Common Mode Continued
Use a voltmeter (aka ohmmeter) to check for common mode problems.
Put one lead into the upper left of a wall outlet socket (neutral), and the other lead into the bottom opening in the socket (ground).
The voltmeter should read less than 0.5 volts.
Since computers are grounded at the computer chassis, common mode problems are the most dangerous because they cause the routing of excess voltage to the computer.
This is a threat to the computer and the person using it.