There is no length restriction; the breakers are designed to protect the cable, which fails thermally. The instant trip is there to kill the circuit immediately if a short circuit is detected (and may not ever trip if the cable impedance is too high) but thermal trip should always work regardless of the fault circuit and protect the cables. If the breaker doesn't trip (and isn't malfunctioning), it's because it didn't see levels of fault current that would pose a danger to the building cabling, merely elevated fault current. It's not smart enough to know in-rush from a cheap compressor from a kid with a fork in the socket; AFCIs aren't even that smart. It just looks at current and trips if there's too much for too long.
On better breakers, all the trip settings are calibrated and manually configrable, so it's just a matter of taking system impedance into account when dialing in the trip settings.
[This message has been edited by SteveFehr (edited 11-01-2006).]
[This message has been edited by SteveFehr (edited 11-01-2006).]
