United States Nuclear Regulatory Commission - Protecting People and the Environment

Cable Insulation Resistance Measurements Made During Cable Fire Tests (NUREG/CR-6776, SAND2002-0447P)

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Publication Information

Manuscript Completed: April 2002
Date Published:
June 2002

Prepared by:
Francis J. Wyant and Steven P. Nowlen

Sandia National Laboratories
P.O. Box 5800
Albuquerque, New Mexico 87185-0744
Operated by Sandia Corporation

H.W. Woods, NRC Project Manager

Prepared for:
Division of Risk Analysis and Applications
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

NRC Job Code Y6037

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An insulation resistance diagnostic system was recently developed and exercised during a series of cable fire tests sponsored by the commercial U.S. nuclear industry. This insulation resistance measurement system was able to identify and quantify the changes in insulation resistance occurring between the separate conductors and the conductors to ground in cable bundles as they were being exposed to fires. Eighteen separate fire tests were conducted during the period January through May 2001 and included a variety of cable and fire exposure conditions. The insulation resistance measurement system was operated at a 120 VAC input to the conductors for 14 of those test runs and at 100 VDC input for three of the runs. One test was run with the insulation resistance measurement system providing 24 VDC to two separate instrument cables being exposed to the fire. The results obtained by the insulation resistance measurement system during these tests showed that cables will fail during a fire in one of three ways: by internal shorting of the conductors in a multiconductor cable, by shorting of the conductors in different cables bundled together, or by individual conductors shorting to ground. No incidents of fire-induced open circuits were found to have occurred during any of these tests. A mockup of a simple 4 to 20 mA DC current loop instrument circuit was included in six of the later tests. The intent was to investigate the potential of misleading or loss of instrument indication from fire damage to the signal cable. This report provides an analysis of the insulation resistance data and current loop results for the 18 tests run in the industry test program.

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