EMPLOYEE WAS ATTEMPTING TO REPAIR A BROKEN CHAINON THE DM 45 INGERSOL DRILL. WHILE STANDING ON THE ROTARY HEAD WHICH WAS APPROX 15' ABOVE THE GROUND THE OPPOSITE CHAIN BROKE AND ALLOWED THE ROTARY HEAD TO FREE FALL APPROX 12' WHILE HE WAS STANDING ON IT. THE IMPACT OF THE FALL CAUSED HIS INJURY.