The installation of surge protective devices (SPD) can provide the required protection against lightning surges for equipment within a structure. To this end, the main parameter to be considered is the probability P SPD that the SPD reduces the frequency of damage of the equipment. This paper shows how the probability P SPD can be evaluated and how to select SPD in order to get the required value of P SPD for achieving equipment protection. Finally, an SPD selection example is described and presented.