The single-failure criterion is set forth in 4.2 of IEEE Std 279-1971 (ANSI N42.7-1972),Criteria for Protection Systems for Nuclear Power Generating Stations. It states: "Any single failure within the protection system shall not prevent proper protection system action when required." This guide describes and classifies single failures, presents acceptable methods of single-failure analysis. The single-failure criterion is a basic principle in the design of reliable systems. There are other approaches to high system reliability, not included in this guide, such as equipment qualification, periodic testing, and the setting and achievement of reliability goals.