Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
SSINS No.: 6835
IN 86-49
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
June 16, 1986
Information Notice No. 86-49: AGE/ENVIRONMENT INDUCED ELECTRICAL CABLE
FAILURES
Addressees:
All nuclear power reactor facilities holding an operating license (OL) or a
construction permit (CP).
Purpose:
This notice is intended to describe (1) age/environment failures of
electrical cables that have occurred at San Onofre Nuclear Power Plant, Unit
1, and (2) actions that can be taken to improve in-service cable
reliability. It is expected that recipients will review the information for
applicability to their facilities and consider actions, if appropriate, for
identifying incipient failures before they occur. Suggestions contained in
this information notice do not constitute NRC requirements; therefore, no
specific action or written response is required.
Description of Circumstances:
On November 21, 1985, San Onofre Unit 1 experienced an incident that was
investigated formally by the NRC and described in NUREG-1190. Offsite power
was lost when a transformer was tripped by its differential relays because
of a fault in the cable to the Class 1E 4160-V bus. Further inspection of
the failed cable showed evidence of insulation degradation and arcing
between two phases to the sheath of the cable. The most likely cause of the
cable failure was determined to be temperature-induced accelerated aging and
degradation of the cable insulation. The source of this heat was a bare
high-temperature (400F) feedwater line and pipe flange in the immediate
vicinity of the cable. The thermal insulation had been removed from the pipe
during previous repair of a gasket leak and not replaced.
Southern California Edison Company established a special Cable Evaluation
Task Force which has investigated and tested both 4160 and lower voltage
cables and has identified several other degraded cables that also would have
failed over time. The faulted cable and other cables are being replaced at
San Onofre Unit 1.
8606120767
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IN 86-49
June 16, 1986
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Discussion:
The November 21, 1985 event and subsequent investigations and inspections at
San Onofre Unit 1 indicated a possible weakness in the surveillance and
maintenance of station electrical cables. This weakness is characterized by
lack of adequate monitoring of representative electrical circuits to obtain
indications of changes in cable characteristics over time that would be
indicative of degraded conditions. The lessons learned from this incident
suggest a need to determine whether adequate surveillance and maintenance of
medium voltage cables are being performed.
Proper selection and installation of cables is the basic requirement for
assuring reliability. However, a good understanding of cable characteristics
and the operational and environmental conditions that could affect these
characteristics is a prerequisite for assurance of the long-term reliability
of cable installations. Cables that provide power to the medium voltage
safety buses are particularly vital components of the plant electrical
system.
Many utilities, as well as other industries such as chemical production and
processing, routinely conduct testing of critical cable circuits. Although
these practices vary, the information obtained from such periodic testing is
used to identify degraded cables and the need for corrective action. Timely
corrective action for cables that provide power to Class 1E buses and other
critical equipment would minimize unnecessary power losses, unplanned
shutdowns, and challenges to the safety systems.
The Insulated Cable Engineers Association (ICEA), National Electrical
Manufacturer' s Association (NEMA), and Institute of Electrical and
Electronic Engineers (IEEE) have developed standards which address the
testing of new and in-service cables. These standards include cable tests
such as: insulation resistance measurements, power factor measurements, and
controlled dc overvoltage (high potential). It is suggested that the
applicable industry standards be consulted for performing these tests.
Another important facet of the periodic maintenance and testing program for
cable circuits is the walkdown inspection to identify actual or potential
environmental conditions (heat, water, chemicals, etc.) in the immediate
vicinity of the cables that could adversely affect the cable conditions.
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IN 86-49
June 16, 1986
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No specific action or written response is required by this information
notice. If you have any questions about this matter, please contact the
Regional Administrator of the appropriate regional office or this office.
Edward L. Jordan, Director
Division of Emergency Preparedness
and Engineering Response
Office of Inspection and Enforcement
Technical Contacts: Paul Gill, NRR
(301) 492-8423
Vince Thomas, IE
(301) 492-4755
Attachment: List of Recently Issued IE Information Notices