With digital communication technologies continuing to grow in performance and reliability, a fully digital substation automation system (SAS) with a sampled values based process bus has become a realisable prospect in the foreseeable future. This, however, demands additional work in engineering configurations and tightens the existing constraints on multi-vendor interoperability, network performance and time synchronisation. As a result, detailed testing and validation of SASs prior to installation is paramount. Integrated testing is the most comprehensive approach to test a SAS, as it evaluates the performance of multiple aspects of the system simultaneously. Real-time power system simulators can interface with multiple intelligent electronic devices in real-time via IEC 61850 station and process buses, while accurately simulating the electrical substation. Hence, they possess the unique capability to carry out closed-loop, integrated testing of SASs. This study presents a methodology to test a fully digital SAS using a real-time power system simulator. Particular emphasis is given on carrying out integrated system testing of a SAS in a laboratory environment. Example test scenarios are provided to highlight the effectiveness of the proposed testing approach.