Adapting a Computer PSU

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One other point to bear in mind is that even fully floating SMPS (industrial ones are available) is that even though the mains "earth" is not connected directly to the secondary 0V line on these, there is usually a resistor between somewhere in the region of between either 2 X 4.7 Megohms in series or a single 10 Megohms , these are across the isolation barrier and are included to stop the secondary from floating up (or acquiring a charge) to a voltage higher than the withstand across the said barrier, also sometimes a high voltage low value Y cap is in this network (the cap is not present on all types).
It may be possible to disconnect the link between Mains Earth and the secondary side with the above in mind, however I am in no position to give any concrete re-design advice here.