Abstract--A back flashover for a shielding failure can be caused by a lightning stroke, which will damage equipment and devices in a gas insulated substation if the devices' maximum voltages are exceeded. In this study, the branch timetable method, which is implemented from the lattice diagram method, is used in a computer program for the calculation of overvoltages in the gas insulated substation, as a result of a shielding failure, so that the gas insulated substation performance under lightning stroke can be determined. The study of the lightning performance of a gas insulated substation is very important for the purpose of designing lightning overvoltage protection systems. Electrical power systems Gas insulated substation (G.I.S.) Shielding failure