There are two types of power districts: Single power district per booster or multiple power districts on a single booster. In either case, you want a PSX or PSX-AR protecting each power district. If you have a single booster for a power district, you still want a PSX-1 protecting that district. The reason is that the protective circuitry in the booster is designed to protect the booster, NOT your $1000 fully lit, full sound, decoder equipped, fully detailed scale model. The PSX series IS designed to protect your models. For the multiple power district per booster, you want to use a PSX-2, PSX-3, or PSX 4. Each PSX feeds a district, and the inputs of the PSX are daisy chained together to a single booster.