United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 93-64: Periodic Testing and Preventive Maintenance of Molded Case Circuit Breakers

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

                                August 12, 1993

                                 OF MOLDED CASE CIRCUIT BREAKERS


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 problem of age-related degradation of molded
case circuit breakers (MCCBs) and to provide sources of information on MCCB
periodic testing and preventive maintenance.  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


During special testing in 1992 of 35 spare Westinghouse Type EB and EHB MCCBs
at the Kewaunee Nuclear Power Plant, 10 of the MCCBs tripped late in thermal
trip tests at 300-percent overcurrent and 1 of the 10 failed to trip below the
maximum allowable trip current in an instantaneous magnetic trip test. 
Subsequent testing during the 1992 outage showed a similar percentage of out-
of-tolerance MCCBs.  Because the MCCBs were available for use in safety-
related and fire protection applications, the licensee reported the
circumstances as an item of generic interest in Licensee Event Report (LER)

The licensee attributed the out-of-tolerance trip values to age-related
degradation and infrequent exercising of the MCCBs, some of which had not been
exercised since initial plant startup in 1974.  Sometimes retests were
successful after the breaker was exercised manually, which is one type of
preventive maintenance recommended by manufacturers and industry standards. 
However, without periodic testing, the degraded condition of the MCCBs was not
detected earlier.  The licensee replaced the Type EB and EHB breakers in
service and plans to implement a program at Kewaunee to test and exercise
safety-related MCCBs periodically starting in August 1993.


                                                            IN 93-64
                                                            August 12, 1993
                                                            Page 2 of 3


An NRC diagnostic evaluation team at the Zion Nuclear Station observed that
the licensee had no program for routinely testing MCCBs (U.S. Nuclear
Regulatory Commission, "Diagnostic Evaluation Team Report for Zion Nuclear
Power Station," August 1990).  Many breakers had been in service for more than
20 years without being exercised or tested so the potential existed for
undetected degradation that could impair the performance of safety-related


As a followup to the Zion evaluation, the NRC conducted an industry-wide study
of MCCB performance history, reported in "Review of Operational Experience
with Molded Case Circuit Breakers in U.S. Commercial Nuclear Power Plants"
(June 1992).  The study found that for the analyzed failure modes, most of the
failure rates fell below those estimated generically in The Institute of
Electrical and Electronics Engineers (IEEE) Std 500-1984, "IEEE Guide to the
Collection and Presentation of Electrical, Electronic, and Sensing Component
Reliability Data for Nuclear Power Generating Stations."  The study also noted
that, except for limited cases, there are no general technical specification
requirements for periodic testing and preventive maintenance of safety-related
MCCBs.  Some plant-specific technical specifications require testing certain
MCCBs used only for overcurrent protection for electrical containment
penetrations, but the testing is limited, and not all of the circuits are
safety-related.  The study found that despite manufacturer recommendations and
industry standards, many plants do not perform periodic testing or preventive
maintenance on safety-related MCCBs, and for the few that do, frequency and
procedures differ widely.

In addition, as part of its aging research program, the NRC sponsored a study
reported in March 1992 in NUREG/CR-5762, "Comprehensive Aging Assessment Study
of Circuit Breakers and Relays," that evaluated various inspection,
surveillance, and monitoring techniques and preventive maintenance practices. 
The study found that MCCB preventive maintenance practices (such as manual
exercising), can mitigate the effects of aging and help ensure continued MCCB
reliability.  However, manual exercising alone was not found effective in
detecting or assessing age-related degradation.  Detecting or assessing
degradation, the study found, could only be accomplished through appropriate
periodic testing and monitoring.  Certain standard MCCB tests (such as
individual pole resistance, 300-percent thermal overload, and instantaneous
magnetic trip tests) performed periodically were found effective along with
the additional techniques of infrared temperature measurement and vibration

An example of the industry standards that address periodic testing and
preventive maintenance is IEEE Std 308-1974, "IEEE Standard Criteria for Class
1E Power Systems for Nuclear Power Generating Stations" (endorsed by .

                                                            IN 93-64
                                                            August 12, 1993
                                                            Page 3 of 3

Regulatory Guide 1.32, Revision 2, February 1977).  Its Section 6.3 as well as
Section 7.4.1 of the current (1991) edition of the standard recommend that
periodic tests be performed at scheduled intervals to detect the deterioration
of the equipment and to demonstrate operability of the components that are not
exercised during normal operation.  Other industry standards and publications
that contain recommendations for testing and maintenance of MCCBs include the

American National Standards Institute (ANSI)/IEEE Std 242-1986, "IEEE
Recommended Practice for Protection and Coordination of Industrial and
Commercial Power Systems" 

�ANSI/National Fire Protection Association (NFPA) 70B-1990, "Recommended
Practice for Electrical Equipment Maintenance" 

�National Electrical Manufacturers Association (NEMA) Standard Publication No.
AB 4-1991, "Guidelines for Inspection and Preventive Maintenance of Molded
Case Circuit Breakers Used in Commercial and Industrial Applications"  

Electric Power Research Institute (EPRI) Report NP-7410, "Breaker
Maintenance," Volume 3, "Molded-Case Circuit Breakers," September 1991 

The recommendations of these publications may not be applicable in every
instance, depending on the specific components installed, their functions, and
their environment.

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.

                                    ORIGINAL SIGNED BY

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

Technical contacts:  Sikhindra K. Mitra, NRR
                     (301) 504-2783

                     Stephen D. Alexander, NRR
                     (301) 504-2995

                     Satish K. Aggarwal, RES
                     (301) 492-3829

List of Recently Issued NRC Information Notices.
Page Last Reviewed/Updated Tuesday, November 12, 2013