This part of IEC 61647 is a test specification standard for Thyristor Surge Suppressor, TSS, components. These components are designed to limit overvoltages and divert surge currents by clipping and crowbarring actions. These components are used in the construction of surge protective devices, particularly in the telecommunications area. This present publication contains information on: - terms, letters, symbols and definitions; - basic function and component description; - service conditions and fault modes; - rating verification and characteristic meausurement.