AFCI breaker trips on the second start of table/miter saw

Question

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?

Answer 1

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.