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85

Information Notice No. 93­85: Problems with X­Relays in DB­ and DHBType Circuit Breakers Manufactured by Westinghouse

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555

October 20, 1993

NRC INFORMATION NOTICE 93‐85: PROBLEMS WITH X‐RELAYS IN DB‐ AND DHB‐TYPE

CIRCUIT BREAKERS MANUFACTURED BY WESTINGHOUSE

Addressees

All holders of operating licenses or construction permits for nuclear power

reactors.

Purpose

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

required.

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

relay.

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.
9310140277.

IN 93‐85

October 20, 1993

Page 2 of 3

Discussion

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

trips.

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.

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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

Attachments:

1. Figure 1, X‐Relay Magnetic Core

2. List of Recently Issued NRC Information Notices

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