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:
.1.1.1 of this guide, as applicable. DG-1359, Page 83 c. Where adequate separation is not provided, demonstrate that fire-induced failures (multiple hot shorts, open circuits, and shorts to ground) of the cables will not cause maloperation and result in an interfacing system LOCA that would adversely affect safe shutdown. The electrical expert PIRT panel, as documented in NUREG/CR-7150, Volume 3, has determined that the potential for a fire to cause hot shorts on all three phases in proper sequence of an AC power circuit to cause a spurious operation of a motor is incredible and need not be considered in the evaluation. The approach outlined in Appendix C to NEI 00-01, Revision 4, provides an acceptable methodology for the determination of components as high-low pressure interface components, when applied in conjunction with this RG. 5.3.3 Multiple High-Impedance Faults The concept of multiple high-impedance faults (MHIFs) deviates from baseline assumptions associated with conventional electrical protective devices coordination. The MHIF failure mode is based on presuming that a fire can cause short circuits that generate abnormally high currents below the trip point of the individual overcurrent-interrupting devices for the affected circuits. Under the assumed conditions, circuit overcurrent-protective devices will not detect and interrupt the abnormal current flow. It follows that if several circuits experience a high-impedance fault (HIF), the cumulative current flow could exceed the trip setting of the supply breaker or fuse, causing it to trip. The NUREG/CR-7150, Volume 1, electrical expert PIRT panel evaluated the potential MHIF phenomenon and concluded that it need not be considered as a failure mode in the safe-shutdown circuit analysis as long as certain criteria are met. Of critical importance is the use of certain robust design criteria outlined in NEI 00-01, Revision 4, Appendix B.1, that ensure HIFs do not persist in an electrical system for extended