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?