ML20207A863
| ML20207A863 | |
| Person / Time | |
|---|---|
| Issue date: | 05/19/1999 |
| From: | Marsh L, Quay T NRC (Affiliation Not Assigned) |
| To: | Lance J ELECTRIC POWER RESEARCH INSTITUTE |
| References | |
| NUDOCS 9905270351 | |
| Download: ML20207A863 (15) | |
Text
.
, .y 4 j p UNITED STATES j
, s# NUCLEAR REGULATORY COMMISSION
- C f, 2 WASHINGTON, D.C. 205f&o001 C'i4l
- May 19, 1999 Electric Power Research Institute ATTN: Mr. Jack Lance 1300 Harris Boulevaio Charlotte, NC 28262
SUBJECT:
INSIGHTS FROM NRC CIRCUlT BREAKER MAINTENANCE PROGRAM INSPECTIONS
Dear Mr. Lance:
1 Because of concerns over the reliability of safety-related low- and medium-voltage power circuit breakers, the U.S. Nuclear Regulatory Commission (NRC) developed an action plan to determine whether regufatory action was needed to ensure that the breakers remained reliable components. As part of the action plan the NRC performed inspections of eight licensee circuit breaker maintenance programs using a special inspection module (Temporary instruction 2515/137, Revision 1). In addition to those inspections, the staff also performed inspections of original equipment manufacturers and third party vendors that perform breaker refurbishments.
The purpose of this letter is to transmit the insights gained from NRC inspections at nuclear power plants and circuit breaker vendor facilities over the two-year period from 1997 to 1998.
The inspection results indicate that the eight inspected licensee maintenance programs for medium-voltage (4 kV to 15-kV) and low-voltage (600-V and below) circuit breakers that supply power to safety-related equipment, are generally adequate and the circuit breakers are still '
reliable components. However, there are some areas of these maintenance programs that could be improved to ensure that circuit breakers continue to be reliable throughout their service lives. Information Notice (IN) 99-13, " Insights from NRC Inspections of Low- and Medium-Voltage Circuit Breaker Maintenance Programs," was issued on April 29,1999, to summarize the inspection results for all licensees.
The enclosure to this letter provides a detailed discussion of the topics covered in IN 99-13 so that licensee personnel responsible for developing and implementing circuit breaker maintenance progrsms may take advantage of the information gathered from the inspection of licensee and vendor facilities. In addition to discussing the topics in IN 99-13 in greater detail, the enclosure also discusses corrective maintenance, refurbishment, and the maintenance rule. I The enclosed insights are for information only, so that licensees may consider them when f making improvements to their circuit breaker maintenance programs, and are not meant to be construed as new regulatory requirements. jO g
The historical background of the circuit breaker reliability issues that led to the NRC performing the maintenance program inspections is enclosed. Following the background, the insights gained from the NRC inspections are described. The insights have been divided into the following categories: (1) general programmatic issues, (2) preventive maintenance, (3) corrective maintenance, (4) licensee / vendor interface, (5) control voltage calculations, (6) operating experience review, (7) refurbishment, and (8) maintenance rule. P RmDr ,,g kg gAd '
y ,,, f C J fl
- l b vh d O
,. , , e 7 n,q 99052 990519 r k$
I "703'51
"""$$4 r EEPM CWY
. Mr. Lance l If you have any questions concerning any of the material in tne attachment to this letter please contact one of the cognizant staff members listed at the end of the attachment.
[ Original signed by] [ Original signed by]
Ledyard B. Marsh, Chief Theodore R. Quay, Chief Events Assessment, Generic Communications Quality Assurance, Vendor Inspection, and Non-Power Reactors Branch Maintenance and Allegations Branch Division of Regulatory improvement Programs Division of Inspection Program Office of Nuclear Reactor Regulation Management Office of Nuclear Reactor Regulation
Enclosure:
Low- and Medium-Voltage Circuit Breaker Reliability Concerns cc: J. Sharkey, EPRI W. Subalusky, INPO I G. Fader, INPO R. Burris, INPO D. Modeen, NEl J. Butler, NEl l
Distribution:
GenkelFileB PDR REXB R/F DSkeen SAlexander KNaidu RCorreia APal AGill JCalvo SMagruder JWilson RBlough, R-l DRP WLanning, R-l DRS LPlisco, R-il DRP BMallett, R-Il DRS GGrant, R-Ill DRP JGrobe,R-Ill DRS KBrockman, R-IV DRP AHowell, R-IV DRS DOCUMENT NAMP C:\DLS\EPRIBKR.LTR To receive a copy of this document, ,,idicate in the box C= Copy w/o attachment / enclosure E= Copy with attachment / enclosure N = No copy OFFICE REXB IQMB C:lOMB k:REXB C:REXB NAME DSkeek SAlexander TQuay JLyk LMarsh k uwem.u s w cmn.a x DATE f/l'//99 s// f/99 @ f/99 / /99 \ g/099 / /99 OFFICIAL RECORD COPY L
Mr. Lance if you have any questions concerning any of the material in the attachment to this letter please contact one of the cognizant staff memoers listed at the end of the attachment.
Ledyard B. Marsh, Chief Theodore R. Quay, Chief Events Assessment, Generic Communications Quality Assurance, Vendor inspection, and Non-Power Reactors Branch Maintenance and Allegations Branch Division of Regulatory improvement Programs Division of Inspection Program Office of Nuclear Reactor Regulation Management Office of Nuclear Reactor Regulation
Enclosure:
Low- and Medium-Voltage Circuit Breaker Reliability Concerns cc: J. Sharkey, EPRI W. Subalusky, INPO G. Fader, INPO R. Burris, INPO D. Modeen, NEl J. Butler, NEl Distribution:
Central File PDR REXB R/F DSkeen SAlexander KNaidu RCorreia APal AGill JCalvo RBlough, R-l DRP WLanning, R-l DRS LPlisco, R-il DRP BMallett, R-Il DRS GGrant, R-Ill DRP JGrobe,R-Ill DRS KBrockman, PAV DRP AHowell, R-IV DRS DOCUMENT NAME: G:\DLS\EPRIBKR.LTR To receive a copy of this document, indicate in the box C= Copy w/o attachment / enclosure E= Copy with attachment / enclosure N = No copy OFFICE REXQ n h IOMB k C:lOMB DC:REXB C:REXB , i NAME DSkebM1SAlexandh TQuay'N JLyons LMarsh JATE G//2/99 FM/99 5 /(W99 / /99 / /99 / /99 l OFFICIAL RECORD COPY m _
Mr. Lance if you have any questions concerning any of the material in the attachment to this letter please contact one of the cognizant staff members listed at the end of the attachment.
fk ' b /*Ay Ledyard B. Marsh, Chief Theodore R. Quay, Chief Events Assessment, Generic Communications Quality Assurance, Vendor inspection, and Non-Power Reactors Branch Maintenance and Allegations Branch Division of Regulatory improvement Programs Division of Inspection Program Office of Nuclear Reactor Regulation Management Office of Nuclear Reactor Regulation
Enclosure:
Low- and Medium-Voltage Circuit Breaker Reliability Concerns cc: J. Sharkey, EPRI W. Subalusky, INPO G. Fader, INPO R. Burris, INPO D. Modeen, NEl J. Butler, NEl i
j l
N
Background:
Low- and Medium-Voltage Circuit Breaker Reliability Concerns -
I The NRC issued Information Notice 98-38,
- Metal-Clad Circuit Breaker Maintenance issues identified by NRC Inspections," on October 15,1998, to alert licensees to issues identified by reactive NRC inspections at plants that experienced circuit breaker reliability issues in 1997.
The nuclear power planta J,6 cussed in IN 98-38 either considered shutting down, extended an j outage, or actually shut down because of concerns over common-mode failure of their safety- !
related circuit breakers. In response to those events, the NRC implemented a task action plan to evaluate whether any generic reguistory action was warranted to address power circuit breaker reliability problems. As part of the plan, inspections were performed at eight plants to determine the overall status of the industry's circuit breaker maintenance programs, using ,
Temporary instruction (TI) 2515/137, Revision 1, " Inspection of Medium-Voltage and Low-Voltage Power Circuit Breakers," issued on March 9,1998. In addition to the Tl inspections, several inspections of original equipment manufacture;s and third party vendors were performed as part of the action plan.
Another part of the NRC task action plan was the monitoring of industry initiatives to address circuit breaker reliability issues. NRC representatives have attended portions of the Electric Power Research Institute's Nuclear Maintenance Applications Center (EPRl/NMAC) Circuit Breaker Users Groups over the last two years. The users groups, formed to develop maintenance and refurbishment guidelines for medium- and low-voltage power circuit breakers manufactured by_ General Electric, Westinghouse, and ITE/ABB, have been aggressively pursuing resolution of the breaker reliability issues and are developing guidance based on industry experience and vendor recommendations. The vendors, although reticent at first, have become increasingly involved over the last year. The EPRl/NMAC groups have already issued guidance for the General Electric Magne-Blast (4-kV) breakers, and plan to have the
)
mainte%nce guidance for all of the 4-kV and 480-V breakers made by the three manufacturers issued by the end of 1999 or early 2000. Although the staff has not reviewed any of the final guidance documents, NRC representatives have seen some of the draft documents at the various users group meetings and they appear to be of high quality.
NRC staff also met with the Nuclear Energy Institute's (NEI) Circuit Breaker Task Force in December 1998. The NEl task force is made up of representatives from NEl, EPRl/NMAC, leaders of the EPRl/NMAC users groups, and the Institute of Nuclear Power Operations (INPO). The staff discussed with the task force a way to share the insights gained from the Tl inspections that were performed in 1998. The staff and the NEl task force believe that it is important to share the inspectors' insights, as well as the inspection results, with the industry, especially since the EPRl/NMAC circuit breaker maintenance guidelines are being drafted and many licensees will be revisiting their maintenance programs to see where improvements can be made once the guidelines are issued. j l
I Page 1 of 11
< \
Circuit Breaker Maintenance Inspection insights
- 1. General Proorammatic issues Licensee preventive maintenance procedures and practices could be improved by ensuring that all of the applicable vendor recommendations or industry operating experience are taken into
. consideration. Inspection results indicate that when licensees deviated from such recommendations and operating experience there was often no documented basis or rationale given. Although vendor recommendations may not be appropriate in some cases, it is important to have a sound engineering basis when deviating from those recommendations.
Di;cussion with the vendor about the reasons behind their recommendations can help to ensure thm a licensee has not overlooked something important when deciding to deviate from a vendor-recommended practice.
Control of the storage of lubricants and cleaning materials (including appropriate resealable containers or dispensers, shelf life, environment, segregation, etc.) is important. Some licensees had not ider tified shelf lives for lubricants and cleaning agents or solvents used in the maintenance of circuit breakers. Procurement and commercial-grade dedication documents did not always identify shelf lives where there were shelf lives associated with the materials. One licensee had identified a resealable container as a critical characteristic for dedication of a lubricant, but receiving documents did not reflect verification of that critical characteristic.
Guidance for useful lives of these materials when in use after original containers had been opened (sometimes referred to as " pot life") was not often established. Guidance fer sterage ;
and handling of these materials after issue to maintenance personnel was typically nm provided l
- (e.g., requirements for storage environments, avoiding prolonged exposure to air or high temperatures, avoiding moisture or other contaminants, etc.). Supply issue procedures did not I
always require that lubricants be issued only on work orders for equipment for which the material was approved and in limited quantities. Maintenance procedures often were not specific about where certain lubricants or solvents should or should not be used on breakers.
" A good training program for maintenance personnel should include: (1) specific qualification for various maintenance tasks on different types of breakers, (2) review of industry operating experience, (3) vendor-recommended modifications or upgrades, (4) vendor manual ievisions, or (5) plant procedure revisions. At some plants, maintenance procedures did not cover inspecting breakers for specific problems identified in industry operating experience. Some licensees stated that they covered such items in training, but the inspectors found that specific items in question were seldom explicitly addressed in lesson plans. Also, maintenance personnel were not always familiar with some of the test equipment. At one plant, the electrical maintenance supervisor instructed electricians to perform runup reduced control voltage tests using a variable power supply and chart recorder, but the electricians were not familiar with the test equipment. After they experienced some difficulty with the equipment, instrumentation and centrol technicians were summoned to assist.
Unique identifiers on individual breakers are important for tracking breaker performance and maintenance history. Individual breakers at some plants do not have unique identifiers, and some of the licensees inspected did not know that groLp or series identifiers, such as shop order numbers, are not unique. Some licensees did not record both the breaker serial number, Page.2 of 11 l
)
r-when present, or the cubicle number in maintenance records to allow for tracking of breaker location, performance, and maintenance history.
At most plants, the racking of breakers in and out of the cub;cle (and local operation when required)is the job of Operations Department personnel. Operations personnel training and/or procedures could be improved by covering (1) verification and adjustment, if required, of cubicle interfaces in the connected position (or calling for Maintenance Department personnel to do this) and (2) functional testing in the connected position (i.e., starting, running, and stopping the load equipment when permitted by plant conditions) to verify post rack-in breaker operability in the fully connected position. This practice provides for verification of proper indications, closing spring recharging, and restoration of all electrical and mechanical interfaces and interlocks.
These functions were sometimes covered to some extent by post-maintenance testing procedures. However, they were often not prescribed if for some reason a breaker was racked out (even if only to the test position), but no maintenance was performed on either the disconnected or racked-out breaker itself or its load equipment.
Having spare breakers on hand (particularly ones that have been refurbished and are certified for safety-related service) can allow flexibility for interchanging breakers in support of refurbishment, preventive maintenance, or in some cases, to replace a failed breaker in a timely manner, if necessary. Maintenance workers sometimes are under pressure to perform preventive maintenance within a short time in order to minimize the time that equipment served by a breaker (or the breaker itself) is out of service. Licensees have reported that much of this pressure is due to a provision of 10 CFR 50.65(a)(3) which recommends assessing and managing the risk associated with taking equipment out of service for planned maintenance.
Having a ready spare to replace a problem breaker could alleviate some of the time pressures.
II. Preventive Maintenance Some licensees did not always adhere to their own preventive maintenance schedules. At several plants breakers were found to be currently overdue for preventive maintenance with respect to licensee-established periodicity (as well as that recommended by the vendor), or were overdue on one or more occasions in the past.
Recording as-found breaker conditions and comparing them to previous as-found and as-left conditions can help maintenance personnel assess the amount of degradation since the last maintenance, or the effectiveness of the latest maintenance. As-found values of preventive maintenance parameters that could provide useful information include, the trip and close voltage, tripping current and times, insulation resistance, and contact resistance. These parameters should be measured and documented before making adjustments, cleaning, or operations that would tend to alter the as-found conditions. Consistently performing maintenance steps in a prescribed sequence designed to minimize preconditioning (because it cannot be completely eliminated) should provide more valid, comparable or trendable results.
Some vendor manuals prescribe functional testing of circuit breakers (i.e., closing and tripping electrically) at the minimum (and maximum) vendor-specified voltage for the closing solenoid, closing spring release solenoid, or tripping solenoid (e.g, Westinghouse MPM-DS). Others -
. simply provide the voltage range within which the solenoids are designed to operate without explicitly prescribing testing at those extremes of solenoid design capability. However, reduced Page 3 of 11 l
c
'N
~
(i.e., less than nominal) control voltage testing as one means of (1) verifying current operability at the minimum expected (design-basis) or calculated control voltage available at the breaker, (2) confirming past operability, (3) determining margins to unsatisfactory performance, or (4) obtaining diagnostic, predictive,' or trendable performance data, has not been routinely ,
performed at all plants in the past. Some licensees have recently begun to obtain quantitative, trendable data on the minimum " pickup" voltages for the control devices which also reflects the condition of breaker tripping and closing mechanisms; or at least to determine if such data are
- trendable and useful in diagnostic condition assessment or performance prediction.
Although reduced control voltage testing is not a regulatory requirement, testing the most
'important breakers at reduced voltage may provide added assurance that these breakers would remain operable under worst-case conditions. Breakers such as the EDG output breaker, offsite power source breakers, or other breakers (including some loads that are sequenced on early or that remain connected to vital busses) may be required to close with minimum design control voitage under conditions such as initial recovery from a prolonged station blackout before battery chargers become available. However, certain others (e.g., later sequenced ECCS equipment breakers) that could see minimum design control ve# age under some j conditions may never be required to operate to perform any of their safety functions at less than nominal voltage because, for example, they are not required to close until after the standby emergency ac power source (e.g., a diesel generator) has restored power to the battery
. chargers, and hence vital 125-Vdc bus voltage, which is most often used for safety-related ;
breaker control power, is restored to nominal (unless for a given plant, the failure of the only l available battery charger must be assumed under the single failure criterion).
Note that most closing spring charging motors on safety-related breakers would not normally be required to operate at reduced voltage because in most design basis event scenarios, e.g.,
LOOP-LOCA, charging motors, which operate immediately after closing in most cases, would have already recharged their breakers' closing springs upon the initial closing after the vital bus (s) (and hence the battery chargers) have been re-energized on the standby emergency ac power source (e.g., a diesel generator). Even in the LOCA folicwed by a delayed LOOP '
scenario, which is not within the design basis of most plants, the charging motors of emergency core cooling system (ECCS) breakers would operate after their breakers closed upon ECCS initiation while normal power was still available. They would not need to operate again to s!!cw their breakers to open upon loss of power and reclose one time as ECCS loads are automatically reenergized by, for example, an emergency diesel generator) load sequencer.
Nevertheless, there may be certain instances that could require a charging motor to operate at reduced voltage (unless manual recharging is being relied upon). For example, during recovery from station blackout, if the EDG breaker or first offs';r power supply breaker should fail to latch closed, and/or remain closed on the first try, the motor wouid need to operate at whatever control voltage was available to recharge the closing spring for subsequent attempts at closing, if warranted. While the motor would then operate at lower than normal speed, manufacturers, e.g., General Electric, have said that it is not deleterious for thm to be tested at reduced voltage if deemed necessary, insulation resistance testing was being performed at some plants using inappropriate test voltage. Often the acceptance criteria required that the resistance be higher than some minimal value such as a thumb rule taken from rotating machinery testing practice, which is one megohm /(kV) + imegohm. However, having a very high value (e.g.,1000 mogohms, or more, Page 4 of 11
@ 2500 volts-dc for 5-kV equipment, as recommended by the National Electrical Testing Association') as an acceptance criterion (with results below this level requiring some action such as notifying the maintenance manager or cleaning) could facilitate early identification and timely correction of a degrading trend before a breaker faibd to meet the minimum acceptable value. Also, some licensees did not require technicians to record the actual values measured, but only required them to indicate that the resistance was greater than some acceptance value, which as previously stated, was often too low. This practice was not conducive to meaningful data recording and evaluation of insulation performance and degradation.
Ill. Corrective Maintenance Procedures or guidance to aid control room personnel who might have to deal with various types of breaker failures in the various modes of plant operation could prove useful. Although it may not be practical to develop detailed procedures for such failures, some general guidance on how to cope with failures of important breakers could be developed. Such predetermined operational considerations and off-normal operating guidance could be very helpful to operators, both from the standpoint of facilitating promptly placing the plant in a safe and stable condition and, to the extent possible at the same time, permitting the isolation of the affected breaker in order to preserve the as-failed conditions. For example, some plants have experienced failures of breakers to open (or open fully) on demand, a much less common, but typically more complicated problem than failures to close. Having predetermined off-normal operating guidance for this contingency could have minimized the time the affected plants had to remain in an unanalyzed condition while operators formulated the strategy for coping with the situation. In one case, having the coping strategies for a stuck-closed residual heat removal (RHR) pump breaker thought out ahead of time might have provided additional time for consideration before an unplanned plant shutdown was deemed necessary. In this case, knowing how much excess initial lead the emergency diesel generator (EDG) could actually handle safely (because one of the subsequent automatically sequenced loads, the RHR pump, could not be disconnected from the affected vital bus), might have obviated the need to declare the emergency diesel generator inoperable, at least initially. Knowing how long it was actually safe to run the affected pump on minimum recirculation flow if, for instance, local temperatures or other peNmeters could be monitored, could have enabled operators to easily and promptly determine how much time was available to shift loads, lock out attemate sources, and de-energize the affected bus, so that some inappropriate and ineffective measures to open the breaker with the bus energized uader time pressure (which resulted in damage to the breaker and violation of personne,i electrical safety precautions) might have been avoided.
Procedures or guidance covering isolation, quarantining, and troubleshooting of failed circuit
. breakers by local visual examination, documentation (logging), and evaluation of the state of indications or the transitions observed in indications, or carefully documenting as-found conditions could be useful. Few licensees have developed symptom-based breaker troubleshooting plans to aid in determining the root causes of failure. Although such procedures are.not explicitly required by NRC regulations, they could facilitate is!!ure en9 lysis and corrective actions, while minimizing the time that the electrical distrit,Jtion system may have to remain in an abnormal lineup. For example, such plans might include predetermined
' National Electrical Testing Association Maintenance and Testing Specifications for Electric Power Distribution Systems and Equipment, MTS-1989.
Page 5 of 11
L strategies to determine the actual position of the contacts, the state of the closing spring, the state of the tripping or closing (or closing spring release) latches; to determine whether an opening or closing operation was electrically initiated, whether the breaker's failure to open,-
close, or remain closed (i.e., if it went " trip free") on demand was mechanical, or whether there might have been an electrical failure such that the closing or opening sequence was never initiated. in the past, instead of performing logical, coordinated failure analysis, some licensees performed routine preventive maintenance on a failed breaker and, if successful, placed the breaker back in service, only to have it, or another breaker in a similar condition, fail for the same, still undetected reason at a later time.
A knowledgeable and experienced breaker technician may be able to identify factors contributing to a failure from just a visual examination of the breaker in its cubicle or cell, but may not always be readily available. However, most on-duty technicians should be able to make basic determinations aided by a well-thought-out troubleshooting guide. Once the breaker is disturbed or removed from the cell, valuable information may be lost. Also, being aware of the latest industry operating experience or vendor information can be very useful in troubleshooting efforts. Certain contributing factors can sometimes be easily verified or discounted if the technician is alerted to the various problems identified by previous failures in the industry or at a specific plant.
Some licensees have developed (or contracted for) special diagnostic techniques, such as video boroscopy; high-speed videography; and time, motion and current data recording. These techniques have proven invaluable in analyses of certain unusual breaker failures when problems were intermittent and routine inspections and tests were inconclusive or ineffective in revealing the root causes. ' Such specisl techniques would not be expected to be employed routinely, but in several cases they have been the only methods that were successful in identifying the cause of the failure.
. IV. Licensee /VendorInterface Licensees committed to implement vendor interface programs to address Item 2.2 of Generic Letter 83-28," Required Actions Based on Generic Implications of Salem ATWS Events," issued July 8,1983, and later, GL 90-03, " Relaxation of Staff Position in Generic Letter 83-28," ltem' 2.2, Part 2, " Vendor Interface for Safety-Related Components," issued March 20,1990. The purpose of vendor interface programs, as stated in GL 90-03 was to ensure that licensees would receive all vendor technical manual updates or revisions in a timely manner and also all other relevant technical information in cMer to have the latest applicable information with which to operate and maintain the key safety-related equipment. Inspection results indicate that
- several aspects of licensee / vendor interface programs could be improved.
Circuit breaker and switchgear vendor manuals should be maintained current by periodically recontacting the vendor (by telephone) to ensure that the licensee has the latest vendor information, including updates to manuals, or other pertinent technical information bulletins, letters, and so on.' In the past, some licensee recontact efforts have not been successful
. because of reorganizations, or name and location changes of several switchgear manufacturers, or by vendors who were unresponsive to periodic recontact attempts and requests for information by licensees. In the past year, however, the major circuit breaker Page 6 of 11 i
s
. ve'ndors have begun participating in the EPRl/NMAC circuit breaker users groups and the licensee / vendor relationship appears to be improving. j Several licensee circuit breaker vendor interface program weaknesses were identified during NRC inspections, including (1) uncoordinated or conflicting procedures; (2) inaccurate or incomplete lists of key safety-related components; (3) inaccurate, incomplete, or out-of-date lists of vendor names and/or locations and cognizant personnel or the most appropriate contacts; (4) insufficiently detailed or specific periodic recontact form letters requesting information, often not sent to tho most appropriate vendor department, location, or personnel; i (5) insufficient followup on requests for information; (6) insufficient involvement by technically I knowledgeable personnel; (7) organizational weaknesses, such as lack of priority, lack of centralized responsibility, and having separate distribution paths; or (8) poor administration.
- Some licensee-identified areas of vendor interface program improvements include (1) periodic l review of plant equipment to ensure that lists of key safety-related equipment are current; l (2) establishing organizational and procedural interfaces and links to ensure that vendor 1 interface personnel are kept informed of equipment changes or modifications; (3) establishing personal contact with the cognizant or most appropriate vendor personnel with the ability and 1 willingness to provide the licensee with the needed information in a timely manner; l (4) substential involvement in the process by personnel technically knowledgeable of the ;
equipment and well acquainted with vendors' technical documentation and staff contacts; and (5) periodic comprehensive reconciliation with the vene'or of lists of equipment and related i technical publications or documentation, preferably by telephone and followup correspondence. ,
V. Control Voltaae Calculations As part of their implementation of NRC regulations, including General Design Criterion (GDC) 17, ' Electric Power Systems"and GDC-18, ' Inspection and Testing of Electric Power Systems,"
of 10 CFR Part 50, Appendix A; 10 CFR 50.63, " Loss of all attemating current power;" and C;iterion lil, " Design Control," of 10 CFR Part 50, Appeedix B, many licensees have performed calculations to determine the worst-case design-basis codrol voltage (nominally 125 Vdc) available at the trip solenoids, closing solenoids, or closing spiing release solenoids on safety-related circuit breakers as part of the design basis of the vital e:setrical power distribution systems. These calculations have sometimes been performed h conjunction with sizing or capacity calculations for vital station batteries, and in conjunction wC. the development of station blackout coping analysis.
In some cases, although formal rigorous calculations for each circuit were not performed based on actual installed cabling, 6esign engineers established the minimum allowable breaker control voltage for the plant as the vendor-specified minimum operating control voltage for the trip and closing solenoids. To translate this design basis requirement into design constraints for construction, they first assumed minimum source voltage (e.g., minimum vital station battery
- voltage without chargers, typically around 105 Vde), then calculated the allowable maximum lengths and allowable minimum sizes for control cabling. During construction, when it became difficult to meet these design requirements in certain cable installations, some licensees used interposing (boosting) relays or used parallel current paths to reduce line resistance and hence minimize voltage drop to meet the design basis requirement that no less than the */endor-specified minimum solenoid voltage would be available to trip and close safety 4 elated breakers Page 7 of 11
~
b However, the NRC inspections revealed that a few licensees had neither performed the calculations based on as-built systems, nor enforced attemative design constraints during construction. Several discrepancies were identified in licensee calculations, including: (1) not starting with the minimum battery voltage; (2) using an incorrect minimum battery voltage that did not take into account loading, state of discharge, and/or aging factors; (3) using incorrect current paths, cable lengths, conductor sizes, and/or ohms / foot values to determine overall cable resistance; (4) calculation of cable conductor resistance using ambient temperature values, but neglecting temperature rise due to heat from surrounding cables in a raceway or without having data to justify the non-conservative lower temperature assumption; and (5) using incotreet loading values in the final determinations of voltage drops.
VI. Operatino Exoerience Review Operating experience review programs should review applicable documents from all pertinent sources. These documents include NRC information notices (ins); INPO SEE-IN documents or Nuclear Network reports; and vendor information, such as service information letters (SILs) from General Electric (GE) Nuclear Energy, service advice letters (SALs) from GE product departments such as the former Specialty Breaker Plant (for Magne-Blast equipment) or GE Electrical Distribution and Control (for low-voltage switchgear equipment); and technical bulletins or nuclear service advisory letters (NSALs) from Westinghouse Nuclear Service Division or its predecessors.
This operating experience sometimes has not been reflected in licensee maintenance procedures for various administrative reasons, including that the information was not distributed to the appropriate licensee personnel or was not received by the plant at all.
However, in most cases, the greater problem involved incorrect determinations of applicability.
The Tl inspections revealed instances of industry operating experience information erroneously ,
determined to be not applicable because of narrowly focused and/or superficial reviews and )'
insufficient involvement by technically knowledgeable personnel. Problems generically applicable to several types of breakers were often not recognized because the plant's breakers did not have the same exact model designation as the one used as an example in the information notice or the vendor technical bulletin. In some cases, licensees failed to recognize specific applicability because reviewers were not familiar with their plant's equipment, did not perform adequate verification of installed equipment, or did not consult with more knowledgeable staff.
Tl 2515/137, Revision 1, lists 62 NRC information notices and bulletins that deal with problems with low- and medium-voltage power circuit breakers. As many as one third of these were ,
erroneously determined not to be app ll cable at one or more plants. Some examples of ins that l were misclassified or received inadequate licensee review serve to illustrate this point. j l
lN 83-50," Failures of Class 1E Safety-Related Switchgear Circuit Breakers," issued August 1, i 1983, alerted licensees to the failure of breakers to close on demand after racking to the connected position. This IN emphasized problems with breaker-cubicle electricalinterlocks and interfaces, but the message was generically applicable. Superficial review and disposition of this IN often resulted in lack of procedural steps in post-maintenance test instructions or ;
operations department instructions, as discussed previously, to ensure that electrical and Page 8 of ii l f
\. j 1
1
a mechanical breaker cubicle interlocks and interfaces have been restored (such as by functionally testing breakers whenever they are retumed to the connected position, plant conditions permitting).
IN 84-46, " Circuit Breaker Position Verification," issued June 13,1984, dealt with position verification of racked-in breakers. The breaker used as an example of the problem was a predecessor of the widely used 4.16-kV ITE/ABB type breaker now known by the "HK" designation, but the IN described it by its old ITE designation, "lTE Model 3." Use of the older nomenelsture apparently led to several licensees' not realizing that they actually hao breakers of the type discussed. In addition, several more licensees failed to realize that the problem and similar remedies were applicable to other types of breakers as well.
- Potential Failure of General Electric Magne-Blast Circuit Breakers and AK Circuit Breakers," issued June 12,1990, alerted licensees to failures of GE Magne-Blast (Type AM, vertical lift) breakers due to deteriorated Teflon @ impregnated fiberglass "Tufloc@" sleeve bearings in their Type ML-13 operating mechanisms. The IN did not point out that this problem was also applicable to Type AMH, hori::ontal drawout, Magne-Blasts with Type ML-13A mechanisms; nor did the subsequently issued GE SAL 318 series; because the internals of the
~ ML-13 and ML-13A mechanisms are the same. Some licensees with AMH braakers assumed the IN and SAL were not applicable to them and did not attempt to verify that assumption with the vendor or the NRC.
IN 93-85, " Problems with X-Relays in DB. and DHB-Type Circuit Breakers Manufactured by Westinghouse," issued October 20,1993, addressed a problem with sticking of the "X" or anti-pump relay used on some Westinghouse type low-voltage breakers. The IN used the Type DB-25 as an example because that was the model of breaker that failed and prompted issuance of
- the IN. The same type of relay is also used on Type DB-50 breakers. Some licensees with DB-50 breakers erroneously dismissed the IN as inapp!icable, because the IN only mentioned the DB-25 breaker.
IN 97-53, " Circuit Breakers Left Rack'ad Out in Non-Seismically Qualified Positions," issued on l
. July 18,1997, discussed the potendal for some safety related breakers to be left in the racked i out position, which could affect the seismic qualification of both the breaker and the switchgear. Some licenseec did not properly evaluate the notice for applicability because no specific circuit breaker type or model numberc were given. !
i Vil. Refurbishment j Some of the plants inspected did not have a schedule for breaker refurbishment (overhaul), I even though the breakers had been in service for 15 to 20 years. Not all circuit breaker manufacturers have promulga5d recommended time-based or operation-based refurbishment intervals or condition-based refurbishment guidelines, particularly for relatively less severe service conditions such as in nuclear power plants. However, industry experience indicates that generally, breaker performance begins to degrade after 12 to 15 years of service. Depending on the operating environment and the maintenance history, a particular breaker may need refurbishment either earlier or later than this range of service. At one plant that had a refurbishment schedule in place, not all applicable circuit breakers (in particular, de supply breakers) were included in the schedule.
Page 9 of 11 l
i I
- - I Refurbishments are accomplished by service shops affiliated with the original equipment I manufacturer (OEM), independent (so-called third-party) contractors, or by licensees ;
themselves, sometimes with outside assistance and/or training. Factors to be considered when '
choosing the most appropriate and expeditious means of refurbishment include: (1) OEM- i affiliated facilities may have the most experience at servicing their particular brand of breaker, I may have access to original design and manufacturing information, and may have the greatest soility to obtain genuine spare parts, but the cost may be higher and the OEM may not be able to meet a licensee's schedule if multiple refurbishments are needed in a short period of time; (2) a third-party contractor may be able to service breakers faster and at a lower cost but may .
riot have fully qualified, experienced personnel for a particular type of breaker or may not have l access to all of the original design information (which is a significant disadvantage when j commercial-grade spare parts must be dedicated), and some third-party refurbishers have had {
difficulty obtaining OEM parts in a timely manner; nevertheless, some third-party refurbishers '
have developed elaborate reverse engineering processes, supplemented by extensive
{
functional testing to compensate for their lack of original design data, and may be able to ;
perform satisfactory dedications and refurbishments; and (3) in-house refurbishment may be i the most cost-effective and it gives the licensee the most control over the process, but it may I not be feasible for a licensee to allocate enough maintenance staff resources to keep up with the demand; in addition, licensee personnel (particularly considering tumover) may not perform '
refurbishments often enough to maintain proficiency and may require retraining by experienced contractor or OEM personnel.
1 Some licensees that have breaker refurbishments performed by OEM-affiliated service shops or by independent contractors, whether at the vendors' facilities or at the plant site, have found it helpful to have one or more of their own knowledgeable personnel observe the work, particularly the first time it is performed. Observation by licensee personnel can help ensure that the work is performed in accordance with the licensee's specifications.
Another area where the refurbishment process can be improved is the quality of the procurement documentation. Some purchase orders (POs) from licens6es to the refurbisher simply stated that the breakers were to be refurbished, instead of prescribing detailed specifications for the work to be performer!. Best results were obtained when technical and quality requirements were discussed in deta!i by the licensee and the refurbisher ahead of time, and then specified in the procurement documents or by reference to vendor proposals, licensee-approved vendor overhaul praedures, and so on. Effective POs also specified any agreed-upon modifications and unades, contents of condition and overhaul reports, disposition of old parts, una m censee's s quality release and/or receipt inspection acceptance i critenn..
Vlli. Maintenance Rule The Tl inspection results indicated that the scoping of breakers met the requirements of the maintenance rule. At most plants, in-scope breakers were classified in two categories:
(1) incoming or feeder breakers, source output breakers, bus tie breakers, and supply breakers to transformers for lower voltage distribution buses were classified as part of one of the electrical power distribution systems, for example, the 4.16-kV vital system or the 480-volt shutdown boards, and (2) breakers that supply power to individual load equipment or motor control centers associated with a particular functional system (e.g., the residual heat removal Page 10 of 11 L
7
.o 4
system), the service water system, or the emergency diesel generator support system, were counted as part of that system. In some cases, a source output breaker might be counted as part of the source system (e.g., the diesel generator) as well as part of the c6nnected distribution system.
In most instances, functional failures and maintenance-preventable functional foilures were appropriately identified, and classified, and the affected system was shifted to a monitoring status under 10 CFR 50.65(a)(1) when warranted. However, there were some ircatances where multiple failures of similar types of breakers for similar reasons occurred within one year, but because the breakers were in different systems, and because one failure in each of those systems in a one year period did not exceed the licensee's established system reliability and -
availability criteria for demonstrating the effectiveness of preventive maintenance under 10 CFR 50.65(a)(2), the failures did not result in the placement of the affected systems in a 10 CFR 50.65(a)(1) status. Not shifting to 10 CFR 50.65(a)(1) status in these instances may have been appropriate for the system because the breaker failure would not typically be related to any attribute of the plant system, with the possible exception that some systems, by the nature of their operational modes, csuse their associated breakers to be cycled more than others.
However, the multiple, and 69metimes common-cause, breaker failures did not result in the incrsased scrutiny and higher pitrity attention afforded by 10 CFR 50.65(a)(1) status.
To address this type of situation, some licensees estCshed circuit breakers of similar types as separate classes of components across system boundaries in addition to ncir conventional classifications. This practice allowed the reliability and availability of similar types of breakers to be tracked at the " component type" level, independent of their load or distribution systems so that in the event of multiple and/or common-cause functional failures (some of which might be
" maintenance preventable"), the affected class of breakers could be evaluated for monitoring under 10 CFR 50.65(a)(1), if warranted. Groupir,g circuit breakers as a separate class of components could aid in performing root cause evaluations (particularly if deficient maintenance was implicated) and also aid in formulating effective and comprehensive corrective action because other failures of breakers of the same type might have previously been attributed to a similar problem.
Technical Contacts Davia Skeen 301-415-1174 E-mail: dis @nrggav Stephen A!cnnder 301-415-2995 E-mal ;_daanre aov Amar Pal :
301-415-2760 I E-mail; amoanre aov I
Page 11 of 11 i