Dell Dimension E521 - Manual (Page 13)

2. Disconnect any telephone or telecommunication lines from the computer.
3. Disconnect your computer and all attached devices from their electrical outlets, and then press the power button to ground the system board.
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NOTICE: To disconnect a network cable, first unplug the cable from your computer, and then unplug it from the network port or device.
NOTICE: Before touching anything inside your computer, ground yourself by touching an unpainted metal surface, such as the metal at the back of the
computer. While you work, periodically touch an unpainted metal surface to dissipate any static electricity that could harm internal components.