Apple PC Serial and Parallel Card - Manual (Page 31)

4 Touch the metal part of the power supply case inside the computer.
Always do this before you touch any parts, or install any components, inside
the computer. Touching the power supply case helps release static electricity
from your body and prevent possible damage to internal components.
IMPORTANT You can collect static electricity just by walking away from and
then back to your computer. If you need to move away from your computer
during installation, remember to touch the metal part of the power supply
case again before you continue installing.
5 Disconnect the power cord.
You can unplug the power cord now that you discharged static electricity from
your body in the previous step.
Power supply case
31 Installing the Card in a Power Macintosh 7200, 7300, 7500, or 7600