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This paper analyzes the protection of onsite emergency power systems in nuclear power plants from sustained degraded or sudden loss of voltage conditions of the off-site power. This analysis develops insights into the problems of existing operating nuclear facilities and establishes corrective actions. The protection philosophy and design of protective scheme developed in this paper are adaptable to back-fitting operating nuclear facilities and also iterative approach to the design of relay protection system for new nuclear power plant that will be on-line in mid 1980's. The design approach emulates the recent Nuclear Regulatory Commission staff positions with regard to susceptibility of safety related electrical equipment during sustained degraded voltage conditions at the offsite power sources.