Probably because your bench or the job is dirty giving a poor electrical connection causing it to arc to the bench, connect the ground / return clamp to the job and make sure everything is clean, or clamp the job down to the bench.
is your return lead fixed to the bench? if so thats the cause, leccy needs the path back to the welder and the return path takes from the workpeice through the bench hence the fusing...
Bench is drastic mate..needs a good scrape and clean then get a 6" bit of copper pipe and flatten in the vice and drill a hole in the end..drill a hole right through the bench then grind the galvanizing off and put a dome headed fencing bolt through and bolt the "copper strap" to the underside and bend it down, this now is your earth strap for the welding clamp. if you keep the bench clean and very lightly oiled then the spatter which is giving you a bad contact will not stick to the top....When your welding becomes better tell them Roofman off the internet put you wise
I use an old, worn flat file that has the tang broken off and the end ground like a wood chisel. It works perfectly for dislodging spatter and other bits off both my bench and the workpiece.