This standard describes the design criteria of the PPS and its interfaces with the Class 1E power system, switchyard, transmission system, and AAC source. Figure 1 is a typical interface diagram of the PPS with related power systems. This standard provides PPS requirements for nuclear power generating stations (NPGSs) and guidance in the areas of AAC power source interfaces with the PPS, physical independence of the PPS power and control circuits, and expanded PPS criteria for multi-unit stations. Revision Standard - Superseded. The design criteria of the preferred power supply (PPS) and its interfaces with the Class 1E power system, switchyard, transmission system, and alternate ac (AAC) source are described. This standard provides PPS requirements for nuclear power plants and guidance in the areas of AAC power source interfaces with PPS, physical independence of the PPS power and control circuits, and expanded PPS criteria for multi-unit stations.