Most common problem I've come across when retrofitting ACFI's in is that the bare ground wire is touching a neutral terminal on a receptacle someplace... You might have a bit of troubleshooting... One way to tell is to 1) unplug everything connected to the problem circuit, turn off all light fixtures. 2) install a regular breaker for the same circuit
3) disconnect the neutral from the panel, cap it off & leave the ground connected 4) restore power & use a plug in tester on the problem circuit... If it doesn't show "Open Neutral" then you have a ground & neutral in contact someplace.. Or possibly, as Samster pointed out, a box where a neutral from another circuit leading to the panel & is tied together (paralleling) back to the panel neutral bar.. (typically would be found in 2 gang switch/recept locations where the lighting & recept circuits are separate)
Hope this helps [Linked Image]

-Randy

[This message has been edited by Lostazhell (edited 05-01-2004).]