Document: NUREG-0800
Document ID: 81256f55-72dd-4777-9472-f373c71c4554
Document Type: srp
Title: AC POWER SYSTEMS (ONSITE)
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0705/ML070550065.pdf
Revision Date: 2023-06
Chapter: 8
Section ID: 8.3.1
CFR Part: 
CFR Title: 

Content:
t 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. 8.3.1-27 Revision 3 - March 2007 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 are included in Branch Technical Position 8-2, which sets forth the basis for not using the diesel generator sets for purposes other than emergency standby power supplies. Operating experience has provided insights into aging-, operation-, and design-related problems associated with medium-and low-voltage switchgear equipment, electrical buses, and circuit breakers used in the onsite ac power system. These include, but are not limited to: - bus failures, involving the integrity of bus bar splice joints, torque relaxation, cyclical bus loading, and incipient damage resulting from a high fault current transient/arcing fault