Abstract An advanced expert system for operation guidance in a 500 kV substitution is presented. The system performance is significantly improved reflecting experience in developing the previous system. When a fault or trouble occurs in the substation, real-time interence from on-line information is executed, and guidance of the whole restoration process is offered on the operator's request. Histories of measured values are also presented using data from the equipment diagnosis system. Faults and troubles are identified by inference using detailed input informaton and knowledge on all equipment, relays, and situations. Frames and rules described in the knowledge base can be maintained easily by end-users with a knowledge-base maintenance utility. The system installed in a 500 kV switching station has been in actual use since February 1991 in good condition.