Document: NRC Regulatory Guide
Document ID: 666e1303-0170-4974-a7d6-af27eb586524
Document Type: regulatory_guide
Title: Risk-Informed, Performance-Based Fire Protection for Existing Light-Water Nuclear Power Plants (Rev. 2)
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML2104/ML21048A448.pdf
Revision Date: 2023-05
Chapter: 
Section ID: RG-1.205
CFR Part: 
CFR Title: 

Content:
g 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 AC circuits in armored cable or (2) for ungrounded 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, “National Electrical Code” (Ref. 26). RG 1.205, Revision 2, Page 24 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. In response to fire-induced MSO concerns, several licensees proposed the use of shorting switches as a design feature to protect against MSOs. The shorting switch is a circuit design that places a short across a coil in the circuit of concern when the circuit is in its “standby” state to prevent spurious energization of the coil. When operation of the component is desired, the motion of the hand switch removes the short before energizing the coil to actuate the component. Any circuit using a shorting switch should have this feature