Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
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.
8804010256
. IN 88-11
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
