shelly

Hi.
one of my customers complains the in the middle of the night their GFI shuts off the power in the house. They are able to restore power by turning off a specific c/b that has connected to it 2 rooms on the first floor and outside lighting. After about 2-3 hours they turn on that c/b and the electricity is is good for 2 or 3 days and than again same story. It could also happen 2 or 3 nights in a roll and than again all is good for a while. According to them there is nothing on in those room or out side than. Twice I have been on site can't find anything wrong. I check with a digital Megger and the ground - neutral is 0.92 meg from the top of each c/b to ground was about 0.41. In the mean time I disconnected the halogen spot light that are outside.
The house has 3 phase electricity. I am out of Ideas HELP
Thanks