Strange GFCI Behavior

[Al G]

The GFCI that feeds my outside lamp post and outlets has started tripping whenever we get a heavy rain. I've traced the problem to the wiring that goes underground from a jbox on the side of the house to the lamp post. Until I can get around to digging and replacing the line I've been resetting the GFCI regularly when we get heavy rain. In the course of doing that I've noticed something.

The lamp post is controlled by a photo sensor. If I reset the GFCI during the day it will trip again in a few minutes if the ground is still wet. If I reset it after dark, when the light will come on, it stays on until morning when the photo sensor turns off the light. Then it trips. It's done this enough times that I don't think it's just coincidental timing.

Does the load on the circuit at night some how hide the fault the GFCI is detecting?

 

[FordTruckWench]

Does the load on the circuit at night some how hide the fault the GFCI is detecting?

Maybe... Assume you're leaking 1mA from the hot. With no load, there's 1mA on the hot and 0mA on neutral. With a load it could be 1.001A on hot and 1.000 on neutral. The latter could well be below a detection precision. ... However, as I understand it, a GFCI will have two coil windings arranged to cancel the common current, leaving only the difference to be checked.

Other possibility: Your motion light is switching on the neutral leg. And there is a fault that bleeds current to ground "after" the load, but "before" the switch. With the light off, i.e. switch open, the neutral side of the bulb has the hot voltage on it. With the light on, i.e. switch closed, this side of the bulb is connected to neutral, and because neutral and earth are almost the same potential, the leakage current is too low to trip the GFCI. (In other words, there could be current that leaks from hot, through the bulb, and out the fault to earth.)

 

[AntonLargiader]

It could just be an electronic defect in the photo sensor.

 

[Al G]

Thanks for the suggestions. I've confirmed that the fault is somewhere beyond the jbox on the house. I haven't investigated anything in the lampost itself. We have a few raining days coming so I should have an opportunity to check a few more things. Although the fact that it only trips following a heavy rain still makes me suspect the problem is underground.