What has gone wrong????

[EddyCurrent]

I will check the cables out in the morning.

as it happens, I am running the PC and bob on a 20' extension, I will check the earth as well on that, the plasma is running on a temporary short extension to a wall mounted isolator that is at 180 deg from the PC supply, the two do not cross.

I'm thinking there could be a voltage difference between both 'grounds' at the ends. In a bathroom for example all metal parts are connected together with a decent size earth wire which is then connected to the earth terminal of a ring main for example, I think you could do the same with your setup where all the earths are connected to a 'star point' on your machine frame. However there is a risk of creating a complex circuit, it would be better if all the equipment could be fed locally from the same supply so that the earth wires between them are very short.