Why would a GFCI fail in one outlet, but not another?
Shouldn't this be physically impossible? Maybe in theory, but let's come up with some real-world hypotheses.
There is a phenomenon of GFCI &quot;nuisance tripping&quot; when the current flow is noisy, or contains higher frequency components. I believe it's because the differential transformer that senses imbalance in the line and neutral currents, is not so perfect at cancelling our those two currents at frequencies above 60 Hz.
So, first hypothesis: loose or corroded connection in the line or neutral wire feeding this outlet. The simple outlet tester or basic voltmeter, would not detect this, so you may have to just repeat your experiment, remembering that the fault could be in this outlet, or anywhere upstream toward the distribution box. Power lines are often &quot;daisy chained&quot; through several outlet boxes. Identify all outlets on this circuit and repeat your experiment on each. That might help determine where the poor connection lies.
The other hypothesis, less likely, is that there is another appliance on this circuit, like a motor with sparking brushes, causing noise on the line. Again, knowing what this branch feeds, you should be able to disconnect any possible culprits and repeat the experiment.
I'm interested to hear what you find.