United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 86-49: Age/Environment Induced Electrical Cable Failures

                                                            SSINS No.:  6835  
                                                            IN 86-49 

                                UNITED STATES
                           WASHINGTON, D.C. 20555

                                June 16, 1986



All nuclear power reactor facilities holding an operating license (OL) or a 
construction permit (CP). 


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. 


                                                            IN 86-49 
                                                            June 16, 1986 
                                                            Page 2 of 3 


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 

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. 

                                                            IN 86-49 
                                                            June 16, 1986 
                                                            Page 3 of 3 

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 
Page Last Reviewed/Updated Friday, May 22, 2015