A plasma machine has to have the +ve electrode clamped or referenced to the material, which is usually also the table.
It will work like this regardless of other connections, but it is the other equipment that must be protected from stray currents that may or may not leak to Earth Ground.
All the CNC tables I have encountered have the +ve conductor bonded to the service ground and generally has a ground stake where all these are terminated, usually right at the table.
This is in order to make equipment such as PC.s or Electronic controllers that also have their power supplies referenced to earth Ground exactly at the same potential, it is called equi-potential bonding.
The last thing you want is a Plasma table that is not at earth ground where stray current may find all sorts of paths through equipment that is.
Ground rods come in all lengths, a 6ft copper plated should work, depends on the soil make up in your area.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.