WELDOR ATTEMPTED TO REMOVE A WIRE FEED WELD ""SUITCASE"" FROM THE LINE #2 WINDBOX EXHAUST FAN AFTER A MAINTENANCE SHUTDOWN. HE WAS IN A BENT OVER POSITION AND ATTEMPTED TO SLIDE THE 45-50# BOX T OWARD HIM. HE IMMEDIATELY FELT A SEVERE PAIN BACK PAIN AND HAS SINCE BEEN OFF WORK FOR LOW BACK PAIN.