United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 96-50: Problems with Levering-In Devices in Westinghouse Circuit Breakers

                                 UNITED STATES
                        NUCLEAR REGULATORY COMMISSION 
                     OFFICE OF NUCLEAR REACTOR REGULATION
                         WASHINGTON, D.C.  20555-0001

                               September 4, 1996

NRC INFORMATION NOTICE 96-50:  PROBLEMS WITH LEVERING-IN DEVICES IN            
                               WESTINGHOUSE CIRCUIT BREAKERS 


Addresses

All holders of operating licenses and construction permits for nuclear power
plants.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees that some 4.16 and 6.9 kV circuit breakers may fail
to close on demand because worn levering-in devices can provide a false
indication that the breaker is completely engaged onto the bus and ready to
close.  The problems affect all 4-15 kV range Porcel-line DHP magnetic circuit
breakers manufactured by Westinghouse Electric Corporation (Westinghouse).  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 are not
NRC requirements; therefore, no specific action or written response is
required.  

Description of Circumstances

On October 19, 1995, during Refuel Outage A1R05 at Braidwood Station Unit 1,
Commonwealth Edison Company (ComEd) discovered that a diesel generator output
breaker (Westinghouse Model 50DHP250) failed to close on demand.  ComEd
determined that the levering-in device on the breaker was worn, permitting the
device to "spin free" before the breaker was fully engaged.

Discussion

When installing a Model DHP breaker, a plant equipment operator first pushes
it into a cell in the switchgear until the rail latch catches and stops
further movement toward the rear of the cell.  At this point the breaker is in
the TEST position and the levering-in device is not yet engaged.  By pressing
down on the rail latch, the operator can push the breaker another small
increment until the levering nut at the rear of the breaker contacts the
levering screw in the cell.  The operator then attaches a manual crank to the
levering shaft at the front of the breaker and turns the crank.  Turning the
crank causes the levering nut to traverse the levering screw, drawing the
breaker into the cell until the breaker stops moving toward the rear of the
cell and the crank and levering shaft spin free, indicating that the breaker
is fully engaged.  Figure 5 is a schematic diagram of the levering-in device.



9608290151.                                                         IN 96-50
                                                         September 4, 1996
                                                         Page 2 of 3


If the levering-in device is worn, the operator receives an erroneous
indication that the breaker is fully engaged and ready to operate, when in
fact the floor tripping mechanism is still engaged.  The floor tripping
mechanism consists of cam plates on the cell floor that lift trip levers on
the underside of the breaker while the breaker is being inserted or removed
from the cell.  With the floor tripping mechanism engaged, the breaker will
not close onto the bus.  The Braidwood licensee subsequently removed the
defective levering-in device and replaced it with a redesigned levering-in
device.

The root cause of the failure was a worn levering shaft key and a worn and
cracked guide tube.  The mating surfaces between the shaft key and the guide
tube were worn and permitted the levering-in device to spin free while the
breaker was still about a quarter inch from being fully engaged on the bus.

An NRC search of industry data revealed that other nuclear power plants have
experienced similar breaker failures because (1) a levering-in shaft was found
to be worn or cracked; (2) the guide tube for the levering-in device was
cracked or broken, with one of its "ears" rounded off and catching under the
guide tube, causing it to bind; and (3) the key on the levering-in device was
found to be rounded off because of wear.

In response to the problems seen at Braidwood, Westinghouse issued the
enclosed Technical Bulletin 96-05-R0, "DHP Breakers: Levering-In Device," on
June 19, 1996. The bulletin explains that the spinning free of the levering
crank and observance of a small space between the face of the breaker and the
steel support barrier of the breaker cubicle are not sufficient to ensure the
breaker is fully engaged.  The best way to check that the breaker is in the
correct position is to actuate the breaker.  When breaker actuation is not
feasible, the Westinghouse bulletin describes a two-step process that the
operator can employ to determine if the breaker is properly positioned.

An analysis of the failures seen in the industry, and discussions with
operators at plants that recently experienced failures, indicate that the
levering-in devices were not maintained properly.  Recommended maintenance
entails cleaning and removing old grease from the device components, and
subsequent application of fresh Westinghouse-recommended grease. Damaged or
worn parts can be detected by inspection and replaced, if necessary, during
routine maintenance, but some plants do not include detailed inspection of the
levering-in device in the maintenance procedure. The vendor manual does not
specifically address inspection and cleaning of the levering-in device.

The Westinghouse technical bulletin recommends that the inspection and
lubrication of the levering-in device be included in the maintenance
procedures.  The levering-in device is designed to insert and withdraw the
breaker 100 times.  Because nuclear plants may subject breakers to numerous
levering operations during initial construction and periodic surveillance
testing, the levering-in devices may require inspection and replacement during
their service life in nuclear applications.

.                                                         IN 96-50
                                                         September 4, 1996
                                                         Page 3 of 3


Westinghouse redesigned the levering-in device in 1982 to extend its service
life.  Figures 1 and 2 in the attached Westinghouse bulletin show the original
and redesigned device installed in the breaker.  Figure 6 is a sketch
illustrating the difference in the guide tubes and levering shafts of the two
designs.  Interface between licensees and vendors in accordance with Generic
Letter 83-28, "Required Actions Based on Generic Implications of Salem ATWS
Events," issued July 8, 1983, can help ensure that information on safety-
related components is complete and up to date.

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


                                          signed by

                                    Thomas T. Martin, Director
                                    Division of Reactor Program Management
                                    Office of Nuclear Reactor Regulation


Technical contacts:  Kamalakar Naidu, NRR             Zelig Falevits, R-III
                     (301) 415-2980                   ((718) 829-9717
                     E-mail: krn@nrc.gov              E-mail: zxf@nrc.gov

                     David L. Skeen, NRR
                     (301) 415-1174
                     E-mail: dls@nrc.gov

Attachments:  Attachment - Westinghouse Technical Bulletin 96-05-R0
              Figure 5 - Schematic of Levering-in Device and Interlock
              Figure 6 - Difference in Original and 1982 Redesigned 
                         Levering-in Devices
Page Last Reviewed/Updated Thursday, November 21, 2013