The transmission line is a corridor that must be passed in the distribution of electrical energy. Transmits electric power on a large scale from Generator Station / Power Plant to distribution station to reach consumer electricity users. To protect the important role of transmission lines, planning the construction of transmission lines with protection and insulation equipment must be reliable. But in fact, the reliability of transmission line protection and insulation systems often exceeds the standard provisions of IEC-71-1 regarding overvoltage and insulation coordination. Insulation coordination involves the selection of the electrical strength of the equipment and its application, in such a way as to reduce the threat of overvoltage that will arise in the equipment technically and economically. In transmission lines that have design discrepancies with the concept of isolation coordination, it can be seen from the excessive number of insulator pieces on the transmission tower, the selection of lightning protection classes is too high, the basic insulation level (BIL) of some equipment is higher, the distance between equipment and lightning rods are too far. Analysis and evaluation of isolation coordination aims to create a system whose parts, each other have such insulation resistance that under every operating condition, the quality of electric power supply services can be achieved at the minimum possible cost.