The Siemens 3RB3143-4UW1 is a SIRIUS solid-state overload relay in the S3 frame size, rated for a thermal current of 50 A and adjustable trip classes 5E, 10E, 20E, and 30E. It protects motors against overload and phase failure by monitoring current through straight-through transformers on the main circuit. The relay trips when motor current exceeds the upper setting value by up to 3.5×, and resets once current drops below the lower setting — no mechanical bimetals to drift.
The 50 A thermal current (Ith) at 480 V and 600 V rated values means this relay handles motor full-load currents up to 50 A in AC-3e duty at up to 1 000 V. The adjustable trip class lets you match the relay's thermal curve to the motor's starting profile: Class 5E for short acceleration times (e.g., centrifugal pumps), Class 10E for standard induction motors, Class 20E for high-inertia loads like fans, and Class 30E for conveyors or crushers with long start times. The auxiliary switch is integrated and rated 2 A at 24 V, 4 A at 120 V, and 3 A at 230 V — enough to signal a PLC input or drop out a contactor coil directly.
Mounts in any position via stand-alone installation — no DIN-rail adapter required, but the 70 mm width and 106 mm height fit standard backplate layouts. The straight-through transformer design means main power conductors pass through the relay's window; no ring lugs or bus bars needed. Terminals accept 1× (0.5–4 mm²) or 2× (0.5–2.5 mm²) solid or stranded wire. Screwdriver shaft diameter 5–6 mm, Pozidriv PZ2 tip. Depth is 124 mm — account for that when spacing components on the gland plate.