Given the assumption that AFCI's do work, isn't one of main reasons it is required to protect the entire branch circuit because of vulnerabilities in NM cable? After all they have allowed metallic wiring methods then AFCI at first outlet.
If reasoning is because of vulnerability of NM cable then why not AFCI requirement for all NM cable regardless of circuit voltage/current rating?
Also in the furnace application, about the only thing that is much risk for electrical fire (presuming good installation and maintenance) is more likely in the supply circuit, the furnace itself would probably contain anything that occurred within.
If anything this is another case where it would likely contain anything long enough for GFPE to catch it once it starts producing fault current.
I don't believe such an appliance is all that likely to start a fire as a result of arcing, not that arcing can't occur, but will likely be contained enough it won't spread before it either develops a ground fault and trips OCPD or burns itself out. At 120 volts to ground it probably burns itself out most cases.
I have seen 277 volts to ground continue to arc many times, can't recall ever seeing 120 to ground not clearing itself, yet they push the code to use these products on 120 volt circuits:ashamed:
