Revision of IEEE standard 741 to provide needed guidance in setting degraded and loss of voltage relays and their associated time delays

Working Group 4.7 of IEEE Subcommittee SC-4, “Auxiliary Power” is currently drafting a revision to Appendix A of IEEE 741-1990, “IEEE Standard Criteria for the Protection of Class 1E Power Systems and Equipment in Nuclear Power Generating Stations”, to provide needed guidance in setting degraded and loss of voltage relays and their associated time delays. The setting of degraded voltage relays has been one of the most significant issues that has emerged from the Nuclear Regulatory Commission (NRC) Electrical Distribution System Functional Inspections (EDSFI). The purpose of the appendix is to provide (1) the requirements for protection of Class 1E equipment for degraded voltage or loss of voltage conditions and (2) to provide the elements of an analysis method for determining the proper settings for degraded and loss of voltage protection systems and their associated time delays. The appendix will also provide a comprehensive description of a protection scheme, utilizing solid state undervoltage relays, which will meet these requirements, including determination of the relay voltage setpoints and their associated time delays. However, it is recognized that due to the diversity of nuclear plant auxiliary system designs, there are other protection schemes which will also provide the desired level of protection when justified by appropriate analyses