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I recently purchased a new table saw and miter saw and they are tripping my AFCI breakers. Curiously, each saw starts up and runs fine the first time I try it and I can stop it without tripping the breaker. If I attempt to restart the saw shortly after stopping it, the breaker trips. I haven't tried waiting a longer period of time yet.

The breaker is a Siemens AFCI and the LED on the breaker confirms that it is an AFCI fault (and not a GFCI fault) that is causing it to trip.

Another possible piece to the puzzle: the saws are in the garage which is super cold at the moment (-30 degrees celcius).

Two questions:

  1. Why does it seem to only trip the breaker on the second attempt?

    • I'm curious if the breaker has some kind of memory from the previous startup load that pushes it over a threshold and causes it trip on the second start, or if it's something specific to the second start of the saw motor.
  2. Is there any kind of "filter" I can put between the saw and the outlet to avoid tripping the AFCI?

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    Does this answer your question? What is causing a motor to trip my GCFI on every OTHER start? Feb 12 at 15:03
  • Thanks for the link @CarlWitthoft. It definitely sounds similar but in that instance it appears to be GCFI fault and not an AFCI fault. Do you think the underlying cause would be the same? Feb 12 at 15:30
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    @RobMacEachern, a lot of AFCI breakers have a weak GFCI functionality built in -- that's the easiest way to detect arcs to ground. So the underlying cause could well be the same.
    – Nate S.
    Feb 12 at 16:46
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    As for filters, yes, there are some that might work. See this answer for one option recommended by Siemens: diy.stackexchange.com/a/216049/91556
    – Nate S.
    Feb 12 at 16:56
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    "What kind of woodworking are you doing at -30°C???" Haha not a lot! Just trying to set up the new saws in the middle of cold snap and cut a few boards. I was gluing in the basement :) Feb 17 at 15:06
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Arc Fault detection is done in the breaker with an electronic board and a set of sensors feeding it, connected to a trip relay. In the electronics, there is a microprocessor that is looking at the current sensor data using an "Arc Signature Filter" algorithm. Every mfr of AFCI breakers and receptacles has their own version of this Arc Signature Filter, it is their "secret sauce" so to speak. The concept behind the Arc Signature Filter is to look at the current and discriminate "normal" patterns of arcing that might come from certain equipment, from abnormal patterns. Those patterns have characteristics of amplitude, frequency and periodic discontinuities that set them apart from what might otherwise be considered normal.

The type of motors used for small appliances and power tools, called "Universal Motors" that have brushes in them, are the very thing an AFCI Arc Signature Filter is looking for. But in reality, it's a matter of how extensively the mfr investigated it with regard to ALL of the different types of appliances and tools that use them.

So now comes the Miter Saw. The big difference here is that, unlike your blender, vacuum cleaner or hand drill that all use Universal Motors, the Miter Saw adds another element to it, called "Plug Braking" to make the dangerous saw blade stop quickly when you release the trigger. It does this by briefly reversing the motor to make it stop. So again, being a brushed motor, that means arcing of those brushes when you STOP, not just when you run. So my suspicion is that this comes into play with the "periodic discontinuities" aspect of the Arc Signature Filter. The fact that you turn it off, then turn it on again, adds up to something that the filter is interpreting as an arcing fault. You could confirm this by waiting a while before turning it on again.

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  • Thanks for the answer. I had the electrician come out and swap the breaker for another Siemens AFCI/GFCI. I've been able to use the table saw reliably since it was swapped, but now the miter saw trips the AFCI even on the first run... so it looks like Siemens still has some work to do on their secret sauce, at least in terms of supporting power tools. Feb 17 at 15:12

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