Main Breaker Tripping But NOT GFCI

[phonedave]

And I believe you were right when you pointed out I was wrong about the GFCI's self-testing (we were talking about the 50A ones near the hot tub). Just sayin

I honestly never looked that deeply into what this auto monitoring entails.

I thought it was just some other monitoring circuit would trip the GFCI if it detected anomalies. I had no idea that it flashed different lights and ran every 15 minutes, and would shut down a GFCI at a planned EOL, etc.