A monitoring system intended to track vibrations associated with the electric generation of an emergency diesel generator (EDG) at a nuclear power plant was originally installed to trip generators during potential out-of-balance shaking. Years after original installation, plant engineers raised questions as to whether the monitoring system would have functioned properly in the event of excessive vibrations associated with earthquake ground shaking. As part of the plant’s examination of this issue, analytical models were developed for three vibration transducers via experimental characterization. A suite of ground motion records were selected and scaled such that their mean and dispersion was representative of the design basis event (DBE) level of intensity at the plant site. The scaled suite of ground motions was used as an input for a three-dimensional analytical model representing the structure and soil-foundation interface where the EDGs were installed. The voltage signal from the analytical transducer model provided input to the physical vibration alarm monitor. This monitor filters the vibration transducer signal and generates an output to a time delay relay that will trip the EDG if the output from the vibration monitor is not reset within five seconds. The analysis and testing completed and described in this paper concluded that a DBE would not have caused the vibration monitoring circuit to spuriously trip the EDGs.

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