In this paper, the Self-Consisting Leader Inception and Propagation Model –SLIM- is used to analyze the shielding performance of transmission lines, with special attention on the terrain topography effect. Transverse and Longitudinal terrain profiles are considered. It is found that a transmission line can be more vulnerable to be struck by lightning on any terrain that leads to increase the height of the conductors. In addition, the striking distance to the phase conductors strongly depends on the landform and the tower geometry, so it cannot be considered that it is constant and equal to all conductors as it is suggested by the EGM, which suggests changes in the current lightning protection standards.