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Independent lightning rods on building roofs or nearby areas will produce transient high potential, which may damage electric and electronic equipment through electromagnetic induction and couple, and step voltage may harm pedestrians. Based on electromagnetic theory, by means of the building down lead current splitting technique, the calculation formula and security thresholds of step voltage are given. In order to relieve the adverse effects of independent lightning rods, the current splitting technique, in which the multi channel several anti fire copper lines are used in the hollow steel pipes of the lightning rods, is proposed, and it has been applied in the lightning protection of cultural relics.