Electrical Utility Cabinets: Evaluating Dynamic Properties

In March 2004 Sudhir Rusotogi and Abhinav Gupa published an article in the Journal of Structural Engineering that evaluated the Ritz vector approach for examining the dynamic properties of electrical utility cabinets. The basis of this suggestion, and therefore the prompt to conduct an experiment, was that a single, simple utility cabinet mode is sufficient to calculate accurate incabinet spectra needed in the seismic qualification of electrical instruments mounted inside the utility cabinet.

What Prompted this Research into Electrical Utility Cabinets?

According to the authors of the study, electrical utility cabinets and control panels in a significant industrial facility, like a nuclear power plant, have safety related electrical instruments such as relays mounted on them. For the safety of the plant and the surrounding area, these instruments must continue to work during an earthquake. As a result, these safety features are seismically qualified by what is called a “shake table test” in which the earthquake input is expressed in the terms of an incabinet response spectrum. These results, that dictate the safety of electrical utility cabinets in the presence of an earthquake therefore, depends upon the dynamic characteristics of the control panels or utility cabinets.

Goal of the Electrical Utility Cabinet Study

  • The hope of the current researchers is to be able to perform qualified evaluations of these utility cabinets utilizing an effective, simple method that is both time and cost saving.
  • The reason for this motivation is simple: the current method of testing the electrical utility cabinet (either by ‘finite element analysis’ or ‘vibration testing’) of several hundred different utility cabinets in a single plant is as mentioned, both time and cost prohibitive.
  • In addition, according the journal’s authors, the actual structural characteristics of a particular utility cabinet may change on numerous occasions during its lifetime as a result of the replacement of electrical instruments mounted on it.
  • Furthermore, the new instruments are generally not identical to the ones being replaced which can result in changes not only to the mass distribution of the utility cabinet but also structural characteristic changes that have needed such things as stiffeners to support them.
  • Therefore, significant resources are required to perform the dynamic monitoring of the electrical utility cabinet, especially with regards to time, money and safety regulations.
  • As a result, simple methods can ultimately overcome these limitations, according to the authors, as well as provide an engineering tool needed in day-to-day decision making for seismic qualification of safety related instruments.

Results of the Electrical Utility Cabinet Study

Ultimately, after an exhaustive quantitative study, the hypothesis that the Ritz vector approach would be effective remained intact. However, there was one modification that the authors stated should be considered: the Ritz vector representing the rigid body utility cabinet rocking should be considered in addition to the Ritz vectors for representing the local mode shapes.