This had everyone stumped for two days. POCO was called to check the house. Everything was fine. They also checked the neighbors house and found an open neutral.
The neighbors house, 200 feet away, was sending enough voltage through the earth to the new inground pool, with rebar and steel mesh in the apron, to be picked up by the GFI receptacle.
The older GFIs would trip on an imbalance between the black and white as well as anything on the ground.
It was the only time I ever saw a GFI trip when it had no power going to it.
