Information Notice No. 88-38:Failure of Undervoltage Trip Attachment on General Electric Circuit Breakers
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
June 15, 1988
Information Notice No. 88-38: FAILURE OF UNDERVOLTAGE TRIP ATTACHMENT
ON GENERAL ELECTRIC CIRCUIT BREAKERS
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose:
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.
Description:
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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June 15, 1988
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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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June 15, 1988
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- 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
office.
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
manufacture,
distribute or
operate radio-
graphic exposure
devices and source
changers.
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
Combustion
Engineering
designed nuclear
power plants.
_____________________________________________________________________________
OL = Operating License
CP = Construction Permit
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