Document: NUREG-0800
Document ID: bc7e4c14-0eaa-4591-a506-23d7893f8a00
Document Type: srp
Title: as establishing acceptable guidance for meeting the requirements of this criterion.
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0520/ML052070498.pdf
Revision Date: 2023-06
Chapter: 8
Section ID: 8.3.1
CFR Part: 
CFR Title: 

Content:
es, both primary and secondary backup protective relaying schemes and their coordination, relay settings, and assigned control power supplies are reviewed by PSBEELB to assureensure that, in the event of an electrical fault, 97 occurring between the preferred power transformer supply breakers and the safety 98 buses, no single failure will result in reducing the number of preferred power circuits to less than the minimum required for safety or prevent the separation of the affected circuit from the respective redundant portion of the onsite power system. In addition, it is verified that no single protective relay or interlock failure will prevent separation of the required redundant portions of the onsite power system from the preferred power system upon loss of the latter. In reviewing the mode of operation where both power systems are being operated in parallel (such is the case during full-load testing of standby power supply diesel-generator sets), the interlock scheme, including electrical protective relay coordination and settings, areis closely examined to verify that the independence 99 of the required redundant portions of the onsite power system is established upon a failure in the offsite power system. The event of concern under this mode of operation is an accident concurrent with a loss of offsite power and a single failure preventing the opening of the feeder-isolation breaker through which the paralleling of the power systems was being accomplished. Because the signal to start the diesel-generator sets is normally derived from undervoltage relays, and under this situation the voltage is maintained above the trip relay settings by the diesel generator under test, the remaining redundant diesel generators will not be commanded to start running. Consequently, the added capacity resulting from the connection of nonsafety-related loads to the diesel generator under test will cause the tripping of this diesel due to overload or underfrequency . The end