Document: NRC Regulatory Guide
Document ID: da55f4e3-0344-4286-b684-2ebd9c82115e
Document Type: regulatory_guide
Title: Independence Between Redundant Standby
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML0037/ML003739924.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.6
CFR Part: 
CFR Title: 

Content:
8/10/71 SAFETY GUIDE 6 INDEPENDENCE BETWEEN REDUNDANT STANDBY (ONSITE) POWER SOURCES AND BETWEEN THEIR DISTRIBUTION SYSTEMS A. Introduction General Design Criterion 17 requires that onsite electrical power systems have sufficient independence to perform their safety functions assuming a single failure. This safety guide describes an acceptable degree of independence between redundant standby (onsite) power sources and between their distribution systems. This guide does not address the suitability of nearby hydroelectric, nuclear, or fossil units as standby power sources at multiple-unit sites. This matter will be evaluated on an individual case basis. B. Definitions Preferred Power System: The offsite external commercial power system. Standby Power System: Those onsite power sources and their distribution equipment provided to energize devices essential to safety and capable of operation independ ently of the preferred power system. Standby Power Source: An electrical generat ing unit and all necessary auxiliaries, usually a diesel generator set, which is part of the standby power system. Load Group: An arrangement of buses, trans formers, switching equipment, loads, etc., fed from the same power source. C. Discussion There is evidence based on operating experi ence and analytical considerations that the par allel operation of standby power sources renders them vulnerable to common mode failures. Cur rent designs are therefore based on the concept of independent, redundant load groups. In these designs, the standby power source for one load group is never automatically interconnected under accident conditions with the standby power source of a redundant counterpart. There can also be compromises of independ ence resulting from automatic bus ties (both a-c and d-c) which connect the loads of one load group to the power source of another in the event the power source of the first load group has failed. The slightly improved defense against random failures achieved by