Document: NUREG-0800
Document ID: d9e9d0a9-6390-4154-94dc-7d5e6d8145cd
Document Type: srp
Title: A-C POWER SYSTEMS (ONSITE)
Source: NUREG-0800
Source URL: https://www.nrc.gov/docs/ML0523/ML052350489.pdf
Revision Date: 2023-06
Chapter: 8
Section ID: 8.3.1
CFR Part: 
CFR Title: 

Content:
r. In this regard an acceptable design will include the capability for restoring preferred power to the respective safety bus by manual actuation only. In assessing the adequacy of the electrical ties between the onsite | and offsite power systems, and the capability of the preferred power. circuits to deliver power to the safety-related buses, both primary and secondary backup protective relaying schemes and their coor- dination, relay settings, and assigned control power supplies are reviewed by PSB to assure that in the event of -an electrical fault, occurring between the preferred power transformer supply breakers and the safety 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 are closely examined to verify that the independence 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