If you are having problems when the plasma unit starts then this will be the HF signal. HF gets everywhere. I have seen it do many weird things to computers and controllers. If your supply cable to your shed is something like a twin and earth or a small armour cable its probable that the earth cbale simply isnt big enough to cope with the HF noise. You also need to ensure that your plasma unit workpiece cable is securely attached to your cutting table and the plasma unit. If the HF noise problem is still an issue you might need to look at turning your shed electrical installation into a TT system. This involves removing the electrical earth from your shed supply and installing a copper earthing rod system outside your shed and connecting this to your shed electrical distribution board. I would recommend getting a qualified electrician to carry this out if you decide to go that way.

Regards

Ian