United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 91-81: Supplement 1: Switchyard Problems that Contribute to Loss of Offsite Power

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

                                 May 19, 1994

                                             CONTRIBUTE TO LOSS OF OFFSITE     


All holders of operating licenses or construction permits for nuclear power


The U.S. Nuclear Regulatory Commission (NRC) is issuing this supplement to
Information Notice (IN) 91-81, "Switchyard Problems That Contribute To Loss Of
Offsite Power," dated December 16, 1991, to alert addressees to a possible
zener diode failure that could cause false operation in stuck breaker failure
unit (SBFU) relays and certain (SA-1) generator differential relays.  The SBFU
and differential relays in question were manufactured by Westinghouse Electric
Corporation (Westinghouse).  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.


The original information notice (IN 91-81) described an event on April 23,
1991, during which the Vermont Yankee Nuclear Power Station (Vermont Yankee)
experienced a loss of offsite power when the 345-kV and 115-kV circuit
breakers in the switchyard opened.  An NRC augmented inspection team sent to
investigate this event determined that zener diodes installed in the SBFU
relays failed due to a voltage spike in the direct current control system,
causing the 345-kV and 115-kV circuit breakers to open.  

In a separate event on October 22, 1984, at Vermont Yankee, SA-1 relays used
in a diesel generator protection circuit failed and caused the diesel
generator breaker to trip and lock out.  The failure of the SA-1 relays was
attributed to the shorting of zener diodes.


In both instances, Vermont Yankee, with the help of the vendor, installed a
redesigned curcuit, eliminating the zener diodes that were vulnerable to
shorting.  Similar vulnerable zener diodes could still be in use in the SBFU  

9405160196.                                                      IN 91-81, Supp. 1
                                                      May 19, 1994
                                                      Page 2 of 2

relays protecting switchyard circuit breakers and SA-1 relays in safety-
related circuit breakers at other operating reactors.  The failure of these
zener diodes could result in failures that affect the availability of either
the offsite or onsite power source that would be required during normal plant
operation or under accident conditions.  

In 1990, Westinghouse sold its relay division, which manufactures the SBFU and
SA-1 relays, to ASEA Brown Boveri (ABB).  ABB, which now has the 10 CFR 
Part 21 reporting responsibility, is not certain that all potentially affected
licensees have been alerted to the problem with the zener diodes.  The relay
division of ABB mailed the attached product advisory letter (PAL 94-98) to its
known customers.  The NRC is forwarding PAL 94-98, without attachments, as an
attachment to this information notice to ensure that all nuclear power plants
using these relays are aware of the problem.

Related Generic Communications

The NRC has issued the following generic communications related to this

  1.  Information Notice 83-63, "Potential Failures of Westinghouse Electric
      Corporation Type SA-1 Differential Relays," September 26, 1983

  2.  Information Notice 83-63, Supplement 1, February 15, 1984

This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
the technical contact 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 contact:  Kamalakar R. Naidu, NRR
                    (301) 504-2980

1.  ABB Product Advisory Letter PAL 94-98, w/o atts
2.  List of Recently Issued NRC Information Notices

.                                                            Attachment 1
                                                            IN 91-81, Supp. 1
                                                            May 19, 1994
                                                            Page 1 of 2

Dear Customer,

   This Product Advisory Letter (PAL 94-98) has been sent to advise you of a
possible component failure that can cause a false operation in the commercial
grade SA-1 Generator Differential relay and the SBFU Static Breaker Failure
relay.  (Class 1E SA-1 is not affected--see note below).

   The SA-1 relay was first manufactured during 1959.  In 1964 the SA-1 relay
was re-designed with a zener diode (Z3) in parallel with the trip SCR for
additional transient protection.  During the years 1964 to 1974, various false
operations were reported with some other solid-state relays that had similar
thyristor (SCR) trip output circuits with zener protection due to the failure
(shorting) of the zener and as a result, this component was removed from the
production SA-1 in 1974.  In 1984 a false trip of a pre 1974 SA-1 was again
traced to the failure of the zener protection diode with the recommendation to
that user at that time, by Westinghouse, to remove the device.

   While there have been no additional reported false operations due to zener
failure since 1984, ABB believes you should be aware of this problem on any
SA-1 relay you purchased between 1964 and 1974 that may still be in service,
as it is recommended this zener (Z3) diode be removed.  Please refer to the
attached information for the location of zener diode (Z3).

   Note:    The Class 1E SA-1 (S/N 1329D62A01) was designed in 1977 without
            SCR zener protection and is not effected with the problem outlined
            in this PAL.

   In 1991 a false operation of an SBFU Static Breaker Failure relay was
reported.  This false operation was traced to the failure (shorting) of the
SCR transient protection zeners Z1 and Z2 in the SBFU trip module (S/N
899C826G01).  These zener diodes were added to this module in 1970, thus any
SBFU purchased before 1970 is not effected with this problem.  In 1973 the
trip module was re-designed to style number 265C975G01 and these protection
zeners were eliminated.  

   Note:    The SBFU was not manufactured as a Class 1E qualified relay. 
There have not been any additional reported false operations of SBFU's due to
zener failures, but ABB recommends the SBFU module (S/N 899C826G01) be
examined for zeners Z1 and Z2 if the unit was purchased between 1970 and 1973. 
.                                                            Attachment 1
                                                            IN 91-81, Supp. 1
                                                            May 19, 1994
                                                            Page 2 of 2

If this module has Z1 and Z2, it is recommended these devices be removed. 
Please refer to the attached figures for the location of Z1 and Z2.

   If after review of this information you should have any questions, please
contact Russ Gonnam or John Wilson at 305-752-6700 or 305-345-5329 (Fax).

   Note:    The manufacturer of the product involved in this Advisory is
            Westinghouse Electric Corporation.  ABB Power T&D Company Inc.
            acquired from Westinghouse in late December of 1989 that portion
            of Westinghouse's business which manufactures electrical equipment
            similar to that involved in this Advisory.  Although ABB Power T&D
            Company Inc. is a successor to certain businesses of Westinghouse,
            Westinghouse has retained complete liability for products
            manufactured prior to February 14, 1989.  The product which is the
            subject of this Advisory was made by Westinghouse and not ABB
            Power T&D Company Inc.  In providing guidance in the form of this
            Advisory, ABB Power T&D Company Inc. assumes no responsibility
            whatsoever for the product manufactured by Westinghouse.

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