Surge overvoltages result in a power system from overcurrents due to atmospheric discharges, induction phenomena, switch operations or faults. In order to reduce the probability of injuries to people and of failure of electrical and electronic systems, power systems within a structure should have adequate equipotential bonding. Surge protective devices (SPDs) are used to achieve such equipotential bonding. Metal Oxide Varistors (MOVs) are a very often used kind of SPDs. Actually, MOVs are affected by ageing, due to the number and amplitude of stresses, but also due to other factors such as overheating, pollution and humidity. The aim of this paper is to present a probabilistic assessment of the ageing process of MOVs when lightning, switching and temporary overvoltages are considered. The expected life thus obtained can be used to decide (logistic/maintenance procedures) when the MOV must be changed before its failure occurs, since the main standards do not give definitive indications about such feature.