Stresses in the manufacture and use of electrical porcelain

In this paper, some of the conditions to which power system insulators are subjected in service are listed together with the ambient temperature range to which they are exposed. The composition of the two main types of porcelain are shown and their method of manufacture described. The physical properties of the two porcelains are compared, and diagrams of the structure of each type of porcelain are given. The effect of glaze on the flexural and tensile strength of porcelain is described. The brittle, nonductile, nature of porcelain is highlighted with the resulting Weibull distribution of tensile strength results rather than the normal distribution generally achieved with ductile materials. Reference is made to the fact that testing on small diameter porcelain samples gives higher levels of stress at failure than tensile stresses on larger diameter pieces. The three main types of insulator are described with an examination of the types of load to which these insulators are subjected in service. The methods of testing of each insulator type are discussed using both nondestructive and destructive techniques. The various types, including acoustic emission methods, of testing systems are mentioned. The crack propagation process in porcelain is described referring to the development from subcritical crack growth through to fast fracture. Finally a method of estimating the likely service life of insulators is discussed