I am not an expert on GFCI, but I do know how they work, however I have had a strange problem in my lab with one in particular that keeps nuisance tripping. Here is the scenario, I have one part of the lab which uses a computer and about 8 to 12 low powered devices which not all on at the same time. They are fed from a 20A circuit to the GFCI and (2) standard outlets are supplied by the "LOAD" side of the GFCI for a total of (3) outlets. Equipment has been running for a few years on this setup with no problems until recently I started to get tripping of the GFCI several times a day. Now, here is where it gets weird, I have monitored the power coming into the GFCI and it appears to be good. I have checked for leakage current which is in the range of less than a few mA. The GFCI should not trip until at least 5mA> from what I know. This problem has never occurred at night, only in the middle of the day and not instantly when equipment is used. I can do all the same activity on the equipment in the evening with no issues at all. I check the power feed coming into the lab with an RFI line meter and get around 150mV or less of HF noise which is not bad considering I have seen upwards of 400-500 mV in new building installations. I have unplugged equipment, tried various combinations and cannot seem to isolate this problem. The current draw is maybe 2 amps at best and averages around 600mA to 1.2A. Since GFCI is not effected by current draw I don't think this is the problem. I guess it could be a bad GFCI, it's not to terribly old though. Their is an APC UPS on one of the "LOAD" outlets but it has been running for over 6 months now without issue.
Any recommendations to try before I pull this GFCI and try a new one, or even replace it with a standard outlet?