Dell XPS 8500 (Early 2012) - Manual (Page 113)

3 Replace the screws that secure the power supply to the chassis.
WARNING: Failure to replace and tighten all screws may cause electrical shock
as these screws are a key part of the system grounding.
4 Connect the DC power cables to the system board and drives.
See "System-Board Components" on page 21.
Postrequisites
1 Replace the computer cover. See "Replacing the Computer Cover" on
page 25.
2 Follow the steps in "After Working Inside Your Computer" on page 15.
Power-Supply Unit 113