EE WAS POSITIONING TRAILING CABLE FOR PLANNED DRAGLINE MOVE. PULLING ON CABLE ACROSS A SLOPE OF ABOUT 4 TO 1 GRADE. 1 FRT TIRE OF THE REELER TRAVELING INTO SLIGHT DEPRESSION & LOADER ROLLED ONTO ITS SIDE. THERE WAS ABOUT 1200FT OF 12,500 VOLT CABLE ON REEL. EE WAS USING SEAT BELT,BUT STRUCK HIS MOUTH & CHEECK ON SOME OBJECT & REQ'D SUTURES IN HIS LIP