The Siemens 3RB2055-1FW2 is a SIRIUS solid-state overload relay, Class 10E, designed for motor protection in industrial control panels. It mounts directly on a contactor or as a standalone unit, giving the panel builder flexibility in layout.
Key ratings and what they mean for fit
Trip class 10E means the relay will trip within 10 seconds at 7.2× the current setting — fast enough to protect standard induction motors during a stall or locked-rotor event, without nuisance tripping on normal starting inrush. The ATEX certification PTB 06 ATEX 3001 Ex II (2) GD permits use in Zone 2 gas and Zone 22 dust hazardous areas when paired with an appropriate enclosure — a compliance detail that often decides the BOM for explosive-environment lines. Rated for continuous operation from -25 to +60 °C ambient, with storage down to -40 °C. The 100 % relative humidity rating (condensing) means it can handle washdown environments without conformal coating on the electronics. Surge withstand at 8 kV (impulse) and burst immunity at 2 kV on power ports (severity 3 per IEC 61000-4-4) means it holds up in industrial electrical noise — no external surge suppressor needed for most panels.
Mounting and wiring
Mounts in any position — vertical, horizontal, or flat — with 6 mm clearance to adjacent devices on the side. The straight-through transformer design keeps the main power conductors passing directly through the relay body, simplifying bus-bar routing. Accepts solid or finely stranded conductors: 0.5 to 4 mm² solid (or 2× 0.5 to 2.5 mm²), and 0.5 to 2.5 mm² finely stranded with ferrules (2× 0.5 to 1.5 mm²). The spring-cage terminals match standard DIN-rail terminal block practice.