This application guide applies to Thyristor Surge Protective Devices components used in systems with voltages up to 1000Vrms or 1200Vdc. These components are designed to limit overvoltages and divert surge currents by limiting the voltage and switching to a low impedance actions. Although telecommunication circuits are the main application of Thyristor SPDs, this guide will also provide useful information for other protection applications. When properly applied it protects telecommunication circuits from failure and damage.