Dissolved gas analysis (DGA) initially developed for the monitoring of transformers was recently introduced for in-service diagnostics of power cables. Criteria successfully developed for transformers are not readily applicable to cables because of significant differences in the design and operating characteristics of both systems. The paper presents the development of DGA diagnostic criteria for the maintenance of Hydro-Quebec's power cable network. The diagnostic of a given cable circuit is based on the voltage rating, the individual gas concentration levels, the key gas ratios, the gas formation trends, the years in service, and the load. A new graphical representation ofthe IEC 599 method for the interpretation of gas ratios was developed in view of its application to power cable diagnostic. This new method, also applicable to transformers, solves most of the undetermined diagnostics given by the IEC method. The whole DGA diagnostic procedure logic is coded in an expert system shell automating the final diagnostic on the condition of the cable insulation.