What it is and what the ratings mean
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 main current circuit uses screw-type terminals; the auxiliary and control circuit uses spring-loaded terminals, which speeds wiring and resists vibration loosening.
Where it fits and how it connects
The relay measures 70 mm wide, 106 mm high, and 124 mm deep. It mounts in any position and requires 6 mm clearance at the side. 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). Operating temperature range is -25 to +60 °C; storage and transport range is -40 to +80 °C. 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 %.