When I plug a light (in a metal enclosure) into an outdoor GFI, the GFI trips.
This happens on two different outdoor GFIs that are near to each other.
My test meter detects no connection between the light's casing and it's hot or neutral wires, even on the meter's most sensitive setting.
The light fixture's ground wire has no detectable resistance to the fixture's casing.
However, if I insulate the light fixture from the ground (by putting it on a table), the GFIs don't trip and the light works.
If I put the light fixture in the kitchen sink (which is grounded), then a different GFI does not trip and the light fixture works.
Why do the two outdoor units trip?
Best Answer
It's an open-and-shut case of a ground fault
All your testing (two GFCIs, isolating appliance from earth etc.) confirms the appliance has a ground fault.
Your tester doesn't detect it because your tester is not a megaohmmeter. The particular failure mode you have here is insulation breakdown; it's fine at the 3 volt meter test voltage, not so good at 120/230V line voltage and will certainly fail at the megaohmmeter's 200V/500V test voltage.
Just dismantle the light and clean everything and look for any obvious problems.