Calculation of Optimum Preventive Maintenance Intervals for Electrical Equipment

Formulas to calculate the optimum interval of preventive maintenance and the optimum interval of diagnostic testing of electrical equipment are presented. Each user can calculate his own custom intervals using the formulas which are based on least total annual costs. The user must have his own data base of equipment failures similar to the example presented. Using 10000 failures of electric utility control and monitoring equipment as a sample, it was found that 25 percent of the failures could have been prevented by preventive maintenance, 65 percent could have been caught by diagnostic testing, and less than 10 percent have failed in the operative mode and could not have been maintained against or caught beforehand. One factor in the formula, which determines the shape of the equipment failure curve, remains to be proven after the next five years of data are collected.