Surge arresters can improve the lightning performance of transmission lines. The flashover rate of a shielded transmission line protected by surge arresters can be negligible when arresters are installed at all phases and all towers. But even if arresters are not installed at all phases, some improvement can be achieved. However, arrester failure can be caused when the energy discharged exceeds the maximum absorption capacity. Therefore, arresters must be selected taking also into account energy discharge stresses. This paper is aimed at analyzing the lightning performance improvement of a shielded transmission line that can be achieved by installing surge arresters and the energy capability to be selected for arresters as a function of the number of phases at which they are installed.