Lightning is main factor of failures in medium voltage distribution lines. In order to protect the lines and improve their lightning performance, surge arresters are installed between phase and earth. When the energy absorbed by the arrester exceeds its withstand capability, the arrester fails. The energy dissipated is depended oil the electrical characteristic of the arrester, the installation interval and the grounding resistance. In the current work a computation of the absorbed energy by the arrester in a typical Hellenic medium voltage line is performed, with a sensitivity analysis for the interval and the ground resistance.