For the transmission of high power over long distances underground the gas-insulated transmission lines (GIL) are a good technical solution as an alternative to an overhead line and in addition to cables. The GIL has two main advantages: low resistive losses because of large cross section of the conductor and enclosure and low capacitive load because of the gaseous dielectric. The latest development activities in this field are mainly focused on the reduction of the cost of the GIL to make its application more economical. The second development goal was to increase the environmental acceptance by using nitrogen (N/sub 2/) as a major insulating gas. In this paper focus is given to the insulation co-ordination of a closed system with integrated surge arresters. The calculations of transient overvoltages are made with typical applications of the integration of GIL in the 400 kV net of overhead lines. The overvoltages in the system are calculated and discussed for system lengths of 1 km and 10 km. Recommendations for such applications for the insulation levels are given. The GIL design configuration, the lightning overvoltages of overhead lines, and the overvoltages in the GIL are discussed under respect of overvoltage protection measures. Together with the use of surge arresters the insulation levels due to overvoltages are discussed. The insulation co-ordination is a very important measure for system optimization. Calculated values found for the insulation level of the GIL were proven with type tests and long duration test set-ups. Some test results are also explained in this paper.