Flying stone - my text is a copy and paste from Boeing's supp procedures. Boeing are clear, if you are using external conditioned air then the pack switches should be off. As to whether damage would occur, well if the check valve were to fail then yes it could - nonetheless it would be prudent not to second guess Boeing's published procedure regardless of my belief of the chance of damage. The original question was whether pack switches should be off or not during a turnaround - I have merely highlighted a situation which I have regularly experienced where the pack switches should be off.