The Siemens 3RB2123-4PB0 is a SIRIUS solid-state overload relay — no bimetals to drift, no heater packs to swap. It mounts directly onto a contactor (contactor mounting) and protects motors against phase loss, overload, and prolonged start conditions. The trip class is field-adjustable among CLASS 5E, 10E, 20E and 30E, so you can match the relay to the motor's starting profile without stocking four different units.
Width is 45 mm — a single module slot on a contactor assembly. Depth is 75 mm, height 73 mm. Auxiliary contact ratings are given per voltage: 2 A at 24 V, 4 A at 120 V, 3 A at 230 V, and 0.55 A at 60 V, 0.3 A at 125 V, 0.11 A at 220 V — these govern the control-circuit load the relay can switch directly (PLC inputs, contactor coils, indicators).
The main circuit terminals are screw-type, accepting solid conductors 0.5... 4 mm² (or 2x 0.5... 2.5 mm²). AWG equivalents for the main contacts are 2x (14... 10). Side clearance of 6 mm is needed; forward, backward, upward, and downward clearance is 0 mm — it can butt against other components on those faces. Operating temperature range is -25... +60 °C; storage/transport range is -40... +80 °C. Shock resistance is rated 15g / 11 ms.