During rigging up mud tanks the Mechanic and roustabout were tightening the flange on the shaft of the agitator. At the same time driller started power in the distribution panel for mud system to switch on lights. He also switched on the agitator by mistake which caused the impeller to rotate. Both mechanic & R/A moved away but the mechanic fell down between the impeller and the suction pipe. The rotating impeller cut his coverall in 2 places causing minor bruises above his right knee & left arm. The splashing muddy water entered his eyes despite his safety glasses causing burning sensation.