Scope. This guide is based on the knowledge and experience of manufacturers, researchers, and end users of nonceramic insulating products that are electrically stressed in outdoor environments. The guide discusses the characteristics of nonceramic materials that are needed for long-term reliability in these applications. The guide makes recommendations for principles of evaluation and tests used in the screening and selection of materials. The judicious use of this guide will significantly improve the probability of success in the applications selected. However, it should be recognized that the application on nonceramics to outdoor electrical insulation is an extremely demanding and complex problem. In any specific application, judgment and experience is required to analyze and balance the many tests and characteristics which are discussed to obtain satisfactory performance and reliability. Since the primary objective of accelerated aging is the prediction of material performance prior to its application and the development of an actual service history, accelerated aging tests should, within a short time, attempt to duplicate the effects of long-term exposure in the field. Extreme reductions in the time to failure are accomplished by increasing the intensity of one or more of the destructive forces of normal operation. For example, increased electrical stress, higher temperature, water immersion, or intense ultraviolet rays, are often used. The acceleration in the time base depends on the type of accelerated aging performed and can vary from a factor of 10 to 350 (1000 h vs 40 yr).