The Siemens 3RB3143-4UB0 is a SIRIUS solid-state overload relay — Size S3, rated for a thermal current of 50 A, designed to protect motors against overload and phase failure. It mounts directly onto a contactor, saving DIN-rail space in the panel. Trip class is field-adjustable among CLASS 5E, 10E, 20E, and 30E, so you match the relay's response to the motor's starting characteristics — fast for low-inertia loads, slower for high-inertia starts like fans or conveyors. The note 'for message tripped' means the signaling contact 97/98 changes state when the relay has tripped, giving a discrete output to a PLC or indicator lamp. That contact is rated 2 A at 24 V, 4 A at 120 V, and 3 A at 230 V — enough for most control circuits without an interposing relay.
Thermal current of 50 A at both 480 V and 600 V means this relay handles the full motor FLA up to that level in a 480Y/277 or 600V delta system — common for North American industrial feeds. The maximum operating voltage is 1 000 V, covering 690 V IEC applications as well. Storage and transport range is -40 to +80 °C. Vibration resistance of 1-6 Hz at 15 mm amplitude and 6-500 Hz at 20 m/s² for 10 cycles, plus shock resistance of 8g / 11 ms, qualifies it for mounting on machinery that sees regular vibration — pumps, compressors, conveyor drives.
Fastening method is contactor mounting — it clips directly onto the matching SIRIUS contactor, no DIN rail needed. Main circuit terminals accept 1x (0.5 to 4 mm²) solid or 2x (0.5 to 2.5 mm²) stranded, with screw-type terminals requiring a Pozidriv PZ2 tip and 5-6 mm shaft diameter. For larger conductors, stranded up to 2x 16 mm² is accepted. Dimensions are 70 mm wide, 106 mm high, 124 mm deep — Size S3 frame. That's a compact footprint for a 50 A solid-state relay; verify clearance for the depth behind the panel door.
For a BOM freeze or long-term planning, this relay has no known PCN or LTB. The SIRIUS platform is Siemens' mainstream motor-protection line, so cross-generational compatibility is stable.