how does an AFCI actually tell a bad arc from a light switch?

ok so last year in class we covered AFCIs and the teacher said they detect "dangerous arcing." but every time you flip a switch or plug something in there's a little arc, and a brushed motor like a drill arcs constantly. so how does the breaker know which arc is the bad one? is it just guessing off some pattern? because from what I hear on here they trip on random stuff and miss other stuff, so I'm skeptical. also the teacher couldn't really answer it so here I am

2 replies

benplumb79

Apprentice plumber · 2 years · Somerset County, NJ

Plumbing guy, but that was a better explanation than I've gotten from two years of listening to electricians complain about them. Thanks for asking it.

7 helpful · Jul 28

codebook_dale

Electrical inspector (former contractor) · 26 years · Ocean County, NJ

Mentor

Good question, and the skepticism is reasonable. In plain terms: the breaker has electronics that watch the current waveform for the signature of a dangerous arc, the erratic high-frequency noise and the pattern a loose or damaged conductor makes over time, as opposed to the brief, predictable arc of a switch or the steady noise of a motor. Combination types look for arcing in series, like a loose connection, and from line to neutral or ground, like damaged insulation. Many also sense ground faults. It is pattern recognition, so it isn't perfect. Some motors and electronics look suspicious to some models. That's part of why the designs keep getting updated.

2 helpful · Jul 27