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ATP-EMTP simulations are performed to estimate the minimum shielding failure current causing flashover in 150 kV and 400 kV double-circuit overhead lines of the Hellenic transmission system. This critical current is of great importance for estimating the shielding failure flashover rate and assessing the insulation coordination of overhead transmission lines. It is shown that the minimum shielding failure current causing flashover varies significantly among the insulator string flashover models employed in simulations, which include volt-time curves and leader development models. The minimum shielding failure current causing flashover calculated on the basis of the relevant IEEE Std is significantly lower than that obtained through ATP-EMTP simulations. The computed shielding failure flashover rate of the 150 kV and 400 kV overhead transmission lines is greatly affected by the method adopted for the estimation of the minimum shielding failure current causing flashover.