Document: NUREG-0800
Document ID: 9cfc38bf-7808-446e-a43d-4ca9ff55df2a
Document Type: srp
Title: AC POWER SYSTEMS (ONSITE)
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML1007/ML100740289.pdf
Revision Date: 2023-06
Chapter: 8
Section ID: 8.3.1
CFR Part: 
CFR Title: 

Content:
ds to the diesel generator under test will cause the tripping of this diesel due to overload or underfrequency. The end result could be the total loss of power to the safety buses. However, this power interruption could be of momentary duration if the remaining redundant diesel generators are commanded automatically to start by undervoltage relay action immediately after total power is lost. The diesel generator under test will be inoperable due to the self- locking feature preventing restarting after an overload or underfrequency trip condition. The reviewer ascertains that the time delay introduced in making power available to the safety buses as a result of this event is within the response time limits assumed in the accident analyses. This should include verification that subsequent failures such as those resulting from improper electrical relaying coordination and self-locking features will not impair the automatic starting of the remaining redundant diesel generators required to meet minimum safety criteria. If the time delay introduced in making power available to the safety buses is not tolerable, it either must be demonstrated that the probability of occurrence of this event is low when compared with the frequency and duration of testing each diesel, or the design must provide diverse automatic signals, other than undervoltage, to ensure the availability of standby power to the safety buses. After reviewing the parallel operation of the offsite and onsite power systems, the staff found the use of the standby power supply (diesel generator) sets to supply power to the electrical system during peak load demand periods to be problematic. The basis for this conclusion is that the frequent interconnections of the offsite and standby power supplies do not minimize the probability of their coincident loss (GDC 17), nor can the design be made immune to common failure modes (Section 5.2.1(5) of IEEE Std. 308). Further details amplifying the basis for this conclusion