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In this letter, the Wedepohl-Wilcox series is combined with the trapezoidal rule of integration for calculating ground return impedances of buried cables. In this hybrid method, the former are used for low frequencies and the latter are for high frequencies. Based on a numerical analysis, a criterion for switching between series and numerical integration is proposed here and compared with the one proposed by Wedepohl-Wilcox. Validation of the improved criterion is made via an electromagnetic transient.