Manuf. like Messer MG systems that have used PC control systems and Hypertherm Plasma units with High frequency ignition successfully, and have used the same procedure that I use with my PC based systems.
First, the service ground and the plasma common (table) is usually taken to a ground stake at the table location, using a sufficiently large gauge cable.
The plasma power supply has to be isolated via a transformer in the hypertherm Plasma unit in order to ground the plasma positive electrode and allow it to be common to earth ground and the table, otherwise it would blow the power supply.
In the electrical enclosure, the PC ground and other auxiliary power supply commons,servo supply commons, are connected to a copper ground plate, together with all encoder shields etc.
This is also bonded to the ground stake.
Often the problem comes when an attempt is made to keep everything totally isolated and separate, instead of common bond.