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:
ovided that the subject analysis incorporates the anticipated temperature rise caused by the self-heating effects of installed power cables with the fire test results. B-3.2 Cable Insulation Tests The nuclear industry uses two principal materials—thermoplastics and thermosetting polymeric materials—as cable insulation and cable jackets. A thermoplastic material can be softened and resoftened by heating and reheating. Conversely, thermosetting cable insulation materials cure by chemical reaction and do not soften when heated. Under excessive heating, thermosetting insulation becomes stiff and brittle. Electrical faults may be caused by softening and flowing of thermoplastic insulating materials at temperatures as low as 149 degrees C (300 degrees F). Thermosetting electrical conductor insulation materials usually retain their electrical properties under short-term exposures to temperatures as high as 260 degrees C (500 degrees F). Insulation resistance (Megger) tests indicate the condition of the cable insulation resistance, whereas the high-potential (Hi-Pot) test ensures that the cable has sufficient dielectric strength to withstand the applied rated voltage. A cable insulation failure usually results from two breakdown modes. One failure mode is excessive dielectric loss resulting from low insulation resistance, and the other failure mode is overpotential stress caused by loss of dielectric strength of the insulation material. To provide reasonable assurance that the cables will function during and after fire exposure, Megger tests need to be performed before the fire test, at multiple time intervals during the fire exposure (i.e., every 20 minutes during the 1-hour fire test and every hour during the 3-hour fire test) for instrumentation cables only and immediately after the fire endurance test to assess the cable insulation resistance levels. This testing will ensure that the cables will maintain the insulation resistance levels necessary for proper