The effect of moisture on the insulating properties of oil depends upon
the form in which the moisture exists. A very small amount of emulsifi ed
water has a marked infl uence in reducing dielectric strength of oil. Free
moisture in oil usually shows up above 50 to 60 ppm depending upon
temperature. Accepted levels of water in oil are shown in Table 4.1. The
amount of moisture that can be dissolved in oil increases rapidly as the oil
temperature increases, as shown in Figure 4.1. Therefore, an insulating oil
purifi ed at too high a temperature may lose a large percentage of its
dielectric strength on cooling, because the dissolved moisture is then
changed to an emulsion, unless vacuum dehydration is used as the purifi
cation process.