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:
s such, the secondary circuit may span many fire areas within a nuclear power plant. These latter cases present a potential safety concern as an open circuit in a CT’s secondary can cause high crest (or peak) voltage on the secondary circuit as the CT attempts to maintain the current relationship dictated by the transformer’s winding turn ratio. This condition can generate voltages that may exceed the dielectric strength of the CT’s insulating materials DG-1359, Page 84 and may then cause arcing to connected or nearby components, potentially damaging the components. Should a fire-induced open circuit occur in the run of instrumentation cable, a high-voltage condition in the secondary circuit would result. In theory, this high-voltage condition could result in a secondary fire because of insulation breakdown. In this context, a “secondary fire” refers to a fire at a remote location, (i.e., in a separate fire area) from the original fire that is responsible for the initial open circuit in the CT’s secondary circuit. The resulting secondary fire introduces a potential concern for fire protection strategies in nuclear power plants for both deterministic and performance-based approaches. The NUREG/CR-7150, Volume 1, electrical expert PIRT panel investigated the above postulated failure mode and considered the concern of a secondary fire resulting from a fire-induced, open-circuited CT was more theoretical than real. Based on the availability of objective test data at that time, the panel determined that the unique combination of low-probability events makes this failure mode incredible for CTs with turn ratios of 1200:5 or below. Upon subsequent testing and evaluation of CTs with higher turn ratios (2000:5 and 4000:5), the NUREG/CR-7150, Volume 3, electrical expert PIRT panel, given the absence of secondary fires or any other fire precursors and the unique combination of low-probability events needed for their occurrence, concluded that secondary fires