Plasma voltage is not related to "Ground". Its the Arc Volts between the Workclamp and the Electrode. So if your table is not grounded then your voltage will not be related to ground. Since Workclamp volts are positive in relation to the electrode then your return voltage in ref to ground is usually negative. The circuit its simple just design up a resistor divider network that divides the raw arc volts by 50 and put in some filtering to filter out the PWM noise on most plasma units. It's HOW you connect and use the volts to your uProcessor that is the key. Do it wrong and you might want to use chips in a socket that can be easily replaced.