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- This paper describes an effect of a single- and a quadruple-lightning impulse currents on electrical characteristics of ZnO blocks, and an expert system to diagnose arrester deterioration.To deduce the parameters needed for diagnosing arrester deterioration, an accelerate deterioration test is carried out. In the experiment, leakage current components are measured. Also, wave height distribution of the leakage current according to the progress of arrester deterioration is analyzed.From the experimental results, the wave height distribution of the leakage current showed conspicuous difference even in an immaterial leakage current increase. Therefore, the use of wave height distribution of the leakage current in deterioration diagnostic technique makes more accurate diagnosis than the conventional method by using only a leakage current value. Finally, the expert system based on the experimental results is developed and the system can diagnose arrester deterioration by measuring the leakage current and its wave height distribution.