The heatsink was live, but was not electrically connected to the MOSFets. He had 2 choices:
1-electrically connect the waterblock so that it's live too
or
2-don't connect the waterblock electrically to mains
He opted for #2. Here's why: If the block had been connected, he would have had to isolate the MOSFETs electrically from the waterblock, which would have involved using either a Mica or Ceramic shim. Now of course that would greatly reduce the cooling ability, so that he didn't have to use any shims...
As he put it: "Ideally I wanted the metal back part of each mosfet directly over the block channel path for the best cooling. "
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