UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, DC 20555-0001

November 25, 2002

NRC INFORMATION NOTICE 2002-34: FAILURE OF SAFETY-RELATED CIRCUIT

BREAKER EXTERNAL AUXILIARY SWITCHES

AT COLUMBIA GENERATING STATION

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 inform

addressees of failures of circuit breaker external auxiliary switches because of inadequate

maintenance of the associated switchgear. 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 in this information notice are not NRC requirements;

therefore, no specific action or written response is required.

Description of Circumstances

On February 14, 2002, at the Columbia Generating Station, the licensee initiated a required

plant shutdown because of a mechanical problem in an emergency diesel generator (EDG)

output circuit breaker. Although the breaker was capable of closing and connecting the EDG to

its safety bus, its cubicle-mounted auxiliary contacts, called mechanism-operated cell (MOC)

switches, failed to change state as designed. The licensee determined that the MOC switch

failures associated with this breaker and other breakers of this type resulted in failure of various

electrical distribution system control functions, including some non-safety loads not being

removed from the EDG bus following an accident signal.

On June 29, 2001, during a refueling outage at the same plant, after breaker replacement, a

standby service water pump breaker MOC switch failed to reposition during breaker closure. 

This failure recurred on November 19, 2001. A MOC switch contact prevented the pump

discharge valve from opening after a pump start, rendering the service water system and

supported systems, such as EDGs, inoperable.

Discussion

The licensee believes the failure was caused by excessive resistance in the linkage between

the breaker and the MOC switches. The licensee had not performed periodic maintenance on

this linkage in the breaker cell housing. Over time, dirt and dust accumulated and grease

hardened to increase the resistance in the linkage mechanism. 

ML023300015
 IN 2002-34

Page 2 of 2

During the preceding refueling outage, ending in July 2001, the licensee replaced 22 safetyrelated 4160-volt circuit breakers, exchanging older design Westinghouse Type DHP-350 (airmagnetic) breakers with newer design, Westinghouse-dedicated, Cutler Hammer Type

DHP-VR (vacuum replacement) breakers using Sure-CloseTM mechanisms to operate the MOC

switches. 

In the DHP-VR, the Sure-CloseTM mechanism operates the MOC switches independently of the

breaker’s main contacts so that the MOC switches can be operated at a slower speed to avoid

excessive kinetic energy and wear. In addition, the external MOC switch linkage drag won’t

stall the main contacts, even if the linkage is poorly maintained. However, because the design

of the Sure-CloseTM mechanism intentionally limits the force on the linkage, it does not have

sufficient force margin to overcome the excessive drag of poorly maintained MOC switch

linkages (for an explanation of Sure-CloseTM operation, see “DHP -VR SURE CLOSE

Mechanism: A Kinematic System Solution for Medium Voltage Circuit Breaker MOC Auxiliaries

Operation,” U. S. Patent 5,856,643, Ronald E. Vaill, available from Cutler-Hammer Nuclear

Programs).

The Columbia Generating Station experience illustrates that the newer design breakers with the

Sure-CloseTM mechanisms operating the MOC switches depend more heavily on the state of

maintenance of the linkage than do the older design breakers coupled directly to the MOC

switches. Older design breakers will eventually cause damage to poorly maintained MOC

switches and linkages. In severe cases, poorly maintained MOC switches and linkages could

stall the older design breaker main contacts with potentially serious results.

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 project manager from the NRC’s Office of Nuclear Reactor

Regulation (NRR). 

/RA/

William D. Beckner, Program Director

Operating Reactor Improvements Program

Division of Regulatory Improvement Programs

Office of Nuclear Reactor Regulation

Technical contacts: V. Hodge, NRR Steven Alexander, NRR

301-415-1861 301-415-2995

E-mail: cvh@nrc.gov E-mail: sda@nrc.gov

Attachment: List of Recently Issued NRC Information Notices