United States Nuclear Regulatory Commission - Protecting People and the Environment

Resolution of Generic Issues 48, "Interlocks and LCOs for Class 1E Tie Breakers" Pursuant to 10 CFR 50.54(f) (Generic Letter 91-11)


July 18, 1991 


TO:       ALL HOLDERS OF OPERATING LICENSES 

SUBJECT:  RESOLUTION OF GENERIC ISSUES 48, "LCOs FOR CLASS 1E VITAL
          INSTRUMENT BUSES," AND 49, "INTERLOCKS AND LCOs FOR CLASS 1E
          TIE BREAKERS" PURSUANT TO 10 CFR 50.54(f) (GENERIC LETTER 91-11) 

The staff of the U.S. Nuclear Regulatory Commission (NRC) has completed the 
evaluation of Generic Issues (GIs) 48 and 49 as part of the resolution of 
GI-128, "Electrical Power Reliability."  GI-48 and GI-49 address vital ac 
buses and tie breakers that connect redundant safety-related buses.  
Additional details are provided in NUREG/CR-5414, "Technical Findings for 
Proposed Integrated Resolution of Generic Issue 128, Electrical Power 
Reliability." 

As a result of its evaluation, the staff concludes that unless licensees 
provide adequate justification that such provisions are not needed at their 
specific facilities, all licensees should have appropriate procedures to 
fulfill the following requirements: 

      1.  Limit the time that a plant is in possible violation of the 
          single-failure criterion with regard to the Class 1E vital 
          instrument buses and tie breakers, 

      2.  Require surveillances of these components, and

      3.  Ensure that, except for the times covered in Item (1), the plant 
          is operating in an electrical configuration consistent with the 
          regulations and its design bases.

The enclosure to the generic letter provides further guidance. 

The staff requires, pursuant to Section 50.54(f) of Title 10 of the Code of 
Federal Regulations (10 CFR) and Section 182 of the Atomic Energy Act, that 
you provide the NRC with certification, within 180 days of the receipt of 
this letter, that you have either implemented the appropriate procedures 
conforming to the guidance provided in the enclosure to this generic letter 
or have prepared justification that such procedures are not needed.  This 
certification shall be submitted to the NRC and signed under oath or 
affirmation.  You should retain any justification for not including such 
procedures in accordance with the document retention programs at your 
facility. 

The actions requested in this generic letter represent new staff positions 
for some licensees; and thus, this request is considered a backfit in 
accordance with NRC procedures.  Because the electrical configurations and 
surveillance procedures at some plants may not satisfy established 
regulatory requirements, this backfit ensures that facilities comply with 
existing requirements.  Therefore, the staff did not perform a full backfit 
analysis. Instead, an evaluation of the type discussed in 10 CFR 
50.109(a)(6) was performed.  The report of this evaluation will be available 
in the Public Document Room along with the minutes of the 163rd meeting of 
the Committee to Review Generic Requirements. 


9107160296
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Holders of Operating Licenses        - 2 -



As the resolution of these generic issues is considered a compliance matter,
a commitment to evaluate the safety significance as part of the Individual 
Plant Examination (IPE) program is not an acceptable alternative to 
responding to this generic letter.  However, you may perform plant specific 
evaluations as part of your IPE program(s) to further evaluate limitations 
imposed in response to this generic letter.  If you elect to do this, your 
IPE submittal must provide the details defined in Section 2.1.6, Subitem 7 
of NUREG-1335, "Individual Plant Examination:  Submittal Guidance." 

The current Westinghouse, Combustion Engineering and General Electric 
(BWR/6) standard technical specifications include the requirements contained 
in this generic letter.  The new standard technical specifications now 
nearing completion as part of the Technical Specifications Improvement 
Program will also include adoption of the requirements of this generic 
letter. 

This request was previously covered by Office of Management and Budget (OMB) 
Clearance Number 3150-0011, which expired June 30, 1991.  (A request for a 
clearance extension has been submitted to OMB and is expected to be 
granted.)  The estimated average number of burden hours is 100 person hours 
per licensee response, including the time required to assess the questions, 
searching data sources, gather and analyze the data, and prepare the 
required reports.  Comments on the accuracy of this estimate and suggestions 
to reduce the burden may be directed to Ronald Minsk, Office of Information 
and Regulatory Affairs (3150-0011), NEOB-3019, Office of Management and 
Budget, Washington, D.C.  20503, and to the U. S. Nuclear Regulatory 
Commission, Information and Records Management Branch, Division of 
Information Support Services, Office of Information and Resources 
Management, Washington, D.C.  20555. 

Please address your response to this generic letter to the U.S. Nuclear 
Regulatory Commission, ATTN:  Document Control Desk, Washington, DC  20555, 
pursuant to 10 CFR 50.4 of the NRC's regulations. 

                                        Sincerely,                                         
                                        
                                        
                                        
                                        James G. Partlow Associate Director 
                                        for Projects Office of Nuclear 
                                        Reactor Regulation

Enclosures:
1.  10 CFR 50.54(f) Request for GI-48 and GI-49
2.  List of Recently Issued Generic Letters

Technical Contacts:  O. Chopra, NRR
                     (301) 492-3265

                     D. Thatcher, RES
                     (301) 492-3935
.

                                                            ENCLOSURE  1 


         RESOLUTION OF GENERIC ISSUES 48, "LCOs FOR CLASS 1E VITAL 

        INSTRUMENT BUSES," AND 49, "INTERLOCKS AND LCOs FOR CLASS 1E 

                 TIE BREAKERS" PURSUANT TO 10 CFR 50.54(f) 


INTRODUCTION 

The phrase "vital instrument bus" may be interpreted in various ways 
depending on the plant.  In this document, "vital instrument buses" refers 
to the ac buses that provide power for the instrumentation and controls of 
the engineered safety features (ESF) systems and the reactor protection 
system (RPS) and are designed to provide continuous power during postulated 
events including the loss of normal offsite power.  Tie breakers are devices 
that cross-connect either redundant Class 1E buses in one unit or Class 1E 
buses in different units at the same site. 

The staff of the U.S. Nuclear Regulatory Commission (NRC) has evaluated the 
concerns of Generic Issues (GIs) 48,  "LCOs for Class 1E Vital Instrument 
Buses," and 49, "Interlocks and LCOs for Class 1E Tie Breakers."  The staff 
has concluded that these concerns can be generally resolved by the 
verification or implementation of appropriate administrative controls in 
plant procedures for the Class 1E buses and tie breakers.  For both issues, 
the primary objective is to verify that plants are not being operated in 
violation of applicable regulations, such as General Design Criteria 17, 21, 
34, and 35 of Appendix A of Part 50 of Title 10 of the Code of Federal 
Regulations (10 CFR).  During its evaluation, the staff identified 
conditions that indicate that the licensees at some plants may be violating 
the single-failure criterion for a substantial period.  Therefore, these 
plants may not meet the requirements pertaining to the design-basis events 
considered in the plant safety analysis report. 

BACKGROUND 

The NRC initiated GI-48 upon discovering that some operating nuclear power 
plants do not have any administrative controls governing operational 
restrictions for their Class 1E 120V ac vital instrument buses (VIBs) and 
associated inverters.  Without such restrictions, the normal or alternate 
power sources for one or more VIBs could be out of service indefinitely.  
This condition could prevent certain safety systems from meeting the plant 
design basis, including loss of offsite power or the single-failure 
criterion. 

Specifically, the VIBs may be subjected to power failure modes that may not 
have been considered during the safety analysis of the plant.  For example, 
this situation could result from removing one or more of the normal or 
alternate power sources for the VIBs from service for repair or maintenance.
Without restrictions, more than one VIB could be connected to an offsite 
.

                                    - 2 -



alternate power source.  The loss of the alternate power source would then 
cause the simultaneous loss of more than one VIB, at least until the diesel 
generators picked up the loads. 

The issue described in GI-49 arose because of an incident that occurred at 
the Point Beach Nuclear Plant, Unit 2.  On June 9, 1980, plant personnel of 
the Wisconsin Electric Power Company (the licensee) discovered that a tie 
breaker between the safeguards buses at the plant was improperly left closed 
after a plant shutdown.  The improper electrical lineup probably occurred 
after a loss-of-ac-power test that was conducted on May 2, 1980, and was 
attributed to personnel error. 

This concern is limited to manually actuated tie breakers that can connect 
either normally independent, redundant Class 1E ac or dc buses at one unit 
or Class 1E buses in different units at the same site.  These tie breakers 
permit convenient maintenance of supply buses and equipment without 
de-energizing plant equipment.  The maintenance is normally conducted when 
the plant is not in operation.  These tie breakers require special 
consideration (such as administrative controls to remove the breakers from 
their cubicles or otherwise lock them open) because, when closed, they can 
compromise the independence of the redundant safety-related buses and may 
prevent loading of both of the emergency diesel generators.  These tie 
breakers could be beneficial under very special conditions (such as a loss 
of offsite power coincident with a loss of a diesel generator or batteries) 
by providing the flexibility to supply power across division boundaries. 

About 5 weeks elapsed before the licensee discovered the improper closure at 
the Point Beach plant.  With the two breakers closed, the two redundant 
buses were connected and the independence of the buses was lost.  If the 
plant had lost offsite ac power with the tie breakers closed, interlocks in 
the circuitry of the diesel generator output breaker would have prevented 
the automatic closure of each diesel generator output breaker. 

The NRC staff's evaluation of the event at Point Beach resulted in the 
identification of the generic concerns of GI-49 regarding procedural 
controls to reduce human error of the type that occurred at Point Beach.  
The staff also noted that the licensee had not implemented tie breaker 
interlocks to prevent manual paralleling of standby power sources, which are 
a provision of Regulatory Guide 1.6, Item 4(d). 

The proposed resolution does not include recommendations regarding the 
addition of tie breaker interlocks.  The interlocks originally raised as a 
concern were to help protect against the possibility of an operator 
committing an error and inadvertently closing a tie breaker between either: 

1.   Two operating diesel generators that may be out of phase with each 
     other or 
     
2.   An operating diesel generator and an incoming feeder line that may be 
     out of phase with each other.

.

                                    - 3 -



Although such interlocks can increase the assurance for some infrequent 
situations, the staff believes that adding such interlocks can also 
adversely affect safety.   For example, in some emergency situations (such 
as a loss of offsite power that coincides with the failure or 
nonavailability of a divisional diesel generator), an operator may need to 
cross-connect power (by way of tie breakers) to an opposite division.  In 
such instances, interlocking circuits could inhibit the operator from taking 
such action.  Therefore, the staff concludes that if proper administrative 
controls that provide assurance that these breakers are normally open are 
placed on the operation of the tie breakers, then the addition of interlocks 
need not be pursued. 

At recently licensed Westinghouse, Combustion Engineering and General 
Electric (BWR/6) plants, licensees have resolved the concerns described in 
GI-48 and 49 by implementing the Standard Technical Specifications. 

RECOMMENDED ACTION 

Ensure that your plant has procedures that include time limitations and 
surveillance requirements for 

1.   Vital instrument buses (typically 120V ac buses),

2.   Inverters or other onsite power sources to the vital instrument buses, 
     and

3.   Tie breakers that can connect redundant Class 1E buses (ac or dc) at 
     one unit or that can connect Class 1E buses between units at the same 
     site.

If plant procedures do not include time limitations and surveillance 
requirements for all of these items, ensure that you have adequately 
evaluated the basis for such a position.  The evaluation should address 
existing regulations and plant design bases, and should specifically 
demonstrate that adequate consideration has been given to the possibility of 
loss of offsite power that coincides with a worst-case additional single 
failure.  In addition, the analysis should consider the time delay required 
for the emergency generators to pick up loads, because in typical plants, if 
an inverter serving a vital instrument bus is not available, a loss of 
offsite power will cause numerous actuations because of the delay time while 
the diesel generators are starting.  Therefore, the analysis should also 
consider malfunctions that do not always have a preferred failure mode, 
(e.g., instrumentation or controls that initiate a switch of emergency core 
cooling from injection to recirculation or initiate isolation of the steam 
generators).  If the alternate power sources for the vital buses cannot 
receive power from the diesel generators, the evaluation should include this 
condition. 

To ensure that you continue to meet the plant design bases, new or revised 
procedures should be evaluated in accordance with the criteria contained in 
10 CFR 50.59. 
Page Last Reviewed/Updated Friday, June 28, 2013