This European Standard addresses the electromagnetic compatibility (EMC) requirements for electrical and electronic apparatus used in railway fixed installations for power supply. It encompasses power distribution systems, protective control circuits, and trackside equipment like switching stations, transformers, and substations. Filters for harmonic suppression or power factor correction are excluded, having specific site requirements. The standard specifies emission and immunity limits for apparatus located within substations, alongside tracks for power control, and for ancillary railway uses. Immunity requirements apply to vital equipment, traction power connections, and apparatus within specified distances. Equipment within railway boundaries but in residential, commercial, or light industrial environments must comply with separate EMC standards. Exclusions include inherently immune power supply apparatus. Compliance with EN 50121-1 is essential, covering both apparatus and fixed installations, with sections for fixed installations not relevant for CE marking purposes.