At the beginning of my Ph. D. thesis, the state-of-the-art technologies are described as used in OL compacting. Physical bases are then discussed for any type of overvoltage phenomena taking place on OLs and principles as well as procedures are given for insulation co-ordination. A special attention is devoted to the deterministic and statistical method. Based on the analysis of the overvoltage conditions experienced in the Slovenian 110 kV network, the insulation level was determined by using the deterministic method. The temporary, switching and lightning overvoltages as well as characteristics of the selected surge arrester were taken into account. An in-depth analysis is made of the lightning characteristics and activity and of the methods used in calculation of the lightning overvoltages causing most of the in-service outages. By using the developed physical model of the compact OL tower equipped with line post composite insulators, standard and non-standard dielectric tests were made at the Institute’s High-voltage Laboratory. Based on the result analysis, the minimal insulation distances were determined. The insulation flashover frequency, resulting from lightning striking into a single-circuit compact OL, was assessed by relying on computer simulations and the statistical method. Different shapes of the OL tower, insulation levels, grounding resistances as well as rated voltages and mounting locations of surge arresters were considered. By analysing the results, the optimal solution was selected.