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While in service, the equipment is under long-term exposure to the operating voltage and short-term but repeated exposure to overvoltages in combination with other factors (temperature, mechanical stress, and adverse environmental state). This results in deterioration of the electric insulation and failure of the equipment due to insulation breakdown. To trace the dynamics of insulation state of the operating equipment, its preventive tests are carried out. Ideally, the preventive tests should be aimed at reliable assessment of the residual operating lifetime of the equipment. However, this problem has been solved only incompletely because of the complexity of diagnostics of insulation aging, description of aging mechanisms, and establishment of a relationship between the insulation state at the time of testing (or fixing the parameters during monitoring) and the remaining operating lifetime. This paper discusses the diagnostic problems to assess residual operating lifetime.