ML19269D912
| ML19269D912 | |
| Person / Time | |
|---|---|
| Site: | Point Beach  |
| Issue date: | 05/02/1979 |
| From: | Trammell C Office of Nuclear Reactor Regulation |
| To: | Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 7906200312 | |
| Download: ML19269D912 (9) | |
Text
{{#Wiki_filter:[p Mck [ UNITED STATES ['. 4 NUCLEAR REGULATORY COMMISSION { .y 4 WASHINGTON, D. C. 20555 s / / DOCKET NO. 50-266 DATE: May 2, 1979 50-301 LICENSEE: Wisconsin Electric Power Company FACILITY: Point Beach Nuclear Plant, Units Nos. 1 and 2
SUMMARY
OF MEETING HELD ON APRIL 24, 1979 REGARDING DASH P0T END CAP CRACKS ON WESTINGHOUSE DB-50 AND D8-75 AIR CIRCUIT BREAKERS On April 24, 1979, the NRC staff met with representatives of Wisconsin Electric Power Company and Westinghouse Electric Corporation (W) to dis-cuss a possible problem with the overcurrent protective device installed on W DB-50 and DB-75 air circuit breakers. A list of attendees is attached (Attachment 1). Highlights of the meeting are summarized below. In 1972, Wisconsin Electric reported an instance of a safety-related W_ DB-50 air circuit breaker opening on overcurrent (time-delayed) when normal equipment starting current was passing through the breaker. Subsequent ex-amination of the breaker disclosed a crack in the cap of the air chamber which serves as a dashpot or air volume type time-delay for the breaker. The crack caused air in-leakage which reduced the time delay from 60 seconds to 5 seconds, thus causing normal motor starting current to operate the over-current trip device. See attachment 2 for a schematic diagram of the over-current tripping device and attachment 3 for typical tripping characteristic curve. A similar problem was found at Ginna. The generic solution was to replace these black end caps (made of cellular-filled phenolic material) with caps of the " navy gray" type on all safety-related circuit breakers. On April 12, 1979, Wisconsin Electric filed a Licensee Event Report again de-scribing a problem with the time-delay overcurrent device, this time on a DB-75 circuit breaker. This breaker was fitted with a navy gray end cap. With the same individual running the test, the time delay was observed to be 18, 4.5, and 6.7 seconds, as opposed to the desired 6-24 seconds. Test per-sonnel felt these trip times to be excessively variable, and therefore the breaker was disassembled for a diagnostic inspection. The navy gray end cap was discovered to have a small hairline crack emanating from the center hole. 2252 257 7906200 3 / 8 /#
t DOCKET N0. 50-266 May 2, 1979 50-301 Plant personnel replaced the end cap with a spare from stock. The replace-ment was not inspected for cracks just prior to its installation. A retest produced the following trip times: 12,18, and 21 seconds (average 17 sec-onds), an acceptable result. The breaker's insbntaneous overcurrent trip times were then checked, with the following result: 0.5 sec., and 4 seconds (unacceptable). The breaker was disassembled again and the end cap was found to be cracked. Again, it was replaced from stock, this time with a navy gray cap known to not have any cracks. The retest was acceptable. Six out of seventeen end caps in stock were found to have cracks. Whereas it is fortunate that the cracked navy gray end caps were discovered in the process of troubleshooting the circuit breaker's misbehavior, it is not altogether clear at this point that the cracks were the proximate cause. W ran tests on the first cracked end cap and found no measurable leakage. Also, the second test was satisfactory even with a cracked cap. The cracked cap would not appear to be the cause of the problem with the instantaneous trip setting, since air in-leakage to the volume chamber would not play a major role in this function. Further, air in-leakage would not logically explain a variance between tests. Point Beach personnel feel that the problem with the instantaneous trip setting was a faulty pick-up setting or improper operation of the dump valve. Nevertheless, as to the time-delay problem, a cracked end cap was found, fi-nally replaced with an uncracked one, and tested satisfactorily. This would tend to point the finger toward the cracked cap as the cause, but breaker disassembly might also have corrected the problem in an unknown way. In any event, it is known that cracks in end caps can lead to leakage and, therefore, can cause reduction in overcurrent time delays. W has instituted several changes into the manufacturing process to provide Increased assurance that cracked caps will not be a future problem.
- 1. The first production piece of a batch will be closely inspected for proper molding.
The mold date will be added to each cap.
- 2. Each cap will be inspected again after the air holes are drilled.
- 3. Each cap will be re-inspected on leaving the W stockroom, and marked.
- 4. Future units will be shipped complete with the associated air valve or as part of a complete overcurrent unit.
2252 258
May 2, 1979 DOCKET NO. 50-266 50-301 W was requested to provide data regarding the repeatability of the over-current setting for this type of device. Wisconsin Electric was requested to provide information regarding the margins between overcurrent settings and actual breaker currents. At the conclusion of the meeting, we said that we believed that some sort of testing of these types of breakers would be required
- to see if any un-acceptably short time delay settings have resulted from end cap cracks. An NRC IE Bulletin or Circular is being considered. Attachment 4 is a copy of the W Technical Bulletin sent to nuclear plant operators having this equip-
- ment, k'
$1 Charles M. Traninell Operating Reactors Branch #1 Division of Operating Reactors Attachments: 1. Attendees 2. Schematic diagram 3. Tripping characteristics 4. W Technical Bulletin All breakers on Point Beach Unit 2 were tested satisfactorily during the March / April 1979 refueling. All breakers on Point Beach I were tested during the October 1978 refueling. 2252 259
Meeting Summary for Docket Files NRC POR Local POR ORBI Reading NRR Reading H. Denton E. Case V. Stello D. Eisenhut B. Grimes R. Vollmer A. Schwencer D. Ziemann P. Check G. Lainas D. Davis B. Grimes T. Ippolito R. Reid V. Noonan G. Knighton D. Brinkman Project Manager OELD OISE (3) C. Parrish ACRS (16) NRC Participants 2252 260 J. Buchanan TERA Licensee Short Service List
. ' ~ ~ ~ ATTACH 5ENT 1 LIST OF ATTENDEES POINT BEACH MEETING _ NRC Wisconsin Electric C. Tramell D. Porter I. Ahmed W. Hermann S. Rhow D. Tondi V. Tnomas G. Lainas Westinghouse Shaw, Pittman, Potts & Trowbridge c B. Johnston H. Stahr J. O'Neill R. Croley W. Furfari K. Jordan 2252 261
~~ Mhchmt 1 a Page 20 _.g._ M n r l B' c::= N A N t 'f e e L ,,;x Fig.10 - Schematic Diagram of Standard g Overcurrent Truppsng Devsco wsth Adjustable Long Delay and ,y Adjustable Instantaneous Element h}s UF'._ q3 \\H ]{ ] [ ~ { \\ / " "d N3 N P G chamber T by the various valves H and K. long delay will begin to operate is deter-When the diaphragm has reached approx-mined by the long delay pick-up adjusting imately 60 percent of its travel, a quick knob P. Figure 10. The magnitude of current at which the instantaneous trip will operate release valve (shown in Fig. 6) is mechan-is determined by the instantaneous pick-up 1cally opened. This quick release feature permits the full force of the magnet to be adjusting knob N, Figure 10. used for tripping the breaker after the timing cycle is completed. At higher cur-2. Dual Selective Overcurrent Series rents when the uttraction between the mov-Tripping Device For Group or Tie ing core C and the stationary core B is Breakers greater than the load on the spring inside of the moving core, the moving core will The operation of the dual selective compress the spring and travel independ-tripping device is similar to that of the ently of the tripping stem F. This spring dual overcurrent tripping device except insures a constant force pattern acting on that for this device the following details the diaphragm E. The Ieset valve is shown are included: as G. Valve H is the long delay valve and it is A. A short delay valve J, Fig.11, con-trols the size of orifice and consequently permanently open to a calibrated setting, the tripping time in the short circuit re-This setting whichcontrolsthetrippingtime This orifice is adjustable by means can be changed by means of dial S, Figure gion. of knob R. 10 The magnitude of current at which the 2252 262
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too - -- FCC l ~. soo + 500 - 1 (,- -) g - - 500 l { eoo J eco f g long doloy pickup settings-odiustobie with i 8 00 calibrated morks et 80,000. 820,140, and l-l 160 percent of trip unit toting i soo - I too l s L g fotal clearing firne @ -\\ 33--100*/ pickup and 30 second long defoy settings k'8 8oo e$ resettable delay @ j~ t QtOO*/o pickup and 20 second long detoy settings l so o',' IOO*/o pickup and 30 second ( F (7 "7 -/#8 E* ""P ""d 30 *"I '*"8 "*' "
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/ 80 /. pickup and 20 second, Al g N flong delay setting -odjuntoble with ~ l a so - long doloy settings calibrated marks of 30 and 20 sec de i I \\ [/ t g g g - i 7j h > y l I I 7._-_. trip unit refing (omps) Q ~ 4 E 5 t600 600b 300 @
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200 \\i{ "dh C8brated marks et 800 and 1200 - e : g s goog 400 @ 225 ineontoneous pickup setting -odiustobre - 5 1 g 3 4 [ percent of trip unit rotsag 3 l g 3 [ @ Total clearing terne is the durotion of k I ~t foult current flow, including orcing -- y t t*me. y / N / / @ Resettoble delcy curve shows the /j j allowoble duration of current flow. g fy 1, .e for which the breaker will not trip -- k / e 7 if the overcurrent subsides within ~'- -{ thof time to 808/o or less of the g brooker long defoy pickup setting. - [N / If the duration of overcurrent en-J coeds the time indicated by the k resettoble deloy curve, the brecker \\ [/ 3 J - overcurrent device may not reset t and may eventvolly trip the g /- l breaker. N y @ Applicobfe only where fault current. j does not onceed 42OOO A(rms sym) o , T-_M i e 1 .o, _ does not esceed SOOOO A(rms sym) I 1__ ~I ~~ ~ ~ (( 1 _ [_ @ Applicable only where fowlt current 3 og @ Applicable only where fouff cuerent \\ es _ _ does not exceed 6SOOO Alems sym) _ \\ \\ --u es og _ -. b Q _4 . j-~ s\\ l i ^ ~ ~ '"o o'~6o o s.Feee to 30 eo 50 e o ooo o o o o o oog g o o o o oo g o g o "'**2 E 2 0 2 S28.- w o88 eke 'o jjE88j current in percent of breaker trip unit retin, E a $ a s e s.8 F;g. 7 - Tripping Characteristics of Standard Series Overcurrent Tripping Device wit.. Long Delay andinstantaneous Elements 2252 263 [ ll
Westinghouse Nuclear ~ Rh) Service Technical Bulletin Division An advisory notice of a recent technical development pertaining to the instatlation or operation of Westinghouse supplied Nuclear Plant equipment. Recipients should evaluate the information and recommendation, and initiate action where appropriate. P.O. Box 2728, Pittsburgh, PA 15230 Subject Number Westinghouse DB Breakers - Overcurrent Devices NSD TB 79-02 System (s) Date Electrical Power - Lo Voltage Switchgear April 17,1979 Affected Plants All with DB-50 or 08-75 Breakers (Probably excludes S.O.(s) all current in-construction W Plants except Diablo Canyon, Salem, 380-385 Ref erEn[Various and Sequoyah) Sheet Of es 1 2 BACKGROUND INFORMATION An operating plant recently advised that periodic overcurrent tests of DB Breakers revealed three breukers with abnormal overcurrent~ trip characteristics. One of the breakers was in a safeguards application, while the other two were not safety related. Inspections of the overcurrent trip units revealed hairline cracks in the end caps of the overcurrent dashpot airchamber which provides the time delay function of the overcurrent trip. Inspection of 17 in-stock spare end caps at the same plant revealed seven with similar hairline cracks. All of the end caps involved heree both from the operating breakers and from spares, had been procurred in 1977-1978 period, and all were of fibre-filled polyester material comonly called " navy gray." This matter relates back to a late 1972 event when similar experiences occurred at two plants (as documented in the AEC (NRC) Operation Bulletin 73-01). Up to that time the end caps were of a black phenolic material and a Westinghouse recomendation was issued for replacement of the black end caps with the " navy gray." It was, and remains, our opinion that the " navy gray" provides improved reliability over the black phenolic material due to lower residual stresses, lower crack propagation probabilitye and increased overall strength. The manufacturer of the end caps believes the end cap cracking may be related to a batch problem, when ingredient materials used have neared the end of their shelf life. It is not believed the end caps, atter fabrication, have a significant shelf life limit, due to the low residual stress and low crack propagation probabilities stated earlier. 2252 264 AdditionalInformation.if Required, may be Obtained from the originator. Telephone 412 256-5493 or(WIN) 236 5493 NN ^N". - -~ bb ,( W. H. Furfari S. G. Caslake, Mgr. Electric Service Electric Service Pdeither Westinghouse Electree Corporation nor its employes rneke any warrenty or reoresentation with respect to the accuracy, completeness or useful-pets of the inf ormation contained in this report or assume any responsibility for habehty or damage which may result from the use of such information.
Westinghouse Nuclear w5D Service Technical Bulletin Division An advisory notice of a recent technical development pertaining to the installation or operation of Westinghouse-supplied Nuclear Plant equipment. Recipients should evaluate the information and recommendation, and initiate action where appropriate. P.O. Box 2728, Pittsburgh, PA 15230 NSD-TB-79-02 Sheet 2 of 2 There have been no other reports of premature tripping of breakers which have the replacement caps. The periodic functional testing performed in accordance with Technical Specifications should reveal significant changes in breaker timing characteristics, and periodic current injection type overcurrent testing (Multi-amp or equivalent), will reveal marginal trends in time delay reductions. The only Westinghouse breakers potentially affected are types 08-50 and DB-75 having series overcurrent trip devices. Reactor trip breakers do not have overcurrent trip devices, and thus, are not affected. Furthermore, the Westinghouse type DS breakers, as used on many of the more recent plants, are not involved as they utilized a totally different overcurrent device. RECOMMENDATION: Although we are not aware of any premature tripping of motors in the field, we recommended that affected plants take the following actions with respect to safety related DB breakers. 1. Review the most recent overcurrent trip device calibration test data for the safeguards breakers. If any individual readings are marginally acceptable on the low side of the acceptance band, or indicate any significant reduction in delay time from previous results, retests should be performed within 30 days. If any individual reading from the retest is below the acceptable band or marginal, the end cap on the trip device should be removed and replaced if inspection under a minimum 3X magnification reveals any cracks. The trip device after assembly should then be recalibrated. 2. All end caps in storage should be inspected for cracks using a minimum 3X magnifying glass. Do not use any end caps having evidence of cracking in safeguards applications. 3. The safeguards breakers not retested under item 1 above should be tested in the same manner during the current or next planned outage. Any personnel involved in service or replacement of the overcurrent device should be cautioned to be certain that the instantaneous overcurrent spring is properly attached after reassembly of the unit. Refer to our Technical Bulletin NSD-TB-73-18. 2252 265 .}}