Abstract: To address the technical-generation mismatch, intersecting operational boundaries and limited switching windows in protection and integrated-automation retrofits of substations, this paper analyzes the interface risks associated with AC sampling, trip and close circuits, signaling, communication and virtual circuits. A secondary-design method consisting of an interface register, a circuit-mapping matrix, a staged cutover sheet and a verification matrix is proposed. Starting from an as-is survey, the method links schematics, terminals, communication parameters, configuration versions and test evidence, while bringing temporary circuits under the same lifecycle control. The analysis indicates that the method improves boundary traceability and reduces the risks of omitted connections, incorrect wiring and configuration-version mismatch. It is applicable to protection and automation retrofits of conventional substations and secondary-system expansion of renewable-energy stations.