The basic principles of FOCT are introduced. The technology of anti-vibration, temperature adaptability and long-term stability are described, and the main solutions are given. Performance testing and application in China are summarized. The integrated installation method of FOCT and high voltage equipment, such as GIS, DCB, is put forward. Integrated equipment will be used for the construction of a new generation intelligent substation. The problems in the application of FOCT in electric energy measurement and relay protection are analyzed. A double sampling technique is proposed to meet the requirements of relay protection. At last, the development and application in the future are prospected.