ABSTRACT
This chapter aims to describe the alternate methods of describing the strength and to present the alternate test methods used to determine the strength. Insulation may be classified as internal or external and also as self-restoring and non-self-restoring. External insulation is the distances in open air or across the surfaces of solid insulation in contact with open air that are subjected to dielectric stress and to the effects of the atmosphere. Internal insulation is the internal solid, liquid, or gaseous parts of the insulation of equipment that are protected by the equipment enclosures from the effects of the atmosphere. The BSL is the electrical strength of insulation expressed in terms of the crest value of a standard switching impulse. To establish more fully the short-duration strength of insulation, a time-lag or volt-time curve can be obtained. BILs and BSLs are specified for standard atmospheric conditions. However, laboratory atmospheric conditions are rarely standard.
