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The relevance of subsequent strokes on the lightning performance of 138-kV lines is assessed. An electromagnetic model was used to simulate lightning overvoltages experienced across insulator strings due to direct strikes to the towers of an existing line in order to determine the critical peak current required to flashover using the integration method. From peak current distributions, estimates of outage rate due to backflashover were developed considering the contribution of first and subsequent strokes. It was found that, depending on the value of tower-footing grounding resistance, the contribution of subsequent strokes can be relevant, notably for tall towers, with height above 30 m.