United States Nuclear Regulatory Commission - Protecting People and the Environment


ACCESSION #: 9701220315



CHARLES H. CRUSE              Baltimore Gas and Electric Company

Vice President                Calvert Cliffs Nuclear Power Plant

Nuclear Energy                1650 Calvert Cliffs Parkway

                              Lusby, Maryland 20657

                              410 495-4455



BGE

                              January 15, 1997



U. S. Nuclear Regulatory Commission

Washington, DC 20555



ATTENTION:     Document Control Desk



SUBJECT:       Calvert Cliffs Nuclear Power Plant

               Unit Nos. 1 & 2; Docket Nos.  50-317 & 50-318

               Failures of General Electric Type AMH-4.76-250 Circuit

               Breakers



REFERENCE:     (a)  Letter from Mr. C. H. Cruse (BGE) to NRC Document

                    Control Desk, dated October 25, 1996, same subject



Enclosed please find a follow-up report to Reference (a). Calvert Cliffs

has experienced two failures of the General Electric type AMH-4.76-250

circuit breaker.  Investigation revealed the breaker's manual trip

paddles had bent, thus preventing proper automatic or manual closure of

the breaker.  We have concluded the bent trip paddles were caused by a

weak trip latch retaining spring.  The weak spring allows the trip paddle

to impact against its buffer pad when the breaker trips.  This impact

causes the trip paddle to bend after repeated breaker open/close cycles.

This problem is being reported in accordance with 10 CFR 21.21 (a)(2).  A

defect has been identified with the trip latch reset springs for these

breakers that represents a potential challenge to the reliability of

these safety-related components to perform their intended safety

function.



Should you have questions regarding this matter, we will be pleased to

discuss them with you.



                              Very truly yours,



CHC/CDS/bjd



Attachment



Document Control Desk

January 15, 1997

Page 2



cc:  D.  A.  Brune, Esquire                  H. J. Miller, NRC

     J.  E.  Silberg, Esquire                Resident Inspector, NRC

     Director, Project Directorate I-1, NRC  R. I. McLean, DNR

     A.  W.  Dromerick, NRC                  J. H. Walter, PSC



ATTACHMENT (1)



10 CFR PART 21 INTERIM REPORT

CONCERNING BENT MANUAL TRIP PADDLES IN

GENERAL ELECTRIC MAGNE BLAST 4-kV CIRCUIT BREAKERS



Calvert Cliffs Nuclear Power Plant, Units 1 and 2



Docket Nos.: 50-317 and 50-318



(i)       Name and Address of Individual Making Notification



          C. H. Cruse, Vice President-Nuclear Energy

          Baltimore Gas and Electric Company

          Calvert Cliffs Nuclear Power Plant

          1650 Calvert Cliffs Parkway

          Lusby, MD 20657



(ii)      Basic Component Affected:



          General Electric Magne Blast Circuit Breakers Type AMH-4.76-250



(iii)     Firm Supplying Components:



          General Electric Nuclear Energy

          6901 Elmwood Avenue

          Philadelphia, PA 19142



(iv)      Nature of the Potential Defect



          During surveillance testing in June and July 1996, the plant

          found two problems with General Electric type AMH-4.76-250

          circuit breakers.  In June, a Low Pressure Safety Injection

          (LPSI) pump breaker failed to close during testing.

          Investigation revealed that there was no gap between the manual

          trip rod and the manual trip lever.  The trip lever was found

          bent.  The bent trip lever prevented the trip latch from fully

          rotating onto the stop pin; thus, there was a smaller than

          normal trip latch "wipe" (i.e., area of contact).  With this

          reduced wipe, there is no guarantee of breaker closure.  The

          trip lever was replaced.



          The function of the manual trip lever is to transmit the forces

          from the manual trip rod to the trip shaft and facilitate

          manual tripping of the breaker.  The function of the manual

          trip rod is to transmit forces from the breakers manual trip

          push button to the manual trip lever.



          In July, another LPSI pump breaker failed to close during

          testing.  In this case, investigation revealed that a support

          "L" bracket attached to the manual trip lever had broken and

          the trip lever was bent, with the same effect as in the first

          event.  The "L" bracket serves to provide additional support

          and stiffness to the manual trip lever.  As the breaker trips,

          the manual trip lever quickly rotates until it comes in contact

          with a buffer pad located on the breaker frame.  This contact

          creates a momentary bending stress in the manual trip lever and

          the "L" bracket.  Each manual trip lever has a reset spring

          that is designed to minimize the force that the trip lever

          contacts the



                                    1



ATTACHMENT (1)



10 CFR PART 21 INTERIM REPORT

CONCERNING BENT MANUAL TRIP PADDLES IN

GENERAL ELECTRIC MAGNE BLAST 4-kV CIRCUIT BREAKERS



          buffer pad, thus reducing the bending stress on the manual trip

          lever.  The breaker was replaced with a spare breaker.  The

          broken parts were removed from the breaker and sent to the

          Baltimore Gas and Electric Company metallurgical laboratory for

          analysis.



          We conducted metallurgical analyses of the broken "L" bracket

          and bent trip levers.  Each of these parts is made of low

          carbon steel.  They appear to have failed due to "strain-aging

          embrittlement," a phenomenon that affects hot-rolled low carbon

          steel parts fabricated by cold bending.  We performed an

          operability evaluation for the rest of the breakers based, in

          part, on these results.



          After the second failure, an inspection program was initiated

          for all 4 kV safety-related breakers to look for the broken "L"

          bracket problem.  Just prior to the start of this inspection

          program, cracks were found in the "L" brackets in two other

          breakers.  The inspection program was accelerated and all

          breakers were checked for the cracking identified on the "L"

          brackets.  During this inspection, two other breakers were

          found with no gap between their manual trip lever and manual

          trip shaft (one was the breaker which had failed in June; its

          replacement lever had also bent).  A method was developed to

          verify operability of the safety-related breakers by insuring a

          gap existed between the manual trip lever and the manual trip

          rod after each breaker operation.  The failed breaker was sent

          offsite to the vendor for detailed testing and root cause

          analysis.



          Based on testing performed, the vendor has concluded that the

          most probable cause was insufficient trip latch reset spring

          force caused by either incorrect or damaged springs originally

          being installed.  Baltimore Gas and Electric Company believes

          that it is also possible the springs suffer from age-related

          degradation.



          We sent the breaker from the July 1996 LPSI pump breaker

          failure to the breaker manufacturer to determine the cause for

          the failure to maintain the gap between the manual trip rod and

          the trip paddle.  A new trip paddle assembly was installed on

          the breaker, and the gap adjusted to 1/4 inch.  The breaker was

          then cycled electronically.



          After 30 operations, the 1/4 inch gap had reduced to 1/16 inch.

          Continued operation showed the gap varied to between 0.015 and

          0.060 inches.  The total number of operations was 200.



          The causes of the gap reduction was determined to be bending of

          the trip paddle.  The cause of the trip paddle bending was its

          impact against the buffer pad (breaker frame) in one direction,

          and then rebounding against the trip rod in the other.



          The trip latch was disassembled, and a new reset spring was

          installed.  The trip paddle to trip rod gap was adjusted to 0.

          115 inches and the breaker was cycled 110 times with no

          resulting change in the gap.  The original spring was then

          tested and found to develop less than the minimum specified

          forces.



          These breakers have no difficulty in opening.  The problem is

          that they could lose their closing ability.  Since Calvert

          Cliffs uses many of these breakers in safety-related

          applications, this could challenge the reliability of safety-

          related components to perform as designed.



                                    2



ATTACHMENT (1)



10 CFR PART 21 INTERIM REPORT

CONCERNING BENT MANUAL TRIP PADDLES IN

GENERAL ELECTRIC MAGNE BLAST 4-kV CIRCUIT BREAKERS



(v)       Date on Which Potential Defect Was Identified



          We determined on August 27, 1996 that this breaker problem

          could potentially represent a challenge to the reliability of

          safety-related components to perform their intended safety

          function.  We issued an interim Part 21 notification on October

          25, 1996, because we suspected this problem was a challenge to

          the reliability of safety-related equipment that would

          ultimately be tied to a defect a non-conforming condition.



(vi)      Number and Location of Components



          Calvert Cliffs has approximately 120 of the General Electric

          type AMH-4.76-250 circuit breakers on site in safety-related

          and non-safety-related applications.



(vii)     Corrective Actions Taken



          We established a team to evaluate and determine the causes of

          the breaker problems after the discovery of the second breaker

          failure in July 1996.



          The vendor has recommended a modification to the breakers to

          prevent additional cases of trip lever bending and "L" bracket

          failures.  We have completed this recommended modification on

          the plant's General Electric Magne Blast 4 kV breakers.  The

          modification includes replacing the trip paddles, support

          bracket and the spring discharge link.  The trip lever material

          was changed from AISI 1005 Carbon Steel to AISI 1018 Carbon

          Steel.  The "L" brackets changed from AISI 1005 steel to

          aluminum.  In addition, the configuration of the parts was to

          be changed.  This modification was designed and tested by the

          vendor to ensure it eliminated the bending problem prior to

          recommendation.  This modification was tested at the vendor,

          and we believe the breakers will continue to be operable with

          this modification installed for the long-term.



          Replacement of the reset spring is not a normal preventative

          maintenance or overhaul activity.  The breakers were not

          designed to have this spring replaced.  Replacement requires a

          modification to the breaker that would include cutting a V-

          notch in an angle support to remove the trip shaft.



          Over the next two to three years, we plan to either replace the

          reset springs during overhauls or replace the breaker.  We are

          confident that the trip paddle modification will maintain these

          breakers in an operable condition until the springs or breakers

          are replaced.



          We have not experienced a similar problem with any other

          breakers at Calvert Cliffs.



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*** END OF DOCUMENT ***

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