I HAVE A COSTUMER WHO HAD A WELDER PLUGED TO A 220V RECPT. AND WHEN HE TRIED TO CONNECT TO A METAL BUILDING FRAME TO WELD WITHOUT THE STINGER TOUCHING ANYTHING THE CLAMP STARTED TO ARC LIKE IT WAS TRYING TO WELD WITHOUT THE STINGER.IT IS A LINCOLN A/C WELDER.ANY IDEAS WHY IT ARCED WITHOUT THE STINGER?