Information Notice No. 93-87: Fuse Problems with Westinghouse 7300 Printed Circuit Cards

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                            WASHINGTON, D.C. 20555

                               November 4, 1993

                               CIRCUIT CARDS 


All holders of operating licenses or construction permits for nuclear power


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to recent failures of Westinghouse 7300 printed
circuit cards related to the use of incorrect fuses.  It is expected that
recipients will review the information for applicability to their facilities
and consider actions, as appropriate, to avoid similar problems.  However,
suggestions contained in this information notice are not NRC requirements;
therefore, no specific action or written response is required.

Description of Circumstances

On April 29, 1992, at the Catawba Nuclear Station, a capacitor failed in a
Westinghouse type NAL printed circuit card.  The card was used in the control
circuit for the reactor coolant system power operated relief valves (PORVs). 
The fault resulted in the loss of the PORV automatic operation function on two
of the three PORVs (although operation in the manual mode was not affected),
and no alarm was initiated to alert the operator to the card failure.  The
licensee, Duke Power Company, found that the card was protected with a 
5.0 ampere (A) fuse, rather than the specified 0.5 A fuse.  Had the correct
fuse been installed, it would have cleared the fault and generated an alarm.

On April 5, 1993, at the V.C. Summer Nuclear Power Station, a fuse blew in a
type NAC card in the component cooling water system.  The card was used for
annunciation and safety-related automatic action for low surge tank level. 
The 0.63 A fuse blew with a card load of about 1.0 A, which is within the
normal range if all of the card outputs are energized.  Thus the blown fuse
was the cause of the card failure, rather than a protective response to
another fault.  The licensee, South Carolina Electric & Gas Company, found
that most type NAC cards have a 2.0 A fuse, but the installed card had the
0.63 A fuse specified in an outdated drawing (Revision 6).


Type NAL signal comparator cards are used extensively in the Westinghouse 7300
series solid state protection system and process control cabinets.  Type NAC
signal comparator cards are not used in the solid state protection system, but


                                                            IN 93-87
                                                            November 4, 1993
                                                            Page 2 of 3

are used in the process control cabinets, particularly where Westinghouse
provided balance-of-plant control equipment.  

Catawba personnel inspected the 7300 circuit cards in both units and in the
warehouse stock (approximately 1000 cards) and found 150 discrepancies in fuse
size, compared with the values specified in the instruction book.   Most
discrepancies were small (e.g., 0.5 A required vs. 1.0 A installed), but 27 of
the MA and MB fuses on type NAL cards were found to be 5.0 A fuses, rather
than the specified 0.5 A fuses.  After the Catawba inspection, Westinghouse
prepared a list of correct fuse values for the various 7300 series card types. 
The list was subsequently issued by Westinghouse to all customers early in
1993 as Infogram IG 93001.

The Westinghouse drawing for a circuit card may contain several groups and
typically has undergone several revisions.  Westinghouse provides each
licensee with an instruction book for each card type, which includes the
schematic diagram and bill of material for the drawing revision in effect at
that time.  (The Catawba manual for the NAL card type, for example, is dated
September 1976.)  When a licensee returns a failed card for repair,
Westinghouse routinely updates the card to the then-current drawing revision. 
The documentation provided typically identifies that an update was performed,
and identifies the new part numbers, but does not otherwise describe the new
parts.  Thus the licensee is advised that the card was changed, but does not
know the values or other details of any new parts.  Although such changes are
relatively minor, they can affect component values.

The Catawba inspection found nine card types with discrepant fuse values: 
NAL, NCD, NCH, NCT, NLP, NMA, NRA, NSA, and NSC.  The Infogram identified six
of these types as experiencing drawing revisions that included fuse changes. 
The reason for the discrepant 5.0 A fuses on NAL cards at Catawba is not
clear, but Catawba sends cards to the vendor rather than repairing onsite.

The type NAC card that failed at Summer used a fuse specified only for an
outdated revision of the card.  According to Westinghouse, at drawing 
Revision 5, fuse M121 had a value of 2.0 A and would not protect certain
lower-rated components on the card.  Accordingly, Revision 6 was generated to
change to a 0.6 A fuse that would protect all of the components.  At the same
time, Revision 7 was initiated to upgrade component ratings and return the
fuse value to 2.0 A.  These changes took place in late 1973, whereas the
Summer cards were not shipped until May 1978.  Westinghouse Infogram IG 93001
specifies the "correct" fuse values for the seven revisions.  However, it does
not warn that in Revision 5 the fuse may not protect board-mounted components,
or that the Revision 6 card may experience a blown fuse during normal plant

                                                            IN 93-87
                                                            November 4, 1993   
                                                            Page 3 of 3

This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
one of the technical contacts listed below or the appropriate Office of
Nuclear Reactor Regulation (NRR) Project Manager.

                                     /s/'d by BKGrimes

                                     Brian K. Grimes, Director
                                     Division of Operating Reactor Support
                                     Office of Nuclear Reactor Regulation

Technical contacts:  George T. MacDonald, RII
                     (404) 331-5576

                     Richard C. Wilson, NRR
                     (301) 504-3220

List of Recently Issued NRC Information Notices

Page Last Reviewed/Updated Thursday, March 25, 2021