All solid and liquid insulations have some measurable loss since there is no perfect insulator. These losses are usually very small in the insulations typically used in electrical equipment and apparatus, and these losses vary as the square of the applied voltage. Gaseous insulations, such as air, do not have a measurable loss until they become overstressed or ionized.
Dielectric loss is measured in watts (resistive components) and is a measure of energy dissipation through and over the surface of the insulation. The dielectric losses of most insulations increases with increase in temperature, moisture, and corona. Insulations may fail due to the cumulative effect of temperature, that is, rise in temperature causes an increase in dielectric loss which in turn results in a further rise in temperature. This phenomenon is self-perpetuating and continues until the insulation fails.