They may sell one to a Joe Homeowner, but that's about it.
The waveform analysis compares the arc signature (the shape of the waveform anomaly to the hundreds or thousands of arcs that we've mapped in the laboratory. Basically, we analyze the curve and see if it is a non-operational arc...that is, an arc that shouldn't be there. I will paste below a short dissertation from one of our senior engineers on the parallel/series issue. I hope this answers more questions than it generates. :-)
Q. Why was the AFCI designed to sense a short-circuit across two conductors, and not a single open arcing conductor?
A. Arcs between conductors are sporadic (pulsing)in nature. For dwelling unit circuits, the rms available current at an outlet prior to a fault is typically 75A or higher. The arc voltage further reduces the circuit current and, in combination with the intermittent current waveform, the fault current is unlikely to trip the circuit breaker instantaneously (trip level about 200A rms) and possibly not even thermally. The safety enhancement of the AFCI is to recognize the presence of these high energy arcs, and to cause circuit interruption (trip the breaker) far more rapidly than would be the case with a conventional circuit breaker. The UL1699 tests are performed with both NM-B and two wire SPT-2. The focus has been on high current "parallel" arcs rather than low current (load limited) "series" arcs because:
(1) they are higher energy arcing/fire sources
(2) they are far more likely to occur (it is very difficult to maintain a low current series arc between copper wires at 120V)
(3) it is possible to detect parallel arcs without nuisance tripping. Here it is noted that low current safe arcs are an everyday occurrence in dwelling units (switches, thermostat controls, electric motors etc.). Further, many low current electronic-appliances have chopped waveforms (drills, ballasts, power supplies etc.). Thus low current unsafe arcs (at a single open conductor) have to be distinguished from low current safe arcs, and other naturally occurring waveforms, without causing nuisance tripping.
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.
Q. What was the reason the NEC decided to require AFCI in bedrooms only?
A. NFPA fire statistics show that a high percentage of electrical fires occur in bedrooms. There are all manner of appliance cords in bedrooms (for example, radios, clocks, blankets, air conditioners, heaters, TVs, vacuums) and also lamp cords. All of these cords can be trapped/abused leading to line to neutral arcs. Further, there are long runs of installed wiring (NM-B) between the loadcenter and the bedroom outlets. These runs can abused during installation (e.g. stapling) and after installation (driving nails into the wall etc.).
Summary
The present Branch/Feeder AFCIs provide a significant enhancement in fire safety. They guard against parallel arcing faults in both the installed wiring (NM-B) and the two wire extension wiring (SPT-2). In the unlikely event of a series arc at a break in NM-B, the arc will be interrupted by the AFCI responding to ground current. The overall device is immune to nuisance tripping.
