The Siemens 4EU4321-0ED00-0A is a three-phase mains choke (line reactor) for variable-frequency drive input circuits, designated Netzdrossel f. FU. It sits between the supply and the drive rectifier to smooth current draw, limit harmonic distortion, and reduce voltage notching that couples back onto the line.
The 459 A rated current and 510 A maximum tell you this choke is sized for a drive with a nominal input current in that range — think a large motor drive, not a small servo. The 8 % voltage drop is the key number for sizing: if the drive requires full bus voltage under load, a choke with a lower drop percentage may be needed, but 8 % is standard for most industrial VFD applications where harmonic compliance (IEEE 519) is the goal. The flat-type terminals accept busbar or large cable lugs, not spring-clamp or screw terminals. Thermal class H per IEC 60085 means the insulation system withstands 180 °C continuous, so it handles the heat rise from high continuous current without derating prematurely.
Mounts on the panel floor or a subplate — no DIN rail, no snap-on. At 0.46 m wide × 0.435 m tall × 0.265 m deep, it needs a clear footprint and airflow around it; don't bury it against other heat sources. The flat-type terminals mean the main power cables land with bolted lugs, so plan for tool access and bending radius.