The study of transmission line protection offers an opportunity to examine many fundamental relaying considerations that apply, in one degree or another, to the protection of other types of power system equipment. Each electrical element, of course, will have protection problems unique to itself; however, the concepts associated with transmission line protection are fundamental to all other electrical devices and provide an excellent starting point to examine and appreciate the implementation of all power system protection. The basic relaying characteristics of reliability, selectivity, local and remote backup, zones of protection, coordination, and speed are present in almost all relaying situations. This guide specifically addresses these concepts for transmission line protection, but the ideas are universally applicable. Since transmission lines are also the links to adjacent lines or connected equipment, the protection provided for transmission lines must be compatible with the protection provided for all of these other elements. This requires coordination of settings, operating times, and characteristics. Individual relays, such as over current, directional, and distance, as well as the total protection package, including the communication channels, are all examined in this guide, with appropriate discussion relating to their application and their particular advantages and disadvantages. Special topics, such as the effects of series capacitors or static volt ampere reactive (var) systems and consideration for tripping versus blocking during system power swing conditions, are also discussed. In addition to the protection of the line itself, consideration is given to the various system configurations and bus arrangements, mutually coupled lines, reclosing, and the impact that system performance and parameters have on the selection of relays and relay schemes. Special protection systems, multi-terminal lines, and single-phase tripping are among the topics covered.