THE EUCLID 74TD 35 TON HAUL TRUCK WAS BLOCKED, LOCKED AND TAGGED OUT FOR REPAIR. A WIRING FAULT WAS BEING TRACED AND ACCESS INTO AND UNDER THE CAB WAS REQUIRED. EE WAS INSPECTING THE WIRING UN DER THE CAB WHILE STANDING ON THE EXTENSION LADDER, HIS FOOR SLIPPED FROM THE LADDER RUNG. HE FELL APPROX 4 FEET TO GROUND LEVEL. HIS INJURIES CONSISTED OF BRUISES TO THE LEFT LEG, THIGH & HI