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have you ohmed the hps to ground when the pressure rises?
The defrost board is just a logic board with inputs and outputs...there is no load.
Thus if the board is burning out the short is external to the board but overheating a component. First you can install 3 amp fuse in the line you are concerned with...
Then I would check the hps itself. If it ran 35 min it is not a direct short but a draw...you can eliminate the hps for testing but it would also be interesting to see if it relates to pressure as the hps gets more pressure on it.
The culprit will be of course one of the safetys..