This application guide applies to thyristor surge protective components (SPCs) used in systems with voltages up to 1000 V rms or 1200 V dc. These components are designed to limit overvoltages and divert surge currents by limiting the voltage and switching to a low-impedance state. Although telecommunication circuits are the main application of thyristor SPCs@ this guide will also provide useful information for other protection applications. This guide is intended to complement and be used in conjunction with IEEE Std C62.37?-1996.1 Purpose This guide is intended to provide assistance in selecting the most appropriate type of thyristor SPC for use in a surge protective device (SPD)@ equipment@ or system application. IEEE Std C62.37-1996 defines thyristor SPC parameters and their measurement. This guide explains the basic functions@ component structures@ surge environment@ comparative thyristor technologies@ and how parameters are interpreted and selected for example applications. 1 Information on references can be found in Clause 2.