Finite insulator life: limiting processes

An insulator comes to the end of its working life either when it fails mechanically, flashes over at unacceptably high frequency or gives evidence of deterioration to a condition likely to lower its factor of safety in service. All insulators are affected to some extent by impact, cycling both thermal and mechanical, ablation from weathering and electrothermal causes, flexure and torsion, ionic motion, corrosion and cement growth. There are, however, strong differences between ceramic and polymeric insulators, as classes. In general, a ceramic insulator will be vulnerable to impact damage, since its dielectric is a brittle material, and to processes which cause concentrations of tensile or shear stress.