You need to work on a power supply that has been taken offline. What is the first thing you should do once the cabinet is open?

Power supplies contain large filter capacitors designed to smooth out fluctuating direct current. These capacitors can store a significant amount of electrical energy at dangerous voltage levels for a long time, even after the device has been unplugged from the AC mains. Therefore, the very first safety step when opening a power supply cabinet is to safely discharge these filter capacitors to ground. Failing to do so can result in a severe or lethal electric shock when working on the internal components. While placing the unit on an insulating mat is a good general practice, it does not remove the immediate stored energy hazard inside the unit.