| Incabinet response spectra are needed for the seismic qualification of safety class I instruments such as relays. Currently used simple methods give unrealistic spectra that can be excessively conservative. Vibration testing and finite element analysis are very sensitive to the mounting arrangement of a cabinet. Both these techniques are time and cost intensive. A simple and easy-to-use method for evaluating accurate incabinet response spectra is developed in this study.;Dynamic characteristics of cabinets are studied by detailed finite element analysis of several typical cabinets. At first, the cabinets are considered to be rigidly fixed at base. The effect of base flexibility is studied separately. It is observed that very few modes (often one) contribute significantly to the response at any instrument location on cabinet. For most cases, this mode is a local mode of the cabinet component (such as door, back wall or internal frame) on which the instrument is mounted. A global cantilever mode may also contribute significantly, along with or without the local mode. Conclusions drawn from these observations are used to develop a Ritz vector approach for evaluating incabinet spectra. In this method, the dynamic properties of significant cabinet modes are evaluated using Rayleigh-Ritz method and incabinet response spectra evaluated using CREST-IRS. Spectra obtained from this method are close to those obtained from finite element analysis.;Next, the effect of base flexibility is considered and the finite element analysis results for 2 cabinets are compared with available modal test data. For a third cabinet (switchgear) both the modal and shake table test data are compared with the finite element analysis. The dynamic properties and incabinet spectra obtained from the modified finite element models compare well with those obtained from tests. This comparison shows the presence of a global cabinet mode which is predominantly a rigid body rotation about the base of cabinet. Conclusions drawn from these new observations are used to include the effect of base flexibility in the Ritz vector approach. A simple equation is developed for accurate evaluation of rotational stiffness of the cabinet mounting arrangement and incorporated in the Ritz vector approach. A computer program, INCABS, is also developed to implement the Ritz vector approach for evaluation of significant cabinet properties. INCABS together with CREST-IRS is used to generate accurate incabinet spectra. |
