External overvoltages constitute a typical cause of faults, damages and interruptions in the electrical networks' high-voltage transmission lines. In order to protect the lines and their equipment against lightning, overhead ground wires in combination with surge arresters are installed, improving the lightning performance of the system and reducing the annual failure rate. The achievement of low values of earth resistance is an essential requirement, since a good grounding system reduces considerably the corresponding insulation breakdowns. However, the behaviour of surge arresters in relation to the grounding resistance is different, as it depends on the position of the lightning hit. The current work is devoted to the sensitivity analysis of the failure probability of surge arresters with relation to the grounding resistance, investigating concurrently the effect of the arresters' installation interval and the energy absorption capability.