What it is and what it does
The Siemens SIRIUS 3RU2126-4DB1 is a thermal overload relay — the part that sits between the contactor and the motor, watching current and tripping before the winding cooks. It's Class 10, meaning it will trip within 10 seconds at 7.2× the full-load current setting, which is the standard for standard induction motors starting against a fan or pump load. The relay is size S0, the most common frame in the SIRIUS modular system, and it mounts stand-alone or directly onto a matching SIRIUS contactor.
Key ratings and what they mean for your motor
At 400 V AC-3e duty it's rated for 11 kW motor output; at 690 V that rises to 22 kW. The current setting range is covered by the 25 A rating at 480 V and 600 V. These are the numbers that decide whether this relay matches your motor nameplate — if your motor FLA sits inside the adjustment range of this unit, it's the right pick. The auxiliary switch is integrated, so you don't need a separate add-on block for the trip signal back to the PLC or the contactor coil circuit. Trip Class 10 is the default for most general-purpose motor starting. If you're starting a high-inertia load (centrifuge, flywheel) that needs a longer acceleration ramp, you'd want Class 20 or 30 — but for conveyors, pumps, compressors, and fans, Class 10 is correct and gives faster protection against a stalled rotor.
Panel fit and wiring
Dimensions are 45 mm wide, 97 mm tall, 95 mm deep — a compact S0 footprint that fits a standard 45 mm-wide DIN-rail slot. Mounting position is any, which helps when panel space is tight. The main circuit terminals accept screw-type connections: 2× (0.5 to 1.5 mm²) or 2× (0.75 to 2.5 mm²) solid or stranded. Use a PZ2 Pozidriv screwdriver with a 5–6 mm shaft diameter. Temperature compensation is active from -40 to +60 °C, so the trip curve stays accurate in a warm cabinet.
Compliance and documentation
The part is RoHS-compliant per the substance prohibition date of October 1, 2009. The MTTF with high demand rate is listed at 2,280 years — a reliability figure that supports holding this as a critical spare without worrying about shelf-life degradation. The note on the product line reads "for message 'Tripped'" — this is the signal the integrated auxiliary switch sends when the relay has operated, used to trigger an alarm or stop the contactor.