Arc Faults:

There are many homes dating back to the 1920's that still have knob-and-tube that has outlasted subsequent wiring technology, such as BX with rubber insulated conductors and early forms of romex. Do AFCI breakers protect existing knob-and-tube wiring systems?

An AFCI circuit breaker will trip and clear the circuit when a line-to-neutral arc occurs (often caused by the melting of the conductor insulation at loose terminals) within three to eight half-cycles, whereas a standard circuit breaker might not open for many hundreds of half-cycles. Note: If the AFCI is dual listed as a GFCI, the two wire receptacle can be replaced with three-wire receptacles and no equipment grounding conductor is required to be run to the receptacles [406.3(D)(3)].

I’ve heard that AFCI devices won’t protect against fires from 2-wire NM cable. Is this true?

False. AFCI’s have superior performance protection over a standard circuit breaker when it comes to low- level line-to-neutral faults. In an old house I would surely install them, even though they are not required by the NEC.