In Japan, for the UHV AC transmission system, 1100 kV, 50 kA two-breaks gas circuit breakers (GCBs) were developed. They apply a resistance closing and opening scheme to suppress switching overvoltages that dominate the tower size of transmission lines. To secure high reliability and economy, specifications for the interruption duties were determined by detailed system analyses considering the phenomena arising from the inherent characteristics of UHV systems. The transient recovery voltage (TRV) duties were specified based on the resistance interruption and high-performance metal oxide surge arresters (MOSAs) arrangement, both of which effectively relax the TRV. Developed GCBs were subjected to various verification tests, and their long-term reliability and performance have been confirmed by the field testing since 1996 at the UHV Test Station of Tokyo Electric Power Company (TEPCO). Copyright © 2009 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.