Our wiring methods in England are slightly different of course, but I have to go along with the general consensus that the majority of arcing faults are series, not parallel.
I see evidence of arcing and overheating from wires loose in a terminal, from worn contacts in low-quality switches, etc. -- All series faults.
By contrast, evidence of parallel arcing is quite rare. Most parallel faults I see are where someone drills/nails/screws straight into a cable and the damage is immediately apparent.
The one type of parallel fault that I do see more often is a trapped hot wire behind a fixture, the insulation deformed and cut through to the point that it shorts to the metal box. In most such cases, the direct short just trips a normal breaker or blows the fuse immediately.
I'll concede that in a non-AFCI & non-GFCI circuit such parallel arcing to ground could cause a fire, but it would be interesting to know if any data are available to show whether a sensitive GFCI is any less effective protection in this instance.
Obviously the GFCI wouldn't help against a hot-neutral arc, but again it would be interesting to know if there are any figures to show what proportion of parallel arcs are hot-neutral rather than hot-ground.
A couple of other points strike me from the engineer's report posted by Brendan. It suggests AFCI should be required for bedrooms due to the amount of appliances and cords in use. But with the exception of an electric blanket, surely everything metioned is just as applicable (if not more so) to the rest of the house?
Just one more thing (with apologies to a certain well-known detective!). If we acknowledge that a potential between-outlets fault in NM cable feeding bedroom receptacles needs AFCI protection, then what about other cables in the same wall feeding outlets on the other (non-bedroom) side or just passing through on route to some other part of the house? Shouldn't these be AFCI as well? Just a thought.
I'm all for anything which aids safety, and as someone who has no practical experience of AFCIs I have no axe to grind either way: I'm just trying to look at all the pros and cons with a critical eye.
