When an ac voltage stress is applied to a ZnO varistor in the pre-breakdown region, a through current (leak-age) constantly flows through the device. This current consists of a resistive component and a capacitive component, the magnitude of the latter being several times greater than that of the former. In spite of this relatively lower value of IR, it is the key parameter responsible for Joule heating of the device. The value of IR is greatly enhanced by external factors such as applied voltage, ambient temperature and duration time of the applied voltage.