The product code ABB REF543KM129 A A A A refers to a specific, fully configured unit of the ABB REF 54x series, which is a bay controller and protection relay designed for medium voltage switchgear applications. The REF 543 is a versatile, modular numerical relay used for the protection, control, monitoring, and measurement of feeders, motors, transformers, and generators in utility and industrial power distribution networks (typically 6-36 kV).