The SPDs of a surge protection system must be able to withstand the discharged current and provide a Ures level (residua l voltage) lower than the Ue peak voltage that the equipment can withstand. Most of the times it is not possible to achieve this with just one SPD, especially because the intensity of the surge is not known, and because of the induction of overvoltages when conductors exceed 10m.

The optimal system of protection is the 3-STEP approach, in which successive stages are combined in the performance of high discharge capacity devices and devices with an optimal voltage protection level (low). This is addressed by the definition of "Types" or "Classes" of SPDs according to the type of transient pulses to which each protected zone in the installation is subject to.