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The paper analyzes the performance of shielding failure lightning protection of 500kV quadruple-circuit overhead ac transmission lines on the same tower with revised EGM method and discusses the shielding effect of ground wires to phase conductors. Calculating the shielding failure trip out rate and modeling reasonably based on the analysis of tower style. The relationships between trip out rate and the influence factors such as tower height, protection angle, ground obliquity and striking distance factor are achieved. A conclusion is drawn that the shielding failure rate will turn larger when tower height, protection angle, ground obliquity become larger and the striking distance factor becomes smaller. The calculation and analysis results show that negative protection angle and lower tower height are effective to improve the performance of shielding failure of 500kV quadruple-circuit transmission lines on the same tower.