United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 93-85: Problems with X-Relays in DB- and DHB-Type Circuit Breakers Manufactured by Westinghouse

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

                               October 20, 1993



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 the possible failure of the 52X-relay (X-relay)
to reset in DB- and DHB-type Westinghouse Electric Company (Westinghouse)
circuit breakers, thus preventing them from reclosing on demand.  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

Description of Circumstances 

On June 27, 1993, power to a motor control center was lost at the Haddam Neck
nuclear power plant.  Connecticut Yankee Atomic Power Company, the licensee,
determined that the possible root cause for this event was a failure of the
X-relay (anti-pump relay) to reset to the de-energized position in a
Westinghouse DB-25 circuit breaker.  

Previously, between July 1 and August 24, 1984, the licensee reported five
incidents in which one DB-25 and four DHB-250 breakers failed to close when
required.  All five of those failures were attributed to malfunctions of the
X-relay.  The licensee believed that dirt had caused the problem and
instituted a program for cleaning the stationary and moving cores of the

On June 11, 1991, and on January 28, 1992, Duke Power Company reported that,
at the Oconee Nuclear Station, X-relays failed to reset and prevented the
DB-25 breakers from closing in the Keowee hydroelectric generator field and
field flashing circuitry.  (The Keowee hydrostation supplies emergency power
to the Oconee station.)  The licensee modified the anti-pump function with an
electrical scheme that does not require the X-relay.


                                                             IN 93-85
                                                             October 20, 1993  
                                                             Page 2 of 3


The X-relay is usually denoted by the symbol 52X in the electrical control
circuit schematic diagram for the breaker.  On receipt of a signal to close
the breaker, the X-relay energizes and one set of its normally open contacts
closes to enable momentary energizing of the breaker closing coil.  After the
breaker closes, the same set of X-relay contacts opens to deenergize the
closing coil even though the X-relay remains energized by the close signal. 
If the close signal is still present after the breaker trips, the X-relay
serves to inhibit repeated closure attempts until the close signal is removed. 
Thus, it provides anti-pump protection to the breaker by preventing repeated
breaker closure attempts when a standing closure signal exists after a breaker

The armature assembly of an X-relay (see Figure 1) fits inside a brass sleeve
and is surrounded by the electromagnetic coil (different from the breaker
closing coil) of the relay.  When this coil is energized, the plunger (moving
core) is drawn up towards the top cap piece (stationary core) of the assembly
and the latch arm operates the relay contacts.  In its uppermost position, an
air gap is maintained between the bullet-shaped top of the plunger and the
cavity in the cap piece, with the shoulder of the plunger mating with the lip
on the cap piece.  When the coil is de-energized, the moving core is designed
to fall, by gravity, into its lowest position.  The failure of the moving core
to fall to its lowest position is the possible failure mode of interest at
Haddam Neck.

Westinghouse and the licensee believe that residual magnetism associated with
continuous energization of the relay or mechanical adherence between the two
parts are probable causes for the plunger failing to fall.  Westinghouse is
testing a design enhancement that uses a brass spacer between the moving and
stationary cores to create a controlled air gap.  Westinghouse plans to issue
a Nuclear Safety Advisory Letter to notify users of this problem.  The
licensee is evaluating a design modification for the X-relay function in the
motor control center.  


                                                             IN 93-85
                                                             October 20, 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:  Kamalakar Naidu, NRR
                     (301) 504-2980

                     Frederick H. Burrows, NRR
                     (301) 504-2901
1.  Figure 1, X-Relay Magnetic Core
2.  List of Recently Issued NRC Information Notices
Page Last Reviewed/Updated Tuesday, November 12, 2013