 3. Is the breaker indicating any sort of information about the fault cause, or am I left guessing whether this is AFCI, GFCI, or overload caused?

Appreciate any suggestions you may have!

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    That's not a manual, that's a sales brochure. Try this. (what an interesting creature BTW). Well, we can cross off "shared/crossed neutral". This breaker is designed to thrive in a shared neutral situation. – Harper - Reinstate Monica Apr 17 at 4:56
  • Appreciate you digging that up. It doesn't give much info about the trip indicator, though. I'm not sure I understand the conclusion that it isn't a shared neutral. That linked manual says that MOD 3 is identified by having a gray case + black switch, and MOD 3 can be used on a shared neutral. – Jaxbot Apr 17 at 4:58
  • Yes, your breaker is a MOD 3 type -- it says so in the upside-down line of text that's currently at the top of the breaker case front – ThreePhaseEel Apr 17 at 11:43
  • After reading the info from harpers link the neutral should not be the problem. You have a fair load for cfl/ LED of 4 amps . Having the dimmer on max really won’t help with most dimmers because although the dimmer is trying to output a true sine wave a harmonic is still generated at zero crossing if it is a Triac and if it is n SCR it turns off 1/2 cycle no matter what. Even without a dimmer I have seen AFCI’s have trouble with lights that use ballasts (cfl, and LED lamps ) 4 amps is not a lot but on a circuit with electronics that would be enough when adding another lamp to cause the issue – Ed Beal Apr 17 at 13:40
  • Might be a loose connection inside the breaker box got worse and some neutral or ground line is not quite doing its job any more. – Carl Witthoft Apr 17 at 15:30

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