For rationalizing equipment insulation level which can subsequently lead to reductions in the cost of substation equipment such as GIS’s and transformers, it is necessary to reexamine insulating test voltages by investigating the method of evaluating lightning surge waveforms in terms of the equivalent standard lightning impulse waveforms. This paper describes the evaluation method for real surges (called non-standard lightning impulse waveform), based on insulation characteristics of gas gaps. The method is applied to typical surges in the lightning surge time region for various UHV and 500kV systems and it is obtained that the equivalent peak values of the standard lightning impulse waveform are possibly reduced by 20 to 30 percents.