Miner/Mant. shop, was climbing down from elevated position on gradall scaler, when he slipped, losing balance, sliding off fender skirt, approx. 4.5' to floor, bracing w/hands. Position was narrow, without elements of 3-point contact, and too low for fall protection device/rig. Inj. recordable because of wrist brace, miner rtn. to work.