Superseded by 421.2-1990. This guide includes criteria, definitions, and test procedures for evaluating the dynamic performance of excitation control systems as applied by electric utilities. Primary purposes are to provide a basis for evaluating closed-loop performance of excitation control systems (including both the synchronous machine and its excitation system) for both large and small signal distutbances; confirming the adequacy of mathematical models of excitation systems for use in analytical studies of power systems; specifying methods for performing test of excitation control systems and their components; and preparing excitation system specifications and additional standards. Portions of this guide will also serve as an educational means for people becoming acquainted with excitation control systems.