05000348/FIN-2007010-03
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Finding | |
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| Title | Adequacy of Root Cause Analysis of the Failed Breakers |
| Description | The circuit breaker that failed to close on demand in the 1A CCW pump breaker DG04 cubicle on September 5, 2007 was an Allis-Chalmers Type 5kV, 1200 Amp MA-350 air circuit breaker. The licensee had developed a fault tree and initially determined the probable cause for failure to be an excessive gap between the first knuckle of the fourbar and the trip latch. Manufacturer\'s specification for this gap dimension is 0.015 - 0.047 inches. When the gap is within these specifications, it reduces the severity of the impact on the latch when the roll begins its closure stroke. If this gap is large, the closing mechanism can build up more speed, hitting the trip latch harder and introducing more recoil, rapid shaking, and flex. The 1A CCW pump breaker was placed in service with a gap setting of 0.045 inches. In the licensees post-failure investigation, the electrical maintenance technician reported the gap to be measured at 0.063 inches. The NRC inspectors viewed the quarantined breaker on September 14. At this time, the stop bolt gap was observed to be between 0.045 -0.047 inches which was approximately the value that had been reported upon its original return-to-service. Although the breaker had been disassembled, there were no adjustments made and the licensee could not account for the discrepancy between the measurement observed by the inspectors and that reported immediately after the failure. The NRC inspector observed that the measured back plate bending would not account for a stop gap growth of 0.018 inches. The inspector also noted that the breaker exhibited a bent roll-pin through its close latch assembly (which works upon the second knuckle of the four-bar mechanism to release the stored energy of the closing springs), with impact markings consistent with markings on the closing mechanism assembly. The licensee had not previously made note of the bent roll-pin. The licensees root cause strategy was logical. However, their evaluation was not thorough enough to identify the bent roll-pin absent questioning by the NRC inspector; the licensee was focused on the out-of-tolerance stop-gap measurement as the cause. Subsequent to the week in which the inspectors were on site, the licensee reconsidered the root cause for this breaker failure to be the interference-based phenomenon related to the close latch assembly. Review of the licensees formal root cause report revealed that though this was the identified root cause, the fault tree which was the only formal analysis tool supporting the conclusion did not reflect this failure mechanism. Based on observations of the licensees root cause analyses for the 1A CCW pump breaker and other breaker failures discussed section 4.OA5.11, the inspectors raised concerns with the thoroughness of the licensees root cause evaluations. Pending further review of the effectiveness of the licensees root cause evaluations in determining adequate corrective actions, this concern is carried forward as an Unresolved Item and is tracked under URI 05000348,364/2007010-003, Adequacy of Root Cause Analysis of the Failed Breakers. |
| Site: | Farley  |
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| Report | IR 05000348/2007010 Section 4OA5 |
| Date counted | Sep 30, 2007 (2007Q3) |
| Type: | URI: |
| cornerstone | No Cornerstone |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 93800 |
| Inspectors (proximate) | T Ross W Lewis G Khouri S Alexanderg Khouris Alexander T Ross W Lewis |
| INPO aspect | |
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Finding - Farley - IR 05000348/2007010 | |||||||||||||||||||||||||
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Finding List (Farley) @ 2007Q3
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