United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 91-55: Failures Caused by an Improperly Adjusted Test Link in 4.16 KV General Electric Switchgear

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF NUCLEAR REACTOR REGULATION
                           WASHINGTON, D.C. 20555 

                             September 16, 1991


NRC INFORMATION NOTICE 91-55:   FAILURES CAUSED BY AN IMPROPERLY ADJUSTED 
                                TEST LINK IN 4.16 KV GENERAL ELECTRIC 
                                SWITCHGEAR


Addressees

All holders of operating licenses or construction permits for nuclear power 
reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information 
notice to alert addressees to a problem that could result from the use of an 
improperly adjusted test link in vertical-lift switchgear manufactured by 
the General Electric Company (GE).  It is expected that recipients will 
review this 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 of Circumstances

On September 24, 1990, during an emergency diesel generator (EDG) 
surveillance test, the load shedding and emergency load sequencing (LSELS) 
logic failed to sequence the loads at the Callaway Plant  Unit 1 because the 
LSELS logic did not receive the required permissive signal from a GE 
air-magnetic 4.16 kV EDG output circuit breaker.  The licensee determined 
that the LSELS logic did not receive the permissive signal because the 
interlock clip on the breaker was bent and could not push the operating rod 
of the cell-mounted auxiliary switch far enough to actuate it.  The licensee 
examined 34 similar breakers installed in safety-related applications at 
Callaway and determined that the clips on 32 of them had been deformed.  The 
clips on two spare breakers were not bent because they had not been used.  

Discussion

The permissive signal for the LSELS logic should have come from contacts in 
the auxiliary switch in the EDG output circuit breaker cubicle (cell), 
indicating that the breaker was in the cell and in the closed position.  
However, this signal was not received because the contacts in the auxiliary 
switch did not change state even though the breaker was correctly inserted 
into the cell and was in the closed position.  During subsequent 
troubleshooting, the licensee observed that the switch contacts failed to 
change state because the breaker's interlock clip was bent.  

9109110056 
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                                                       IN 91-55
                                                       September 16, 1991
                                                       Page 2 of 2


The bent clip was unable to transmit sufficient motion to the plunger bolt 
and push the operating rod of the switch far enough to change the state of 
the contacts.  The licensee determined that the clip deformed because an 
improperly adjusted test link was used previously during the operation of 
the breaker in the test position.  The improperly adjusted test link had 
insufficient clearance between its top and the bottom of the auxiliary 
switch housing.  As a result, the plunger did not travel the designed 
distance and the top of the test link struck the switch housing and exerted 
an excessive force on the plunger mechanism and bent the interlock clip.

GE's evaluation of this deviation concluded that this problem could only 
exist where users have optional cell-mounted auxiliary switches and an 
improperly adjusted test link.  Although GE concluded that the test link was 
a maintenance tool which did not directly perform a safety-related function,
they also concluded that the original design of the test link, coupled with 
less than optimum maintenance practices, could result in bent interlock 
clips.  This damage could degrade the safety-related function of the 
auxiliary switch.  As corrective action, GE has developed a modified test 
link which should prevent recurrence of this maintenance problem.  
Addressees may wish to contact GE directly for further information 
concerning the modified test link.

Attachment 1 describes how the test link interacts with other components to 
actuate the auxiliary switch with the breaker placed in the test position.  
This attachment also includes sketches which illustrate the orientation of 
the auxiliary switch and the plunger and illustrate the modified test link.

This information notice requires no specific action or written response.  If 
you have any questions about the information in this notice, please contact 
the technical contact listed below or the appropriate Office of Nuclear 
Reactor Regulation project manager. 




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


Technical contact:  Kamal R. Naidu, NRR
                    (301) 492-0980


Attachments:
1.  Operation of Test Links used in 4.16 kV GE Circuit Breakers
2.  List of Recently Issued NRC Information Notices                
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                                                       Attachment 1 
                                                       IN 91-55 
                                                       September 16, 1991 
                                                       Page 1 of 4


             OPERATION OF TEST LINKS USED IN 4.16 KV GE CIRCUIT 
                                  BREAKERS


Description of Switchgear 

A typical General Electric Company (GE) vertical-lift air-magnetic series 
(4.16, 7.2 or 13.8 kV) switchgear unit consists of a stationary cell and a 
withdrawable (drawout) cubicle containing the circuit breaker.  The line and 
load side (bus or cable) terminations are connected to female receptacles in 
the stationary cell.  These female receptacles mate with the male primary 
"stabs" of the breaker when the drawout cubicle is inserted into the 
stationary cell and the breaker is mechanically lifted into position.  The 
stabs are the primary disconnects.  The switchgear design permits the 
breaker and its drawout cubicle to be withdrawn from the stationary cell to 
an intermediate "test position" from which the breaker can be operated 
without being connected to the bus and unnecessarily operating other 
safety-related switchgear.  

Operation of the Test Link

When the breaker is fully inserted in the cell and lifted vertically into 
the operating position, an interlock clip contacts a plunger bolt through a 
cam.  When the breaker is being closed, the clip, which is bolted to the 
breaker crank-shaft, rotates, pushing the plunger upward by moving the cam.  
The plunger bolt pushes the operating rod of the cell-mounted auxiliary 
switch vertically to change the state of its contacts.  After the circuit 
breaker is placed in the test position, a test link is inserted between the 
plunger bolt and the operating rod to bridge the gap between them to actuate 
the switch when the breaker is closed.  The length of the test link has to 
be properly adjusted so that the depth that the rod is inserted into a 
recess in the test link provides the correct clearance between the top of 
the link and the bottom of the switch housing.  A test link is improperly 
adjusted when this clearance is insufficient to permit the plunger to travel 
its designed distance and causes the top of the test link to strike the 
switch housing before the plunger completes its travel.  In striking the 
switch housing, the link exerts an excessive force on the plunger mechanism 
and bends the interlock clip.  Page 3 of attachment 1 includes an 
illustration of the orientation of an interlock clip in relation to the 
breaker in the operating and test positions with the existing test link.

Page 4 of attachment 1 includes an illustration of the new modified test 
link.  The test link is installed by sliding the open-ended slot "A" over 
the brass bushing "B" on the switch housing and then lowering the socket at 
the bottom end of the link onto the top of the plunger "D."  The length of 
the test link can be adjusted by rotating the hexagonal nut "E" so that 
there is a 1/16-inch gap between the top surface "G" of the test link and 
the bottom of the rod "F."  If the test link is improperly adjusted 
(allowing an insufficient gap) the clip can be bent.
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                                                       Attachment 1 
                                                       IN 91-55 
                                                       September 16, 1991 
                                                       Page 2 of 4


When the circuit breaker operates from the "open" to the "closed" position, 
the test link will move the operating rod approximately 15/16 inch 
vertically and actuate the switch.  When the breaker moves from the "closed" 
to the "open" position, the operating rod will drop down 15/16 inch 
vertically, and both the test link and the plunger will drop down 1 inch 
(15/16 plus 1/16) through the return springs.  However, a deformed clip 
cannot reliably transmit the required 15/16-inch vertical motion to the 
plunger to operate the switch during normal breaker operation (or during 
subsequent tests with the link installed).  Note in the plan view of page 4 
of attachment 1, that while the slot in the operator engages the brass 
bushing, the test link keeper "K" engages the side of the housing.  In this 
position, the test link is secure, and testing may proceed.  

Improper Compensation for Deformed Interlock Clips

Unaware that the clips have been deformed by operating the circuit breaker 
in the test position with an improperly adjusted test link, operators may 
add washers to the plunger to restore the correct gap between the plunger 
and the operating rod.  If enough washers are placed on the plunger, a bent 
interlock clip can remain undetected because it then may not prevent the 
breaker from operating the switch in the test or operating positions.  
However, adding washers to the plunger is intended to be only a minor 
adjustment and not to compensate for a bent clip; such compensation is 
unreliable and not recommended.

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