About 2 years ago during a kitchen update, I replaced a GFCI outlet with a new one, 20-amp (both pass through and the GFCI outlet itself). Branching downstream from it are two more outlets, one on the other side of the sink, and one right behind it on the outside of the house. It's an up-to-date "lock" GFCI, is wired correctly and has always tested and functioned correctly. Last week having a new roof put on, the roofers plugged a pretty heavy-duty looking air compressor into the outside outlet. After about an hour, the GFCI tripped. Now, plugging just about anything into the outside outlet is causing it to trip after a few minutes' use, for example yesterday, my shop vac. I have used the shop vac in that outlet many times before with no problem. So, my question is, did the current draw by the air compressor damage the GFCI (it still tests fine). If so, I guess I better replace it.
I have noticed that GFCIs seem to be worn out by heavy, prolonged loads. I can't find any authoritative sources for this effect. Probably the GFCI manufacturers know all about it.
The bridge circuit which detects the flow imbalance depends on some precision electronics which are heat sensitive. The heavy current itself does not affect them, but the side effect of that current heating up the components will lead to premature failure.
My experience is that when GFI receptacles trip due to overloads rather than a ground fault, their lifespan is diminished greatly. I have replaced dozens of them over the last few years. To be honest, I think they are sensitive to overloads and simply are damaged. Easier to replace them then deal with a lot of nuisance tripping.
OK, here are the results of my test. I ran the shop vac for about 15 minutes directly off the GFCI, no problem. Then I went outside and plugged it into the load outlet -- dead. I think it tripped the second I plugged the vac in, even though I didn't hear the vac start at all. The reason I think this is because using an LED nightlight as a tester, I see the LED illuminate for like a microsecond when I plug it into the load outlet. And there's more -- even after the load outlets go dead, the GFCI outlet itself still has power, only the two load outlets are dead. My conclusion is, it has to be replaced.