EE WAS TROUBLESHOOTING AN ELECTRICAL PROBLEM WITH A CONVEYOR'S TAIL PULLEY. A CO-WORKER WAS STANDING ON THE TAIL PULLEY'S FRAME & HAD STARTED TO REMOVE BOLTS SECURING THE SWITCH COVER. INJURED POSITIONED AN 8' STEPLADDER AGAINST THE TAIL PULLEY GUARD & STARTED TO ASCEND THE LADDER. THE BOTTOM OF STEPLADDER KICKED OUT & HE FELL ONTO THE UPPER FRAME OF THE GUARD, FRACTURING RIBS.