The method of partial discharges (PD) diagnostics is used to assess the insulation condition of high-voltage equipment in order to control quality and identify the initial process of its aging. The paper presents methods for measuring PD and describes their main advantages and disadvantages. Particular attention is paid to the optical method of registering PDs. An experimental study was carried out with a cable sample and a glass insulator to measure the PD intensity in defective places and record their optical radiation. Voltage to the tested objects was applied from 0 to 26 kV. To register the PD, an electronic-optical flaw detector FILIN-6 device was used. Also, according to the results of the measurement, amplitude-phase diagrams of the PD were obtained, by which it is possible to determine the degree of the defect. It is determined that the threshold level for the appearance of partial discharges is their power of about 100 mW. In this case, an increase in the discharge power to 160 mW leads to the appearance of a discharge visible to the naked eye. The specified partial discharge pow-ers may be relatively harmless only for equipment for voltage classes from 35 kV. Thus, registration of partial discharges by optical means makes it possible to detect defects at a relatively early stage only for equipment for high operating voltages.