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:
h 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 these bus ties is more than offset by the additional vul nerability to common mode failures which they create. A special case of the foregoing is the bus that is automatically transferred to one or the other of two redundant standby power sources; this is commonly referred to as a swing bus. This arrangement also compromises the independ ence of redundant power sources and their load groups while adding little to the defense against random single failures. The inclusion of a swing bus in an otherwise well designed system often results from an in compatibility between the number of standby power sources (whether a-c or d-c) and the number of redundant load groups. For example, an engineered safety feature system design which depends on the operation of at least two of three electrically driven pumps and which derives power from either of two redundant standby power sources must provide for the swinging of one of the three pump motors in order to meet the single failure criterion. A compatible design, such as one based on three' power sources, would not utilize the swing fea ture. The necessity for a swing bus can also result from an incompatibility between the a-c and d-c power sources themselves. An example would be a three diesel generator, three bus system utilizing d-c control circuits. If only two 5.1 d-c sources are provided, the switching of diesel generator control circuits between the d-c sources becomes necessary in order to provide the necessary redundancy. Again, a compatible design such as one based on three d-c sources, one for each generator, would not utilize a swing bus. A diesel generator that swings between the load groups of different units at a multiple unit site is not an example of the foregoing since such load groups are not redundant to each