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General relationships for the estimation of the maximum shielding failure current of overhead transmission lines have been derived by performing shielding analysis on the basis of several lightning attachment models including a recently introduced statistical one. The interdependence of maximum shielding failure current, transmission line geometry and factors employed in lightning attachment models is discussed through an application to typical 150 kV and 400 kV lines of the Hellenic transmission system. The maximum shielding failure current depends on transmission line geometry and shows a great variability among the lightning attachment models that are used in shielding analysis; electrogeometric models, thus also the IEEE Standard 1243:1997, yield higher values. These results are of great importance when considering that the maximum shielding failure current of transmission lines, besides being employed in estimating their shielding failure flashover rate, is an important parameter for insulation coordination studies.