A few weeks ago, the 20A breaker unexpectedly tripped after the jets had been running about an hour or so. After resetting the breaker, it would trip again a few seconds after the jets were turned on. I didn't run it much after that except for a few 10-minute cleaning cycles. Last weekend, it ran for 50 minutes then tripped again without warning. This time, I reset the breaker and all was well until I started the jets at which point the breaker would immediately trip. I temporarily moved the jet pump circuit to the 30A "heater" breaker. It tripped. I then temporarily moved the jet pump circuit to a 15A non-GFCI breaker. It did NOT trip. At no time do I measure the jet pump drawing over 9.9 amps, so I'm not tripping out on over-current. It must be tripping out on ground fault. The reason I tried other breakers was in case the 20A GFCI breaker was "weak". All "hot" wires to ground measure at least 1 megohm. I re-seated all the wires in the input terminal, re-seated all "slip on" terminals, and loosened and re-tightened all readily-visible grounding screws. I will say that ONE of the line terminals in the control box read 220K ohms when I had my positive lead on ground and negative lead on the terminal, but this calculates to 1 mA of current - well below the 5 mA of current required to trip the GFCI breaker on a ground fault. Since re-seating everything, I haven't been able to reproduce the problem. Should it reoccur, is there a better/suggested way to diagnose ground faults? I've been around electricity and electronics all my life. I work with 3-phase, 480V equipment at work up to several hundred amps. I program industrial PLCs. I know how to safely work around high voltage and I fully understand the importance of grounding and the need for ground fault protection devices, but I'm open to more efficient troubleshooting ideas.