This test should read zero or almost zero. Test from the plasma table frame to the outside of the printer-style USB plug outside shield.
(upload://5D8fjn4zHInuHAvK9wlce94kCLK.jpeg)
Check out CNC USB Isolator 1Kv -(Bullet Proof USB) UC-100 USB, USB Ground Loop Eliminator on eBay!
The ground loop isolator goes between the computer USB and the control box USB input.