The following is a summary from a part 21 report received via facsimile:
Susquehanna identified a deviation with a phase to phase to ground short that was caused by misalignment of the cubicle stabs when racking in the cubicle. The cubicle was racked into the cell and 2 of the 3 phase stab fingers engaged properly with the bus. The A phase stab fingers missed the center of the bus, however an electrical connection was maintained. This condition was estimated to be present for 3 years before the failure occurred. The deviation was originally evaluated as an installation issue, however after further evaluation, the deviation has been determined to be a failure to comply with the specification requirements. The specification requirements were reviewed and it was determined that the design of the connection stabs did not fully comply with the requirements.
The reported deviation will complete an electrical connection between the stab finger assembly and the bus, but at a possible reduced current carrying ability as only 1 side of the stab finger assembly is touching the bus. This causes less surface area contact between the stab finger and the bus, which causes less current carrying ability. The cubicle will likely function, however over time, if left uncorrected, it is possible that the connection could overheat, depending on the load of the cubicle, since the connection has a reduced surface area and/or has a higher resistance than what would be typical if the stab finger assembly was properly engaged.
The issue could extend to all F10 series MCC cubicles supplied by NLI [Nuclear Logistics Inc.] with the current design of stab assembly P/N: 5600-SA3-1. This design has been provided to Susquehanna Station, Limerick Generating Station, and Perry Nuclear.
Please contact me with any questions or comments:
Tracy Bolt, Director of Quality Assurance
AZZ Nuclear
7410 Pebble Drive
Ft. Worth, TX 76118