Pool and Spa
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OK here is how I diagnose a tripping breaker....
First I do what you already did which is disconnect power from tub, if breaker stays on then we know it is a component. Now I turn off breaker then disconnect the heater wires from board, I turn breaker on if breaker stays on then we know it is a bad heater, if breaker still trips move on to ozone, then blower if equipped, etc until you find the one part that will not trip breaker then replace that part. In most cases I find the heater bad next ozone unit, or blower, lastly the board.
I would do it just to know that it does or does not trip with heater disconnected. Have you replaced the 20 amp breaker just to be sure that you do not have a weak breaker and it is giving a false trip? I also noticed a black mark in one of the pictures of the board.
Yes when you see black spots on a green circuit board it usually means a burned out circuit but not always. At this point I would replace the relay board for the heater: Check out here