Document: NRC Regulatory Guide
Document ID: 9a8db182-3194-4663-aeb3-64f2e4edd6aa
Document Type: regulatory_guide
Title: Availability of Electric Power Sources (Rev. 1)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML1008/ML100840581.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.93
CFR Part: 
CFR Title: 

Content:
e technical specifications and should verify that the remaining offsite power source can accommodate the plant shutdown in accordance with the plant-specific technical specifications. 3. The available offsite ac power sources are two less than the LCO. This degradation level means that the offsite power system is not available or does not have the capability to achieve a safe shutdown and mitigate the effects of an event; however, the onsite ac system is not degraded. Thus, this level generally corresponds to the total loss (inadequate capacity, voltage, or frequency) of the offsite power sources. Because of the normally high availability of offsite sources, this level of degradation may appear to be more severe than the next two degradation levels discussed below. However, two factors tend to decrease the severity of this degradation level as compared with the next two degradation levels. First, the configuration of the redundant onsite ac power system that remains available for this degradation level is not susceptible to a single bus or switching failure, whereas the next degradation level may be susceptible. Second, the time required to detect and restore an unavailable offsite power source is generally much less than that required for detecting and restoring an unavailable onsite ac power source. DG-1244, Page 7 This level of degradation can be caused by a variety of events, including the loss of two offsite circuits, the loss of the electric grid, or any other condition that renders offsite power unavailable for safe shutdown and emergency purposes. Because the onsite power system has not been degraded and because a simultaneous loss of offsite power and a loss-of-coolant accident were postulated as a design basis, a brief interval of continued operation is allowed if the onsite sources of ac power, independent of grid condition, are operable and can act as a substitute train of ac power in accordance with the plant-specific technical specifications. Note that