Page 337 cites UL testing, where arc resistance was found to be only 30 milliohms:
https://www.nfpa.org/assets/files/AboutTheCodes/70/70_A2022_NEC_P02_FD_PIReport_1of2_rev.pdf
(You may need to sign in to view, its free)
Anyway, here is the value and equation as dervied from UL:
The assumption was made (key word) that the minimum available fault current in a residential US panelboard would be 500 amps, and as such a breaker with a 300 amp magnetic pickup may provide parallel arc fault coverage for 50 feet or 2.08mm2 wire and 70 feet of 3.31mm2 wire.
Regarding the UK Ze is known in addition to R1+R2, so the full circuit already has parallel arc fault coverage.
Reason why I'm posting this is to further supplement my warning to the UK: AFDDs provide no more protection than existing MCBs, RCDs and plug tops fuses.
AFCIs were created for us merely to compensate for the lack of loop impedance.
https://www.nfpa.org/assets/files/AboutTheCodes/70/70_A2022_NEC_P02_FD_PIReport_1of2_rev.pdf
(You may need to sign in to view, its free)
Anyway, here is the value and equation as dervied from UL:
The assumption was made (key word) that the minimum available fault current in a residential US panelboard would be 500 amps, and as such a breaker with a 300 amp magnetic pickup may provide parallel arc fault coverage for 50 feet or 2.08mm2 wire and 70 feet of 3.31mm2 wire.
Regarding the UK Ze is known in addition to R1+R2, so the full circuit already has parallel arc fault coverage.
Reason why I'm posting this is to further supplement my warning to the UK: AFDDs provide no more protection than existing MCBs, RCDs and plug tops fuses.
AFCIs were created for us merely to compensate for the lack of loop impedance.
