An integrated fuzzy expert system is presented to diagnose various faults that may occur in a regional transmission network and substations. Fuzzy reasoning method is applied, and it is discussed in detail. Discrimination of false operations or nonoperations of protective devices as well as the fault identification scheme are also analyzed, together with the fuzzy inference process. The proposed system is designed to improve efficiency, generality, and reliability of the solution. The system will replace a fault diagnosis system that had been tested as a part of an intelligent support system on a local control center in Korea.