Outdoor GFI tripping, wires have voltage
I wired a detached greenhouse with one 120V and one 240V circuit, and all wires run though the same conduit. The conduit has one outlet mounted on the side of the house, with a GFI. The outlets inside the GH are protected by the house GFI. The outlets are for lights and a fan. The 240V line is for a heater. Wire size, box load, load/line lugs downstream from the GFI are all proper.
Recently, the GFI started tripping when reset. I disconnected the downstream lugs, and the GFI works properly. When I tried to test resistance with a digital (Fluke) multimeter, I find low AC voltage, around 2VAC between hot and ground, and hot and neutral. With the 240V breaker tripped, there is less voltage, perhaps 0.6VAC.
That weirded me out! I traced the voltage leak all the way back to the breaker. With one lead on the neutral bus, one lead on the breaker script, breaker tripped, I find between 0.6 and 2.3 VAC!
I know GFI's measure the difference in current, but I can't even begin to check resistance in the downstream circuit with the low voltage leak.
Is this a peril of a digital meter?
I can't remove the GFI, but I need power back for the fan!
Anyone have an idea where to begin?