Influence of humidity exposure time on dielectric properties of insulation material surfaces

The exact knowledge about the water adsorption characteristics of insulation materials is of significance for dimensioning of creepage distances with respect of flashover under humidity conditions. In order to describe each insulation material with respect to the water adsorption characteristics the insulation materials under test are classified into water adsorption groups. For this classification an appropriate test procedure is required. Hereby, however, two problems arise: The fundamental problem is involved with the preparation of the test samples especially the realisation of an appropriate electrode configuration. For this we will discuss two alternatives: The first one is using printed conductors, the second one is fixing the electrodes by application of high pressure upon the insulation material surface. Another problem is the duration of the test, because insulation materials, depending on surface quality and chemical compound, can change water adsorption characteristics during humidity exposure time. Therefore in this paper also the influence of humidity exposure time on the dielectric properties of the insulation material surface is discussed. Within the scope of this paper the investigated test specimen are consisting of three different resins (epoxy, polyester and polyimid resin) as well as a laminate consisting of presspan and polyester foil.