SPD characteristics for equipment protection

The installation of surge protective devices (SPD) can provide adequate lightning protection for equipment inside a structure. To this end, the main SPD characteristics are the SPD protection factor and its protection level. This paper shows that a coordinated SPD system, characterized by both the nominal discharge current and the impulse current, can have different protection factors for each equipment risk component, as defined according with IEC 62305 series. This paper also shows how the criteria for the selection of the SPD protection level can be defined in order to limit the overvoltage at the equipment input port to a value less than or equal to the equipment withstand voltage. These criteria, which take into account the installation conditions of the electrical and signal lines, are applied to an example of structure to be protected against lightning.