Document: NRC Regulatory Guide
Document ID: 1731a882-b0c5-418d-b65e-55646c26be5c
Document Type: regulatory_guide
Title: 03/2007
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML0703/ML070380553.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.9
CFR Part: 
CFR Title: 

Content:
nal in about 1–2 second(s). Protection of the emergency diesel generator from excessive overspeed, which can result from an improperly adjusted control system or governor failure, is provided by the immediate operation of a diesel generator trip, which is usually set at 115 percent of nominal speed. Similarly, to prevent substantial damage to the generator, the generator differential current trip must operate immediately upon occurrence of an internal fault. Other protective trips can also safeguard the emergency diesel generators from possible damage. However, these trips could interfere with successful functioning of the diesel generators when they are most needed (i.e., during design-basis events). In addition, experience has shown that on numerous occasions, these protective trips have needlessly shut down emergency diesel generators because of spurious operation of a trip circuit. Consequently, it is important to take measures to ensure that spurious actuation of these other protective trips does not prevent the emergency diesel generators from performing their safety function during the emergency mode of operation. The uncertainties inherent in safety load estimates at an early stage of design or prior to the combined license stage are sometimes of such magnitude that it is prudent to provide a reasonable margin in selecting the load capabilities of the emergency diesel generators. This margin can be provided by estimating the loads conservatively and selecting the continuous rating of the emergency diesel generators that exceeds the sum of the loads needed at any one time. A more accurate estimate of safety loads is possible during the operating license or combined license stages of review because detailed designs should have been completed and component test and preoperational test data are usually available. However, the design-basis event loads during the operating license or combined license stages should be within the continuous rating of the emergency