Document: NRC Regulatory Guide
Document ID: 8e45dce1-e1e7-4415-b1dd-7e2a610e545b
Document Type: regulatory_guide
Title: Fire Protection for Nuclear Power Plants (Rev. 4)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML2023/ML20231A835.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.189
CFR Part: 
CFR Title: 

Content:
er of non-latching intra-cable circuit failures should be considered, unless otherwise limited. In addition, for latching fire- induced hot shorts, any number of intra-cable circuit failures should be considered unless otherwise limited. d. Sequentially Selected Fire-Induced Circuit Failures—If the MSO requires a selective sequence of five or more separate target cables, each with specific fire-induced cable failures, and the adverse condition will not occur if the sequence is not produced by the fire-induced circuit failures (e.g., hot short, short to ground, open circuit) with at least two of these failures being hot shorts, then the MSO does not need to be considered regardless of fire-induced failure durations, circuit configurations, or fire-induced failure types. To be beyond what needs to be considered for MSOs, the total number of sequential failures must exceed the threshold established above without including (1) the more probable failures of conductor grounding of grounded alternating current (AC) circuits in armored cable or (2) for ungrounded direct current (DC) circuits, the more probable failures of intra-cable short or GFEHS in armored cable as one of the sequential failures. The metal armor of armored cable is assumed to always be grounded in accordance with NFPA 70. e. In general, the duration of a hot short may be assumed to be limited to 20 minutes in AC circuits, and 40 minutes in DC circuits. DG-1359, Page 78 For circuits not sealed in or latched for equipment important to safe shutdown, licensees should consider multiple fire-induced circuit failures in at least two separate cables. For circuits not sealed in or latched for equipment important to safe shutdown that involves high-low pressure interfaces, licensees should consider circuit failures in at least three cables. This applies when there are defense-in-depth features, such as automatic suppression and limits on ignition sources and combustibles. When there are no defense-in-depth