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As an insulating medium filled inside electrical equipment, transformer oil must possess excellent electrical properties in order to fulfill its intended functions properly. The main electrical properties of new oil include: breakdown voltage, dielectric loss factor, volume resistivity, and gas evolution tendency. The dielectric strength or breakdown voltage of transformer oil is a measure of its ability to withstand voltage within electrical equipment without being damaged; it is one of the main methods used to assess the quality of transformer oil. It actually measures the instantaneous breakdown voltage of insulating oil. In pure insulating oil, there are always some free electrons that become liberated under the action of high-energy external radiation, or that are emitted from the cathode due to strong local electric fields. Under the influence of an electric field, these electrons undergo impact ionization, which ultimately leads to the breakdown of the insulating oil. Since this breakdown is caused entirely by electrical effects, it is referred to as \"electrical breakdown\". Insulating oil used in engineering is never completely pure, containing various impurities. The breakdown of impure insulating oil occurs because impurities form \"bridges\" that span between the electrodes; these bridges have high electrical conductivity, which increases the leakage current and leads to severe heating. This in turn enhances the ionization process, ultimately resulting in ionization breakdown along the path of these bridges. This process is closely related to thermal processes, which is why it is called \"thermal breakdown\".