This paper presents the Protective Device Coordination Expert System (PDCES) software which helps electric distribution engineers correct miscoordinated protective device pairs. PDCES was prototyped as a component of PG&E’s Electric Distribution Planning Analysis (EDPLAN) system, an in-house engineering tool for analyzing electric distribution feeders. PDCES evaluates the causes of miscoordination, suggests and ranks coordination remedies, and, at the user’s discretion, sequentially identifies the most appropriate course of corrective actions. It then triggers EDPLAN to reset protective device parameters to alleviate feeder miscoordination. Protective devices studied by PDCES include relays, fuses, reclosers and interrupters. PDCES contains 14 knowledge bases and more than 300 rules, and invokes 2 graphical displays: a distribution network’s fishbone diagram and a geographical configuration. An expert system shell was used to provide rapid prototyping, friendly user interface, knowledge base construction, knowledge inference and external program capabilities.