This informative document contains additional and leading informations to the overvoltage protection of information technology equipment (IT installations) and the surge protective devices used hereto. Relevant normative specifications are contained in the standards DIN EN 61663-1 (VDE 0845-4-1) and DIN EN 61663-2 (VDE 0845-4-2) and furthermore in DIN EN 61643-21 (VDE 0845-3-1) and the Technical Report IEC/TR 61643-22