ELECTRICIAN WAS ASSIGNED TO DISCONNECT THE WHEELMOTORS ON #228 PRODUCTION TRUCK LOCATED IN THE MINE SERVICE BUILDING. HE USED 30"" HIGH PORTABLE STAIRWAY TO GAIN ACCESS TO THE AXLE BOX OPENING. AS HE ATTEMPTED TO ENTER THE OPENING. HE HELD ONTO A HANDHOLD WELDED ONTO THE TRUCK BY THE MANUFACTUTER. THE WELDS SUDDENLY LET LOOSE AND HE FELL, INJURING HIS RIGHT KNEE.