Information Notice 1986-62, Potential Problems in Westinghouse Molded Case Circuit Breakers Equipped With a Shunt Trip
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118 OKIGfi*AL SSINS NO: 6835UNITED STATESNUCLEAR REGULATORY COMMISSIONOFFICE OF INSPECTION AND ENFORCEMENTWASHINGTON, D.C. 20555July 31, 1986IE INFORMATION NOTICE NO. 86-62: POTENTIAL PROBLEMS IN WESTINGHOUSE MOLDEDCASE CIRCUIT BREAKERS EQUIPPED WITH ASHUNT TRIP
Addressees
- All nuclear power reactor facilities holding an operating license or a con-struction permit.
Purpose
- This notice is to alert recipients to a potentially significant problem involv-ing the failure of shunt trip coils in Westinghouse molded case circuit break-ers (breakers) type LBB 22250 MW. Breaker types LB 22250, HLB 22250, and DA22250 have similar operating mechanisms which can affect the operation of theshunt trip coils. However, to date no failures of the shunt trip coil havebeen reported for these breakers. It is expected that recipients will reviewthis information for applicability to their facilities and consider actions, ifappropriate, to preclude a similar problem from occurring at their facilities.However, suggestions contained in this information notice do not constitute NRCrequirements; therefore no specific action or written response is required.
Description of Circumstances
- On February 6, 1986, an open occurred or was discovered in the shunt trip coil(STC) circuitry which could have resulted in the failure of the associatedbreaker to trip open on an abnormal voltage condition at the Peach BottomReactor. The breaker is identified as an LBB 22250 MW type molded case circuitbreaker manufactured by Westinghouse. The breaker is installed in a transferpanel to protect the 120-V 60 Hertz power supply to the reactor protectionsystem (RPS) from undervoltage, overvoltage and underfrequency conditions.In RPS applications, loss of the STC prevents the breakers from trippingautomatically on abnormal voltage conditions. Tripping of this breaker onabnormal voltage conditions is essential because it interrupts a potentiallydamaging abnormal voltage supply to the RPS relays, scram solenoids, and othersafety-related electronic devices. However, failures of the STC does not, byitself, prevent a reactor scram. Also, the breaker is equipped with a magneticovercurrent trip device that protects the circuits in the event of a fault.The STC in series with a contact is activated through a toggle linkage by thebreaker's moving main contact arms. The failure of the STC occurred when thecontact did not open when the breaker tripped resulting in the overheating of8607280196 IN 86-62July 31, 1986 the STC and ultimately in an open circuit in the shunt trip circuit. If thebreaker had been subjected to an abnormal voltage condition after reset theopen shunt trip circuit would have prevented breaker trip.On January 16 and 24, 1986 the same types of problems were identified in similarbreakers at the Peach Bottom Unit 3 plant. On November 9, 1984, Limerick 1reported an identical failure. It is essential that the STC remain operable toassure the circuit breaker trips in the event of an abnormal voltage condition.Administrative measures have been established to periodically test and verifythe operability of the STC at the Peach Bottom and Limerick nuclear power plants.Discussion:The failures described above have occurred in LBB 22250 MW type breakers with aSTC. Westinghouse supplied the breakers to ASCO Electrical Products, Incorpo-rated who installed them in safety-related RPS power supply monitoring panelsrequired for boiling water reactors (BWRs). LB 22250, HLB 22250, and DA 22250type breakers, which have similar operating mechanisms to LBB 22250, may havebeen supplied to other manufacturers for use in safety-related applications.The performance of the breaker is affected only when used with a STC. The STCis energized by the closure of a normally open contact which is actuated througha toggle linkage by the breaker's moving main contact arms. Westinghousestated that the moving contact arms may be impeded from being fully displacedto the "open" position by excessive material in the rivet which holds thehandle post to the operating mechanism.The 2-pole molded case circuit breaker is equipped with an A contact that isoperated by toggle operating links controlled by the breaker operating handle.The A contact is an auxiliary switch which is open when the breaker is in theopen or tripped position and is closed when the breaker is closed ("ON" posi-tion). This A contact is in series with the 125-V dc STC. When the breaker isclosed, the auxiliary A contact is closed and the STC is ready to receive asignal to trip the breaker if any one of the RPS power monitoring relays sensesan abnormal voltage condition. The STC is not rated for continuous duty andwill overheat and be damaged if subjected to full voltage for more than.a fewseconds.The STC can be made to fail, after the breaker has tripped by either of thefollowing actions or conditions:1. If the operating handle is pushed to the "ON" position without resettingthe breaker, when a trip signal is present. This will not close thebreaker, but will close the A contact, thus continuously energizing theSTC.2. If the previously described interference between the operating handle andthe toggle operating links exist the act of resetting the breaker .ad,before clearing the trip signal, attempting to close the breaker. Thetoggle operating links may jam in the "ON" position, unless the operatinghandle is pushed toward the "OFF" or "RESET" position, again causing theSTC to be continuously energize IN 86-62July 31, 1986 The breaker will trip even if the operating handle is forcibly kept in the"ON" position when a trip signal is received or present if the STC isoperable. However, because the operating handle is in the "ON" position,the contact will not open to disconnect the power supply to the STC.Westinghouse recommends the following tests to ascertain the operability of thebreaker.1. Perform a continuity check on the STC after each breaker operation via thetwo leads exiting the breaker.2. -Operability of the STC protection can be determined by completing thefollowing tests:a. Manually close the breaker and continue to hold the operating handlein the "ON" position.b. Apply the shunt trip rated voltage through the two leads exiting thebreaker until the breaker trips or one second elapses.c. While continuing to hold the handle in the "ON" position, perform acontinuity check of the shunt trip circuit. Continuity indicates amalfunction in the contact. An open circuit indicates that thecontact has performed its intended function.Westinghouse revised the circuit breaker mechanism assembly drawing to specifythe removal of excessive rivet material and prevent recurrence of this problem.No specific action or written response is required by this notice. If you haveany questions regarding this notice, please contact the Regional Administratorof the appropriate NRC regional office or this office.DirectorDivisi of Emergency Preparednessand gineering ResponseOffice of Inspection and EnforcementTechnical Contacts: K. R. Naidu, IE(301) 492-4179James C. Stewart, IE(301) 492-9061
Attachment:
List of Recently Issued IE Information Notices Attachment 1IN 86-62July 31, 1986LIST OF RECENTLY ISSUEDIE INFORMATION NOTICESInformation Date ofNotice No. Subject Issue Issued to86-6186-6086-31SUPl. 186-5986-5886-57Failure Of Auxiliary Feed- 7/28/86water Manual Isolated ValveUnanalyzed Post-LOCA Release 7/28/86PathsUnauthorized Transfer And 7/14/86Loss Of Control Of IndustrialNuclear GaugesIncreased Monitoring Of 7/14/86Certain Patients WithImplanted Coratomic, Inc.Model C-100 and C-101Nuclear-Powered CardiacPacemakersDropped Fuel Assembly 7/11/86Operating Problems With 7/11/86Solenoid Operated Valves AtNuclear Power PlantsReliability Of Main Steam 7/10/86Safety ValvesDelayed Access To Safety- 7/10/86Related Areas And EquipmentDuring Plant EmergenciesCriminal Prosecution Of A 6/27/86Former Radiation SafetyOfficer Who Willfully DirectedAn Unqualified Individual ToPerform RadiographyAll power reactorfacilities holdinga CPAll power reactorfacilities holdingan OL or CPAll NRC generallicensees that possessand use industrialnuclear gaugesAll NRC licenseesauthorized to usenuclear-poweredcardiac pacemakersAll power reactorfacilities holdingan OL or CPAll power reactorfacilities holdingan OL or CPAll PWR facilitiesholding an OL or CPAll power reactorfacilities holdingan OL or CPAll holders of NRClicense authorizingthe possession ofbyproduct, source,or special nuclearmaterial86-5686-5586-54OL = Operating LicenseCP = Construction Permit