As long as you and the workclamp are at the same potential then no current flow. Both laying on the ground is the same spot. Workclamp to the frame and you laying on the ground with no shirt on and there is a possibility of current flow through you body if you had a bare hand on the car frame AND good contact to the ground around you. The most dangerous time would be if you fired the torch in the air and the current is looking for a place to go. While cutting the lower impedance of the car frame to the electrode would cause most of the current to travel in that direction.
To shock yourself with a plasma is not as easy as just doing one thing wrong. Like most accidents the cause is often doing multiple things wrong at the same time (e.g. not wearing gloves + not wearing a shirt + having the workclamp on the frame and firing the torch in the air) All industrial equipment have elements of danger. The CNC table can hurt you bad. Saws and routers take off body parts as easily as wood or metal. Flying objects can damage eyes. In one rare case a friend of mine was almost killed when cutting with a table saw and a carbide tooth from the blade penetrated him in the chest and hit his heart. He almost bled to death before he crawled to a phone a dialed 911. When they got him to the ER his blood pressure was so low they thought he was dead.
I once watched a guy cut off the end of his finger while using a manual paper cutter. This was in 1970 and before the manual cutters had guards. Sometimes dumbass has it's own punishment (:-)
TOM Caudle
www.CandCNC.com