Issues in Generating Accurate In-Cabinet Response Spectra for Seismic Qualification of Electrical Equipment

In the context of seismic safety assessment, high frequency motions are not likely to cause damage to structural systems as the displacements associated with such motions are relatively very small. Yet, past experience around the world has shown that electrical instruments such as relays, breakers, and contact switches can exhibit loss of functionality due to high frequency motions. The spectra to define input motions used in shake table testing of such instruments is therefore called in-cabinet response spectra (ICRS). In this paper, we review the current methods used for evaluating ICRS and evaluate applicability of each method and its appropriateness for practical use. It is recommended that the methodology in EPRI NP-7146 SL based on a constant amplification factors is not appropriate under the current environment for seismic qualification. The constant amplification factors can be excessively conservative in some cases thereby rendering many electrical systems or relays to be seismically unfit for use in a plant. In other cases, the constant amplification factors can be unconservative. Therefore, they can result in incorrect ICRS for such cases. Application of the simplified method to an actual plant would require additional work collecting relevant information, finite element analyses, validation with experimental data, and development of the code or spreadsheets for implementing the approach.