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:
Thus, if the first offsite source is lost (because of a fire or ice storm), then the licensee should evaluate the susceptibility of the second source for a potential common-cause failure and continued reliability and availability and, if appropriate, should treat it as equivalent to the loss of both offsite sources. If the second offsite power source comes from an unaffected pathway, is underground, or possesses other unique design features, it may not be susceptible to a common- cause failure. Licensees should make a concerted effort to restore the offsite ac power source during this restricted time period and should ensure that the remaining power source can accommodate the plant shutdown in accordance with the plant-specific technical specifications. 2. The available onsite ac power sources are one less than the LCO. This degradation level means that one train of the onsite ac power system is not available for safe shutdown or to mitigate the effects of an event. In a single-unit plant, this typically means that one diesel generator is inoperable. In certain multiple-unit plants that share onsite ac supplies, this typically means that the available onsite supply does not have the capacity to mitigate the effects of events in both units. Because any inadvertent generator trip could potentially result in a total loss of ac power, the licensee should severely restrict the time allowed for continued operation. In the absence of one onsite power source, the intent of GDC-17 is twofold: • Avoid the risk associated with immediate shutdown. • Minimize the risk associated with this level of degradation by severely limiting its exposure time. Licensees should make a concerted effort to restore the onsite ac power source during the completion time period allowed by the 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