A lightning arrester with a series gap for transmission lines has been developed to prevent faults due to lightning. It has shown excellent performance since 1986 on 77 kV service lines in heavy lightning regions. A 275 kV lightning arrester has also been developed, and a field test was started on a service line in 1988. The authors describe the requirements for lightning arresters for transmission lines, design concepts, specifications, and operational records on the 77 kV lines for three years. The gapped-type lightning arrester is considered the most practical given the existing technology. To obtain light and small lightning arresters, an ethylene-propylene rubber shed was adopted for external insulation and a special pressure relief mechanism was introduced in place of the pressure relief guide of the station arresters. The follow current can be cut off within a period of 0.25 to 0.5 cycles even under contaminated and wet conditions. >