United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 88-38: Failure of Undervoltage Trip Attachment on General Electric Circuit Breakers

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

                                  June 15, 1988

                                   ON GENERAL ELECTRIC CIRCUIT BREAKERS 


All holders of operating licenses or construction permits for nuclear power 


This information notice is being provided to alert addressees to potential 
problems resulting from the failure of the undervoltage trip attachment on 
General Electric (GE) circuit breakers, Types AK-2-15 and AK-2-25.  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 do not 
constitute NRC requirements; therefore, no specific action or written response 
is required. 


Two failures of GE circuit breakers installed as reactor trip breakers (RTBs) 
have been reported recently.  On March 16, 1988, during a routine monthly sur-
veillance test, the undervoltage device (UVD) on one of four dc reactor trip 
breakers at Three Mile Island Unit 1 (TMI 1) failed.  This UVD was mounted on 
a Model AK-2-15-1 dc breaker.  On March 23, 1988, a similar failure occurred 
at Rancho Seco.  The Rancho Seco licensee reported that the UVD on a GE Model 
AK-2-25 RTB, one of four dc RTBs, failed during a monthly surveillance test.  
The shunt trip test for each of the breakers was performed without any 
problems during these tests.  The UVD and shunt trip device are tested 
independently during monthly surveillances.  The analyses of both failures 
revealed that mechanical binding of the UVD trip linkage had occurred and 
prevented the operation of the breakers.  

Figure 1 shows the normal pre-trip position of the UVD.  On a valid 
undervoltage trip signal, the coil is deenergized, releasing the armature.  
The spring attached between the armature and the UVD frame causes the armature 
to pivot in a clockwise direction between the roller rivet and coil core 
support.  The small circular disk welded to the armature then contacts the 
trip paddle causing it to engage and rotate the circuit breaker's trip bar.  
Figure 2 shows the normal post-trip configuration of the UVD.  

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                                                                 Page 2 of 3 

The investigation conducted by the TMI 1 licensee revealed that the clearance 
between the end of the armature and the trip paddle was such as to allow the 
paddle to become positioned to prevent the armature from moving.  This 
improper clearance resulted from a manufacturing anomaly which produced a trip 
paddle of shorter dimensions than design specifications dictate.  Thus, the 
position held by the trip paddle at the time the UVD coil was deenergized 
impeded the motion of the armature and prevented the breaker's operation. 

The TMI 1 licensee is investigating changes in the preventive maintenance pro-
cess to ensure that the appropriate clearances are maintained for all UVDs 
mounted on RTBs. 

The Rancho Seco licensee's investigation determined that the failure mode of 
the UVD also involved internal binding of the UVD's linkage.  However, the 
root cause of the binding at Rancho Seco differed from that at TMI 1.  The 
Rancho Seco licensee found that the clearance between the armature and trip 
paddle was inadequate.  The licensee also determined that the armature disk 
was improperly welded in a position both vertically and horizontally off 
center of the end of the armature.  This altered the armature/trip paddle 
points of contact from the prescribed configuration shown in Figure 3a to the 
undesirable configuration shown in Figure 3b.  Another factor relating to the 
incident is the lack of adequate clearance between the armature disk and the 
trip paddle in the energized position.  

Because of the mechanical oscillation inherent in an ac powered electromagnet, 
the armature of the UVD vibrates continuously.  Since the armature disk and 
trip paddle were in contact, these vibrations eventually created a groove in 
the upper portion of the armature disk and in the end of the trip paddle,  As 
long as the grooves in the disk and trip paddle were in a certain 
configuration, the device operated properly.  However, when the disk/trip 
paddle groove relationship was disturbed (side movement), the disk would bind 
on the trip paddle resulting in failure of the UVD to trip the breaker (Figure 
4).  The affected breaker was removed from service and a spare breaker was 
installed.  All RTBs were taken out and visually inspected for the anomalies 
identified above; no abnormal UVDs were identified.  The licensee has also 
modified its preventive maintenance procedure to ensure that all known 
problems with these breakers have been addressed to preclude the recurrence of 
these problems. 

Previous Generic Communications: 

-  IE Bulletin No. 79-09, "Failures of GE Type AK-2 Circuit Breaker in Safety 
   Related Systems." 

-  IE Bulletin No. 83-04, "Failure of the Undervoltage Trip Function of 
   Reactor Trip Breakers." 

-  IE Bulletin No. 83-08, "Electrical Circuit Breakers With An Undervoltage 
   Trip Feature in Use in Safety-Related Applications Other Than the Reactor 
   Trip System." 
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                                                                 Page 3 of 3 

-  Information Notice No. 83-18, "Failures of the Undervoltage Trip 
   Function of Reactor Trip System Breakers." 

-  Information Notice No. 85-58, (and supplement), "Failure of a General 
   Electric Type AK-2-25 Reactor Trip Breaker." 

A GE Service Advice Letter (SAL) 175 (CPDD) 9.3 (April 15, 1983) set forth 
maintenance procedures to forestall the types of failures that were known 
prior to issuance of the SAL. 

No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact the technical con-
tact listed below or the Regional Administrator of the appropriate regional 

                                 Charles E. Rossi, Director 
                                 Division of Operational Events Assessment 
                                 Office of Nuclear Reactor Regulation 

Technical Contact:  E. N. Fields, NRR
                    (301) 492-1173 

Attachment:  List of Recently Issued NRC Information Notices
.                                                            Attachment
                                                            IN 88-38 
                                                            June 15, 1988 
                                                            Page 1 of 1

                             LIST OF RECENTLY ISSUED
                            NRC INFORMATION NOTICES 
Information                                  Date of 
Notice No._____Subject_______________________Issuance_______Issued to________

88-37          Flow Blockage of Cooling      6/14/88        All holders of OLs
               Water to Safety System                       or CPs for nuclear
               Components                                   power reactors. 

88-36          Possible Sudden Loss of RCS   6/8/88         All holders of OLs
               Inventory During Low Coolant                 or CPs for PWRs. 
               Level Operation                              

88-35          Inadequate Licensee Performed 6/3/88         All holders of OLs
               Vendor Audits                                or CPs for nuclear
                                                            power reactors. 

88-34          Nuclear Material Control      5/31/88        All holders of OLs
               and Accountability of                        or CPs for nuclear
               Non-Fuel Special Nuclear                     power reactors. 
               Material at Power Reactors 

87-61,         Failure of Westinghouse       5/31/88        All holders of OLs
Supplement 1   W-2-Type Circuit Breaker                     or CPs for nuclear
               Cell Switches                                power reactors. 

88-33          Recent Problems Involving     5/27/88        All Agreement 
               the Model Spec 2-T                           States and NRC 
               Radiographic Exposure                        licensees 
               Device                                       authorized to 
                                                            distribute or 
                                                            operate radio-
                                                            graphic exposure 
                                                            devices and source

88-32          Promptly Reporting to         5/25/88        All NRC material 
               NRC of Significant                           licensees. 
               Incidents Involving 
               Radioactive Material 

88-31          Steam Generator Tube          5/25/88        All holders of OLs
               Rupture Analysis                             or CPs for 
               Deficiency                                   Westinghouse and 
                                                            designed nuclear 
                                                            power plants. 
OL = Operating License
CP = Construction Permit 
Page Last Reviewed/Updated Friday, May 22, 2015