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HomeNRC LibraryDocument CollectionsGeneric CommunicationsInformation Notices1993

# Information Notice No. 93-91: Misadjustment Between General Electric 4.16-KV Circuit Breakers and their Associated Cubicles

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

                               December 3, 1993

NRC INFORMATION NOTICE 93-91:  MISADJUSTMENT BETWEEN GENERAL ELECTRIC 4.16-KV 
                               CIRCUIT BREAKERS AND THEIR ASSOCIATED CUBICLES

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 the potential for multiple common-cause failures
of logic functions due to misadjustment between General Electric (GE) 4.16-kv
Magne-Blast circuit breakers and their associated cubicles.  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

During surveillance testing at the Duane Arnold Energy Center on
September 16, 1993, the "B" standby diesel generator output breaker (1A411) to
essential bus 1A4 failed to close after simulation of a loss of offsite power
(LOOP) and loss of coolant accident (LOCA) signal.  During normal plant
operation, the normally closed feeder breaker (1A402) from the startup
transformer and the normally open feeder breaker (1A401) from the standby
transformer provide essential bus 1A4 with two offsite sources of power.  One
of the "b" contacts (closed when the breaker is open) from each offsite power
feeder breaker stationary auxiliary switch (mounted in the breaker cubicle) is
used in the diesel generator output breaker automatic close-permissive circuit
to indicate whether the two offsite power feeder breakers are open or closed. 
A closed indication from either breaker 1A401 or 1A402 will prevent the diesel
generator output breaker from closing automatically (unsynchronized) onto
essential bus 1A4.  Breakers 1A401, 1A402, and 1A411 are GE Magne-Blast types.

Troubleshooting revealed that the auto-closing logic for the diesel generator
output breaker was not being completed because a "b" contact from breaker
1A401 was open, indicating that the breaker was closed when the breaker was
actually open.  When the breaker 1A401 is open and in its fully elevated
operating position in the cubicle, there is supposed to be a small gap or
clearance between the breaker plunger and the stationary auxiliary switch 

9311190456.

                                                            IN 93-91
                                                            December 3, 1993
                                                            Page 2 of 4

operating rod (see Attachment 1, Figure 2) to ensure positive closure of all
"b" contacts in the stationary auxiliary switch.  However, the plunger of
breaker 1A401 was found to be actually pushing its stationary auxiliary switch
operating rod up about 4.8 mm [3/16 inch] with the breaker open.  This
apparently occurred when the breaker was last elevated into its operating
position which then opened one or more "b" contacts (including the one in the
diesel generator breaker control circuit) from their normally closed position. 
The misalignment rendered the "B" diesel generator inoperable from 
July 21, 1993, to September 25, 1993, because it was not capable of
automatically closing onto the 1A4 bus.

Upon investigation, the licensee determined that it had performed routine
preventive maintenance on breaker 1A401 (a vertically racking, GE model 
AM-4.16-350-2H) on July 21, 1993, which consisted essentially of lowering the
breaker to the bottom of the cubicle, inspecting and lubricating it, and
cycling the breaker after elevating it to its test position.  However, cycling
the breaker in the test position does not operate the stationary auxiliary
switch unless a special test link is fitted.  Therefore, this action alone
does not verify the various control functions performed by the stationary
auxiliary switch contacts.  The "B" diesel generator monthly tests were
completed satisfactorily during the operating cycle, but these tests use a
synchronized manual closure of the diesel generator output breaker, which
bypasses the breaker automatic close permissive (dead bus) circuit.  Even
though the once-per-operating-cycle integrated LOOP/LOCA test detected the
failure, the potential existed for the "B" diesel generator to be inoperable
for an entire operating cycle, or when required during an outage.  The
periodic routine preventive maintenance performed on breaker 1A401, which may
have resulted in the breaker misadjustment and opening of a stationary
auxiliary switch contact, was not scheduled in conjunction with a surveillance
test (such as the integrated LOOP/LOCA test) that would verify proper
operation of the auxiliary switch contacts.

Discussion

Misadjustment between the stationary auxiliary switch and the breaker plunger
on 4.16-kv breakers has the potential to cause multiple common-cause failures
of many different logic functions that rely on these auxiliary switch
contacts.  The circuit breaker plunger to auxiliary switch operating rod
adjustment may be affected by breaker maintenance, switchgear maintenance,
differences in operating plunger height of interchangeable breakers,
differences in breaker cubicles and breaker position switches, and non-
repeatability in breaker height when racking in breakers.

During breaker maintenance on July 21, 1993, the licensee for the Duane Arnold
plant checked and adjusted the height between the breaker lifting rail and the
operating plunger in accordance with the applicable GE breaker technical
manual (GEK-7320F, Revision 2/80, "Magne-Blast Circuit Breakers Types AM-4.16-
350-2C and -2H").  However, the licensee did not verify the open breaker
plunger-to-operating-rod clearance of 0 to 3.2 mm [0 to 1/8 inch] as
recommended in the applicable GE switchgear technical manual (GEH-1802,
"Instructions - Metal-Clad Switchgear Types M26 and M36 for Magne-Blast 
.

                                                            IN 93-91
                                                            December 3, 1993
                                                            Page 3 of 4

Circuit Breakers Types AM-4.16 and AM-13.8").  In order to ensure a minimum
gap for positive "b" contact closure, the manufacturer (now called the GE
Specialty Breaker product department) provided additional guidance in its 
Service Advice Letter (SAL) 073-323.1, issued February 1, 1978, specifying a
gap of 1.6 mm [1/16 inch] to 3.2 mm [1/8 inch].  This SAL also provides the
recommended sequence of adjustments. 

Further conversations between the licensee and GE determined that there are
additional dimensional checks (not contained in either the breaker or
switchgear technical manual) which would provide additional assurance of
proper auxiliary contact operation after breaker maintenance.  These
additional dimensional checks for a closed breaker (see Attachment 1, 
Figure 1) are:  

� 7.9 � 1.6 mm [5/16 � 1/16 inch] from the top of the circuit breaker plunger  
 nut to the bottom of the auxiliary switch operating rod casing.  
� All "a" contacts are closed with at least 23.8 mm [15/16 inch] travel of the 
 operating rod. 

NOTE: Total specified travel of the plunger is 27 � 1.6 mm 
      [1 1/16 � 1/16 inch].  If the total plunger travel is set at the minimum 
     specified value of 25.4 mm [1 inch] and the plunger-to-operating-rod gap  
     is set at the maximum specified value of 3.2 mm [1/8 inch], the amount    
     of travel of the operating rod would only be 22.2 mm [7/8 inch], or less  
     than the required 23.8 mm [15/16 inch] necessary to fully ensure closure  
     of the "a" auxiliary contacts when the breaker closes.  

For an open breaker (see Attachment 1, Figure 2) the additional dimensional
check is:

�  The operating rod collar should rest on the operating rod casing, and the   
   operating rod should not be displaced by the plunger of an open breaker.

GE stated that it will issue a SAL to clarify the instructions and dimensions
contained in the previous publications.  Meanwhile, licensee inquiries on this
subject may be directed to GE Nuclear Energy Power Delivery Services in 
King of Prussia, Pennsylvania (Phone:  215-992-6049/Fax:  215-992-6191.)

Related Generic Communications

� NRC Information Notice 91-55, "Failures Caused by an Improperly Adjusted     
  Test Link in 4.16 KV General Electric Switchgear," dated 
  September 16, 1991.
.

                                                            IN 93-91
                                                            December 3, 1993
                                                            Page 4 of 4

No specific action or written response is required by this information notice. 
If you have any questions regarding this matter, please contact one of the
technical contacts listed below or the appropriate Office of Nuclear Reactor
Regulation (NRR) project manager.

                                     /S/'D BY BKGRIMES

                                     Brian K. Grimes, Director
                                     Division of Operating Reactor Support
                                     Office of Nuclear Reactor Regulation

Technical contacts:  Jay A. Hopkins, RIII
                     (319) 851-5111

                     David S. Butler, RIII
                     (708) 790-5796

                     Stephen D. Alexander, NRR
                     (301) 504-2995

Attachments:
1.  Magne-Blast Circuit Breaker Plunger and 
      Stationary Auxiliary Switch Adjustments
2.  List of Recently Issued NRC Information Notices
.
```

*Page Last Reviewed/Updated Tuesday, November 12, 2013*

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