Q. Could the breaker be designed to sense a single arcing conductor? Would it still be called an Arc-Fault Circuit Interrupter?
A. The Branch/Feeder AFCI responds to "series arcs" in NM-B. The test is in UL 1699. In practice, manufacturers utilize the fact that NM-B contains a bare ground conductor. Any break in a single conductor causes current leakage to this ground wire, and the AFCI opens the circuit prior to the arc burning through the NM-B insulation. Since leakage to ground is the response criterion, downstream switching arcs do not cause nuisance tripping. Again it is noted that a series arc in NM-B is highly unlikely.
UL1699 contains an AFCI "Combination" category. This category includes series tests in SPT-2 as well as NM-B. The series test demands that the arc interrupt after 200 to 1000ms (12 to 60 cycles) depending on the current (15A to 5A). In particular this time interval allows discrimination between "operational arcs" (e.g. switching) and "unsafe arcs". There are no commercially available Combination AFCIs.
By Combo AFCI's is dual AFCI/GFCI function implied?
Does this mean Ul1699 lists AFCI's for NM w/ground only? and older circuits would not be listed for use?
