In the determination of lightning-induced voltages on overhead power lines, ground losses play a role in both the evaluation of the lightning radiated electromagnetic fields and of the line parameters. The aim of this paper is to assess the effect of these losses through each of these two mechanisms. The authors show, in particular, that for lines whose length does not exceed a certain `critical' value, the surge propagation along the line is not appreciably affected by the ground finite conductivity, and, therefore, it can be neglected in the computation process. Further, it has been shown that, for typical power lines, the wire impedance and the ground admittance, both frequency-dependent, can be neglected in the computations for typical overhead power lines