The dielectric strength of this kind of oil is usually between 15kV and 35kV. The instrument can test it normally too. During rising of voltage, a few bubble particulate or impurities will be adsorbed to electrode gap to cause discharge. The bubble will be broken through and pushed out from electrode gap, and the oil will come and fill the electrode gap. Therefore, the voltage should be raised continuously up to break-down, the test data is reliable.