Within the framework of the study, a comprehensive methodology for electrical equipment technical state assessment based on complementary methods of technical diagnostics, providing the opportunity infrared and ultraviolet periodic inspection and partial discharges monitoring, was developed and validated for a industrial energy facility - 110 kV outdoor switchgear. As a result of the study, various defects were identified in various types of equipment with varying dynamics of their development. Each diagnostic method individually covers only a certain group of defects, which can be identified only in 20-25% of cases when using another method from the listed above. Moreover, the accuracy of defect identification using all three methods of technical diagnostics, as has been demonstrated on current and voltage transformers, increases up to 90%. The implementation of ultraviolet control in conjunction with thermal imaging control can improve the accuracy of defect identification by 15%.