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That is the function of your GFCI breaker. That little bit of corrosion you saw was a short but the reason that didn’t turn into a meltdown and possible life threatening problem is the GFCI shuts the power off the instant it detects a short to ground. If this was wired straight to the breaker in the main panel without the GFCI in line it would have eventually tripped the 50a breaker in the panel when the heater had totally melted down in flames and sent 220v thru the tub water and the combination of paths for the electric to take had totaled more than 50a. This would have taken some time could have caused a fire in the tub or electrocuted people sitting in the water.
So in an interesting turn of events:I contacted Watkins via phone. This gentleman was supposed to talk to a tech and get back to me. Never did. So 15 days later I called again and spoke a nice women. She did speak to her tech and even pat her me to him. We discussed all the trouble shooting I have done (primary pump replaces, isolated pump 2, replaced breaker and wires, and confirmed it wasn't heater as it happens during filter session) and he agrees that it could be the main board. They are sending me a new board, preprogrammed with the newest software for free even though it's about a year out of warranty. Hopefully it fixes it but if not, he said to call back. I guess they are in the holiday spirit I'll keep posted, though itll take a few months to confirm it's working right