Background:
I had a contractor finishing my basement, and part of that was installing underfloor heating using several heating mats in parallel. It worked initially but soon after it just started tripping on a ground fault (built in to thermostat). I think my contractor is a little out of his element so I took some time to try diagnose the issue.
There are no obvious issues in terms of element resistance or total current draw, so I started to check for ground leakage. I don't have a megger and so I used my DMM to check resistance to ground for each element, nothing measurable.
Here is my question:
As a more detailed check for leakage to ground I used my DMM to measure the current. For each mat I DISCONNECTED the neutral side, connected one lead of my DMM to the hot side of an element and then touched the other lead to a hot wire (not from thermostat). So in theory this should just measure current leaking from the element to ground, right?
I ended up measuring between 4-8 ma for each element. Seems a little high if this was correct.
Best Answer
GFCI's are set to trip between 4-6ma of leakage current. This is a pretty sensitive setting to measure.
It is possible the GFCI in the thermostat has gone bad. Like all electronics, it is prone to failure at some point.
I would try a new thermostat to see if it holds. If not, you need to investigate the wiring further.
Good luck!