United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 88-11: Potential Loss of Motor Control Center and/or Switchboard Function Due to Faulty Tie Bolts

                                  UNITED STATES
                          NUCLEAR REGULATORY COMMISSION
                      OFFICE OF NUCLEAR REACTOR REGULATION
                             WASHINGTON, D.C. 20555

                                  April 7, 1988


Information Notice No. 88-11: POTENTIAL LOSS OF MOTOR CONTROL center 
                                  AND/OR SWITCHBOARD FUNCTION DUE TO FAULTY 
                                  TIE BOLTS 

Addressees: 

All holders of operating licenses or construction permits for nuclear power 
reactors. 

Purpose: 

This information notice is being provided to alert addressees to the potential 
for failure of bolts used to splice bus bars in motor control centers and 
switchboards.  If these bolts should fail, a loss of electrical function is 
possible.  The event described highlights the importance of inspecting motor 
control centers and switchboards.  It is expected that recipients will review 
the information for applicability to their facilities and consider actions, as 
appropriate, to preclude similar problems from occurring at their facilities.  
However, suggestions contained in this information notice do not constitute 
NRC requirements; therefore, no specific action or written response is 
required. 

Description of Circumstances: 

During a recent inspection of motor control centers and switchboards, classi-
fied as Class 1E as well as some not classified as 1E, at Brunswick Units 1 
and 2, the licensee, Carolina Power and Light, found that numerous 5/16 inch 
silicon bronze carriage bolts connecting the bus bars had failed.  These bolts 
are used to ensure electric connection and not structural integrity.  The per-
centage of failed bolts approached 35 percent in some inspected equipment.  In 
one case both bolts in a splice had failed and there was evidence of high-
resistance-induced overheating. 

Discussion: 

In 1973, during bus bar installation, the licensee noted many failures of the 
bolts used to splice the bars and connect battery cables.  The licensee dis-
cussed this problem with General Electric, and it was discovered that the 18 
ft-lb torque identified on a drawing was incorrect.  The correct value was 9 
ft-lb.  This drawing had been provided to the architect/engineer, United 
Engineers, and was used during construction in 1973.  In addition, no 
lubricant was specified or used; however, had a lubricant been used, torquing 
to 9 ft-lb would have increased the bolting stress, which could also lead to 
bolt failure.  



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                                                            April 7, 1988 
                                                            Page 2 of 2 

Although the licensee replaced all of the broken bolts at that time, it is not 
known if bolts that did not fail were replaced or merely retorqued.  Records 
that might have provided this missing information were destroyed in a fire. 

After 10 years of power production, the licensee instituted a program to in-
spect the internals of motor control centers and switchboards.  This program 
had a rotating inspection schedule that allowed for 10 percent of the 
equipment to be inspected at each refueling outage.  By 1986, approximately 40 
percent of the equipment had been inspected.  After finding one or two failed 
bolts in each inspected ac motor control center (approximately 4 percent 
failure rate), the licensee concluded that an accelerated inspection program 
was necessary.  The accelerated inspection, which also involved the dc 
switchboards, revealed as many as 19 failed bolts out of 54 bolts in one of 
the dc switchboards, or a failure rate of about 35 percent.  The dc 
switchboards were not part of the initial inspection because they are shared 
between both units and simultaneous outages were required to perform the 
inspection. 

Even though the NRC staff believes that the incorrect torque values were used 
only at Brunswick, because of the potential consequences and similarities in 
electrical distribution systems, it is possible that similar equipment may 
have been improperly installed in other nuclear power plants.  The 
consequences of such improper installation may include total station blackout 
during a seismic event or an electrical fire resulting from localized heating 
at a weakened splice.  The existence of this problem can be determined by 
checking installation instructions, verifying torque values, and/or by visual 
examinations.  Retorqueing of these bolts may not be adequate to ensure their 
continued service; replacement may be necessary. 

No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact the technical 
contact listed below or the Regional Administrator of the appropriate regional 
office. 




                                 Charles E. Rossi, Director
                                 Division of Operational Events Assessment
                                 Office of Nuclear Reactor Regulation 

Technical Contact:  Raymond F. Scholl, NRR
                   (301) 492-1171

Attachment:  List of Recently Issued NRC Information Notices
.                                                            Attachment
                                                            IN 88-11 
                                                            April 7, 1988 
                                                            Page 1 of 1

                             LIST OF RECENTLY ISSUED
                            NRC INFORMATION NOTICES 
_____________________________________________________________________________
Information                                  Date of 
Notice No._____Subject_______________________Issuance_______Issued to________

88-10          Materials Licensees:  Lack    3/28/88        All NRC licensees 
               of Management Controls Over                  authorized to use 
               Licensed Programs                            byproduct 
material. 

87-44,         Thimble Tube Thinning in      3/28/88        All holders of OLs
Supp. 1        Westinghouse Reactors                        or CPs for nuclear
                                                            power reactors 
                                                            that employ a 
                                                            Westinghouse NSSS.

88-09          Reduced Reliability of        3/18/88        All holders of OLs
               Steam-Driven Auxiliary                       or CPs for nuclear
               Feedwater Pumps Caused                       power reactors. 
               by Instability of Woodward 
               PG-PL Governors 

88-08          Chemical Reactions with       3/14/88        All NRC licensees 
               Radioactive Waste                            generating or pro-
               Solidification Agents                        cessing low level 
                                                            radioactive waste.

88-07          Inadvertent Transfer of       3/7/88         All NRC broad 
               Licensed Material to                         licensees and 
               Uncontrolled Locations                       licensees 
                                                            authorized to 
                                                            possess byproduct 
                                                            material as sealed
                                                            sources in 
                                                            teletherapy units 
                                                            or "self-
                                                            contained" 
                                                            irradiators. 

88-06          Foreign Objects in Steam      2/29/88        All holders of OLs
               Generators                                   or CPs for PWRs. 

88-05          Fire in Annunciator Control   2/11/88        All holders of OLs
               Cabinets                                     or CPs for nuclear
                                                            power reactors. 

88-04          Inadequate Qualification      2/5/88         All holders of OLs
               and Documentation of Fire                    or CPs for nuclear
               Barrier Penetration Seals                    power reactors. 
_____________________________________________________________________________
OL = Operating License
CP = Construction Permit 
Page Last Reviewed/Updated Tuesday, November 12, 2013