Problems like this are where a good ohmmeter is valuable. I had a similar GFCI problem where it would trip immediately whenever the light was switched on. I disconnected the "hot" supply wire at the panel, and got very low impedance...0.7 ohms. The 300W bulb is about 4 ohms, so that's what it should have read. I removed the underwater fixture, disassembled, took out the bulb, and the fixture and wire, unattached to anything, gave me 36k ohms from the hot to ground. GFCI will trip with any leakage below 48k ohms. I found burn marks on the cable just behind the fixture. I'm guessing the installing electrician test ran the light before the pool was filled and it scorched the sheathing at several spots where the "extension loop" was wrapped around the fixture. I now have to get a new fixture / cable. This time, I'll use a LED bulb.

"FYI the gfci doesn't really check ground fault in the sense some people on here think. A gf I actually checks current leak." Do you have a link to some sort of documentation for this voodoo theory? If that were the case, any time one GFCI in your house tripped because of a current leak, they ALL would trip. If the difference between the current leaving and returning THROUGH the current transformer of the GFCI exceeds 5 mA, the solid-state circuitry opens the switching contacts and de-energizes the circuit. http://m.ecmweb.com/content/how-gfcis-work The GFCI measures the DIFFERENCE in current between the hot and neutral that goes THROUGH the current transformer in the breaker. It can not sense a current that does not go through the current transformer. The GFCI may not reset if the main breaker is not turned back on first, or the LINE side neutral wire is not connected to the neutral bar yet, but it won't sense current leakage on a different circuit. If that were the case, how far away will it sense this "other" leakage? 100 feet? Your neighbor's house? The leakage in all the power transmission lines? Try this experiment, take the line wires off your doorbell transformer, and just wrap them around the transformer instead of hooking them up, and tell me if the doorbell still work ;) Or take your fluke and try to test for current without putting the wire INSIDE the current transformer. It is more likely that you just got unlucky with the new breaker being bad after the first one gave out.

first, the fact that you replaced an outlet that was on an apparently unknown circuit underscores the importance of mapping all of the circuits in your house and always testing hot and neutral before touching them. if it had been a faulty outlet and you killed the power to the wrong circuit, it would have not been pleasant when you tried to replace the outlet. the first thing that came to mind with your description is that the one functioning outlet is the last in the series and the wires on the "load" side (so to speak, even though it isn't GFCI) somehow disconnected or the outlet itself is bad. that means everything on the other side of it won't work. just a thought. please don't touch any wires where there is a remote possibility that they are live. always test first.

Check for damage to insulation of the conductors leading to the recept. Such damage may be caused by chewing rodents. I've not witnessed it myself, but I've seen photos of thoroughly dead mice bridging the lugs in a panel.