gmat
I think it must be something like that as there has to be a reason. Like you say the metal back part of the Mosfets on my PSU is now & was before the water-cooling at 0v, it's the heatsink itself that was live mains voltage. The sink also wasn't being used as a bridge joining two parts of the PCB together either, as I checked for this.
The mica shims prevented the back part of the mosfet touching the sink directly, (as far as electrical conductivity is concerned),
I don't pretend to understand fully how a PSU works, but the water block is now earthed and the mosfets mounted in the same way with the same mica shims. It appears to work ok without the original heatsinks "liveness". Like you say a scope and someone that knows how to use it is what's really required
