The Siemens 3RB2143-4UD0 is a SIRIUS solid-state overload relay for three-phase motor protection. It mounts directly onto a contactor (fastening method: contactor mounting), saving DIN-rail space in the panel. The trip class is adjustable across CLASS 5E, 10E, 20E, and 30E — meaning you can match the thermal memory to the motor's starting characteristic: fast trip for short-start pumps (5E), slow trip for high-inertia loads like centrifuges (30E). That adjustability is the key differentiator from a fixed-class bimetallic relay. Rated for 3 poles and carries an ATEX Ex II (2) GD protection type, so it is approved for use in potentially explosive atmospheres — the relay can be installed in the safe area and protect motors in Zone 1/21 or Zone 2/22.
The relay measures 70 mm wide, 106 mm high, and 124 mm deep. The main circuit accepts stranded wire 2x (10... 50 mm²) or 10... 70 mm², and AWG 2x (10... 1/0) or 1x (10... 2/0). The auxiliary circuit accepts solid 2x (0.25... 1.5 mm²) and AWG 2x (24... 16). Shock resistance is 15g / 11 ms, and surge voltage resistance is 8 kV. EMC immunity includes burst at 2 kV on power ports and 1 kV on signal ports, severity degree 3 per IEC 61000-4-4. Relative humidity during operation can reach 100 %.