05000285/FIN-2011014-03
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Finding | |
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| Title | Cause of Breaker 1B3A Trip Not Identified |
| Description | The team identified an unresolved item related to an apparent lack of 480 Vac electrical bus protection and coordination associated with the unexpected tripping of feeder breaker 1B3A as a result of a fire in the 1B4A switchgear. During the fire event in the 1B4A switchgear on June 7, 2011, the feeder breaker to the 1B3A switchgear tripped unexpectedly, de-energizing a redundant train of safe shutdown equipment. The licensee performed a root cause analysis of the events associated with the fire in switchgear 1B4A and originally concluded that breaker 1B3A tripped on overcurrent based on inspection of the breaker following the event; however, additional investigations could not confirm this conclusion. Six safety-related feeder breakers and six safety-related bus-tie breakers had been replaced in November, 2009 under permanent plant modification EC 33464. The modification replaced General Electric AK-50 low voltage power circuit breakers with Nuclear Logistics Incorporated/Square-D Masterpact circuit breaker/cradle assemblies and digital trip devices. The 480 Vac electrical distribution system is illustrated in Figure 1 of Attachment 4 of this report, and is comprised of nine load centers; three load centers are fed from the 4160 Vac bus 1A3 and three load centers are fed from 4160 Vac bus 1A4. There are three island buses which can be energized from either 480 Vac bus via bus-tie breakers. The 480 Vac electrical distribution system design was such that an electrical fault in the 1B4A load center should trip the normally-closed bus-tie breaker BT-1B3A, isolating the fault from the 1B3A bus. The bus-tie breakers had electronic trip settings with time-overcurrent trip values coordinated with those of the bus feeder breakers. The team reviewed Calculation EC-91-084, Breaker and Fuse Coordination Study, Revision 8, which was developed to show that adequate overcurrent protection and coordination existed on the safety-related buses. The team reviewed the time-current characteristic curves, breaker vendor materials, licensee breaker calibration data and time-voltage plots of the 4160 Vac bus voltages but was unable to confirm the licensees original conclusions that breaker 1B3A tripped on overcurrent. The licensee elevated Condition Report CR 2011-6621 to condition level A, requiring a root cause analysis to investigate the breaker 1B3A spurious trip. Condition Report CR 2011-5613 was written to document the unexpected tripping of breaker 1B3A. The licensee removed breaker 1B3A from service and on October 12, 2011 and sent it to the vendor for additional testing and analysis. The licensees analysis of breaker 1B3A had not been completed during the inspection period. Further inspection is required to determine whether performance deficiencies exist and if they are more than minor. |
| Site: | Fort Calhoun  |
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| Report | IR 05000285/2011014 Section 4OA5 |
| Date counted | Mar 31, 2012 (2012Q1) |
| Type: | URI: |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 93812 |
| Inspectors (proximate) | K Clayton R Deese J Kirkland F Ramirez A Klett B Tharakan L Carson A Rosebrook C Young C Alldredge N Greene J Wingebachd Lovelessg Miller J Josey S Achen S Graves |
| INPO aspect | |
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Finding - Fort Calhoun - IR 05000285/2011014 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Fort Calhoun) @ 2012Q1
Self-Identified List (Fort Calhoun)
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