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7 8 57 58 59 60 61 68 69 74 75 SO EVENT DESCR;PT1CN O2 l See Attachment l
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[NJ 7 89 10 11 12 17 43 44 47 48 CAUSE DESCDPTION
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ATTACHMENT TO R0-50-315/77-05 k
EVENT DESCRIPTION On January 11, 1977, at 1037 hours0.012 days <br />0.288 hours <br />0.00171 weeks <br />3.945785e-4 months <br />, with the Reactor in Refueling Mode 6, 600 volt bus 11C was crosstied with Bus 11A and the 11C feed breaker, 11C1, was opened.
At 1348 hours0.0156 days <br />0.374 hours <br />0.00223 weeks <br />5.12914e-4 months <br />, 600 volt bus llB was crosstied with bus 110 and the llB feed breaker, 11B11, was opened.
Both feed breakers and their supply transformers were tagged out for Doble testing of the transformers.
This left all 600 volt buses energized, but only buses 110 and 11A would be energized from an emergency diesel generator in case of a blackout.
Per Technical Specification 3.8.2.2, Refueling Mode 6 requires one 600 volt bus to be operable and aligned to an operable diesel generator.
Per Technical Specification 3.8.2.1 one 600 volt bus is defined as 11A and 11B or 11C and 110.
At 1513 hours0.0175 days <br />0.42 hours <br />0.0025 weeks <br />5.756965e-4 months <br /> feed breaker 11A1 tripped due to overload.
This deenergized buses 11A and 11C.
The clearance of 11C transfomer was released and bus 11C energized from it's normal source at 15f' hours.
This made 600 volt buses 11C and 110 operable.
An investigation of bus 11A found the middle horizontal bus Far burned open between bus cubicals 1 and 2.
The bus bars for the other two phases in the burned open area were opened so that cut,icals 1 and 2 were no longer tied together.
To accomplish this bus 11C was deenergized at 1758 hours0.0203 days <br />0.488 hours <br />0.00291 weeks <br />6.68919e-4 months <br /> due to the proximity of bus 11C to the work area for opaning the phases in bus 11A.
While doing this work bus 11D was the only bus aligned to an operable diesel generator, however, bus llB was energized from bus 110.
Bus 11C was energized from it's nomal source at 1917 hours0.0222 days <br />0.533 hours <br />0.00317 weeks <br />7.294185e-4 months <br />.
Bus 11A, except for cubical 1, wa energized from it's normal source at 2029 hours0.0235 days <br />0.564 hours <br />0.00335 weeks <br />7.720345e-4 months <br />.
Cubical 1 of bus 11A was energized at 2031 hours0.0235 days <br />0.564 hours <br />0.00336 weeks <br />7.727955e-4 months <br /> by closing bus tie breaker 11AC.
This made all feeders from bus 11A operable.
The Technical Specification requirement for leaving an operable 600 volt emergency bus aligned to an operable diesel generator was not met for a period of 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> and 15 minutu when the buses were crosstied for Doble testing and for a period of 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> and 20 minutes for the emergency repair work to the failed bus 11A.
The total elapsed time from the start of the testing to completion of the temporary repair work was 5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> and 29 minutes.
Bus 11A and 11C were again removed from service on February 12 for the permanent repair of bus 11A.
The elapsed time that both buses were deenergized was 7 hours8.101852e-5 days <br />0.00194 hours <br />1.157407e-5 weeks <br />2.6635e-6 months <br /> and 48 minutes.
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Page 2 CAUSE DE';CRIPTION The cause of the first event was personnel error when the original switching was made for testing of the transformers.
The cause of the second event was equipment failure.
The 11A bus bar burned open at a bolted connection which either had loosened during use or had never been properly tightened during manufacture.
The latter reason is believed to be the most likely cause because all other bus connections that have been checked are of proper bolt tension and show no evidence of overheating.
ADDITIONAL FACTOR _S_
The personnel ' error was due to a misinterpretation of the requirements of Technical Specification 3.8.2.2 which only specifies "1 - 600 volt emergency bus" being required in Modes 5 and 5.
The Technicel Specifications involved will be reviewed by all licensed personnel prior to March 1,1977.
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