During recent work we've been upgrading breakers to dual arc-fault / ground fault. Square D QO series (USA).

I've noticed a couple of them tripping pretty frequently when certain appliances are used.


  • Kitchen circuit consistently trips when the Keurig coffee machine completes its cycle

  • A different circuit often trips when a power saw is used; not as certain but it may be when it is powered off

So the question is: are these faulty breakers? Faulty equipment? Something else?

If its bad equipment, OK so be it, I'll replace them, maybe warranty issues. But I'm skeptical they are the problem - eg the coffee machine is pretty new-ish. Neither item had any (evident) problems on the older circuits.

Is there some kind of tester, perhaps, which could tell me why the trips are occurring? Or some other diagnostic method? For one thing I can't tell if this is the arc-detection, ground-fault detection, overcurrent, or some other reason.


  • Are we talking about QO or Homeline re: the DFCIs? Jul 28, 2019 at 16:26
  • @ThreePhaseEel QO Jul 28, 2019 at 17:26
  • Do you have a stopwatch or some other way of timing out a 2 to 5 second interval? Jul 28, 2019 at 17:35
  • @ThreePhaseEel Possibly the clock function on my phone would work well enough... I'm intrigued! Jul 28, 2019 at 18:02

2 Answers 2


Trip diagnostics a-la QO

The QO dual function and combination arc fault breakers use a somewhat peculiar procedure for trip reason readout, as detailed in the installation instructions:

  1. Turn the breaker OFF.
  2. Push and hold the TEST button for the remainder of the procedure
  3. Start a stopwatch at the same time you turn the breaker back ON
  4. Stop the stopwatch when the breaker trips:
    • Immediate trip: ground fault or arc to ground
    • Trip after 2s: arc fault
    • Trip after 5s: no fault or overcurrent/short circuit

Further diagnosis if this doesn't yield anything

If the diagnostic procedure above doesn't yield anything, I would turn the breaker ON with the culprit load plugged in and switched ON (running), but no other loads on the circuit, and then measure the time from turn-ON to breaker trip. A fast trip (within a second or two) would indicate a short-circuit somewhere (bolted fault), while a slow trip (several seconds to minutes) would indicate that the load in question is overloading the circuit.

You can also try plugging the Keurig into a GFCI outlet on a different (non dual function protected) circuit and seeing if it trips the GFCI there; if it does, then it's definitely toast. If you get an arc fault reading for the kitchen circuit, though, I would focus on the wiring as the source of the trouble; you can use classical "divide and conquer" troubleshooting to isolate the faulty wiring run if need be, by the way.

  • 2.23 seconds. Thanks, this gives me a lot of confidence I can talk to the manufacturer of the defective product (coffee machine) and deal with a refund / warranty etc. Aug 17, 2019 at 11:05

GFCIs detect ground faults within appliances. AFCI's detect arcing faults, usually in wiring in the walls, but sometimes in appliances. But the first law of GFCIs is these things only happen to other people. Your appliances don't fail!

Seriously, people really do think that, and will spend hours and hundreds of dollars chasing every problem but that.

Note the part where you said "Neither item had any (evident) problems until I installed the detection devices".

It's probably not overcurrent, because you had that protection before. Now it's time to move each of these loads to a different circuit, and see if the problem moves with the load. Kitchens are supposed to have 2 small-appliance circuits, so that would be the easy one. For others, use 3-prong extension cords.

In particular, don't put GFCI/AFCI protection on refrigerators and freezers. The risk of a shock (or arcing wiring problem) is not as severe as the risk of food spoilage.

I'm a little concerned about the idea of going whole-hog converting every circuit. Though I certainly don't want to see a kitchen appliance with a ground fault, you risk running into challenging problems in existing wiring or hardwired appliances. For instance a mis-tapped neutral could give you hours of bug-chasing!

  • Thank you for the good input & advice, and perspective. We're not converting every circuit deliberately, but if we're doing major work anyway it seems worthwhile at that time (not to mention required in many instances). Aug 17, 2019 at 11:08

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