What it is and what the key ratings mean for fit
The Siemens 3RB2045-2UB0 is a SIRIUS solid-state overload relay — a CLASS 20E device, meaning it will trip within 20 seconds at 7.2× the set current, which is the standard speed for protecting standard induction motors against locked-rotor conditions without nuisance tripping during normal start-up. It mounts directly to a contactor (contactor mounting), so it integrates into a motor starter assembly on a DIN rail or panel plate without needing a separate enclosure for the relay itself. The main circuit is rated for up to 1 000 V at AC-3 duty, so it handles the switching of squirrel-cage motors under starting and running conditions up to that voltage — typical for 400 V and 690 V industrial networks. The ATEX certification (PTB 06 ATEX 3001 Ex II (2) GD) means it is approved for use in potentially explosive atmospheres as a associated apparatus (category 2) for gas and dust, which is a common requirement in chemical, pharmaceutical, and grain-handling lines. Wiring the main circuit accepts solid conductors from 0.5 to 4 mm² (or 2× 0.5 to 2.5 mm²) and stranded up to 70 mm² (or 2× 10 to 50 mm²) via screw-type terminals — sized for motor leads from fractional-horsepower up to about 30 kW depending on the contactor frame. The depth of 124 mm fits standard 160 mm deep enclosures, but verify clearance for the outgoing cables and any auxiliary contact block mounted on the side.
Deployment context and approvals
The relay is designed for contactor mounting — it snaps onto the same DIN rail or panel as the contactor, saving panel space versus a separate overload block. The IP00 protection on the front means it is intended for installation inside an enclosure (IP54 or higher) where it is not exposed to dust or water jets. The shock resistance of 15g / 11 ms is sufficient for mounting on a machine frame or in a mobile equipment panel, not just a stationary cabinet. Approvals include ATEX Ex II (2) GD for gas and dust hazardous areas, plus surge immunity of 8 kV and burst immunity at 2 kV (power ports) / 1 kV (signal ports) per IEC 61000-4-4 severity level 3 — so it is suitable for industrial environments with moderate electrical noise, such as motor control centers near VFDs or welding equipment.