The Siemens 3RB2123-4ND0 is a SIRIUS solid-state overload relay for three-phase motor protection. It mounts directly to a contactor — no separate DIN-rail footprint needed — and provides adjustable trip classes 5E, 10E, 20E, and 30E, so you can match the thermal curve to the motor start profile (fast for pumps, slow for high-inertia loads). The ATEX Ex II (2) GD marking means it's certified for use in potentially explosive atmospheres, covering both gas and dust zones. Main circuit connections use screw-type terminals; the auxiliary and control circuit uses spring-loaded terminals for faster wiring.
The adjustable trip class is the headline feature: set it to 5E for quick-trip protection on short-start motors, or 30E for high-inertia loads like centrifuges. The 3-pole design covers standard three-phase motor circuits. Surge voltage resistance is rated at 6 kV, and the relay withstands burst transients at 2 kV on power ports and 1 kV on signal ports per IEC 61000-4-4 severity level 3 — enough for industrial environments with contactor switching and VFD noise.
Fastening method is contactor mounting — it clips directly onto the matching SIRIUS contactor, saving DIN-rail space. Dimensions: 45 mm wide, 73 mm high, 75 mm deep. Mounting position is any. Main circuit accepts 2x (0.25... 1.5 mm²) solid or finely stranded with ferrule, or 2x (14... 10) AWG. Auxiliary circuit uses spring-loaded terminals for 2x (24... 16) AWG.