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I have 4 GE AFCIs in a GE subpanel in my basement. Occasionally, when I turn on a bank of dimmable CFL lights via a CFL-rated dimmer switch, I get a trip in a particular AFCI breaker that's on the other phase. This doesn't happen every time, and it seems to happen more when the dimmer isn't set to full brightness. Resetting the breaker gets the power back on with no issues, but I'm tired of this.

I'm at a loss as to why a different circuit on a different phase is tripping, and even more confused why it happens only some of the time. These are not MWBCs, but obviously share the neutral in the panel feeder. Nothing else causes any other trips anywhere else in the house that aren't obvious, like 4 hair driers on a 15A circuit.

What's going on here? How can I find the cause of this?

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Some breakers will inform you whether they tripped due to an arc fault, ground fault, short circuit, or overload. Do yours have this feature? Is the switch in question in close proximity to the service panel? AFCI breakers may be susceptible to nuisance tripping due to radio frequency and/or electromagnetic interference. –  Tester101 Feb 6 '13 at 22:25
    
No, it doesn't appear to give that information, only that it tripped. The switch is about 10 feet from the panel. –  Jeff Shattock Feb 7 '13 at 0:35
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It is possible that there is a shared neutral wire between the circuits. how long has the wiring been in there? –  codydog Feb 7 '13 at 2:00
    
It's been in for about 18 months, but I don't remember when this started happening. I certainly don't remember messing up the neutrals, and the inspector didn't say anything about that, but that doesn't mean it didn't happen. –  Jeff Shattock Feb 8 '13 at 23:22
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2 Answers

The dimmers are spewing radio-frequency hash into the power line, which confuses the AFCI into a false trip. It makes sense that it's more reproducible on an intermediate dimmer setting, because that's when the dimmer chops the line voltage aggressively, not when it's fully on or fully off.

Bypass capacitors could help with this: 1 to 10 nF non-polar capacitor (ceramic or film, not electrolytic or tantalum!) rated to withstand 500 volts or more. This diagrams shows three caps: between hot and neutral, hot and ground, and between neutral and ground.

The basic idea is that 60 Hz AC will not pass through these capacitors because the frequency is too low, but radio frequencies pass through easily. So the capacitors provide a short circuit shunt path for high frequencies, reducing how much of it spews back upstream from the dimmer.

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This looks simple, and it is, but it has to be done properly. Capacitors have long, uninsulated terminals, creating opportunities for short circuits if you're not careful.

Instead of these ad-hoc capacitors, you can install an EMI filter upstream from the light dimmer. EMI filters have a more sophisticated circuit which includes inductors.

One more thing: the Lutron dimmer company sells a filtering component that they call a lamp debuzzing coil.

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This is an interesting answer but installing ad-hoc capacitors in this manner has to be against electrical codes. The products you mentioned are definitely relevant though. –  Steven Feb 26 '13 at 3:42
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Look at this circuit as theory. Find listed devices doing about the same thing like the EMI filters. –  Skaperen Feb 27 '13 at 14:51
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Have you tried swapping the breakers around? If the problem follows the breaker, then you know the breaker is the problem. If the problem stays with the branch circuit, then you know the circuit is the problem.

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Sorry to have abandoned this for so long. I did try this, no dice. The dimmer circuit still trips. –  Jeff Shattock Jun 19 '13 at 22:06
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