THERE are today two general types of lightning arresters in use for the protection of distribution apparatus. They are the valve type covered by AIEE Standard 28, and the expulsion type, for which AIEE Standards are being formulated. Both have been used widely for many years. Typical specimens are illustrated in Figure 1. The valve type is the older. The first distribution-valve arrester was an Autovalve produced in 1922. The first expulsion-type distribution arrester was the deion arrester (United States Patent, 2,050,397) used on surge-protected distribution transformers in 1931. By far the largest number of expulsion-type arresters manufactured have been applied as part of completely self-protected transformers. Many of these have used a somewhat different physical construction and mounting as shown in Figure 2 than that used for the arresters installed separately from the transformers. There exist today several variations of both the expulsion and valve types.