The Siemens 3RU2126-4PC0 is a SIRIUS-branded thermal overload relay, size S0, designed for motor protection in a control panel. It mounts directly onto a contactor, saving DIN-rail space — the fastening method is contactor mounting, so it stacks on the load side of the matching S0 contactor rather than taking up its own rail slot. That's the standard for standard-duty motor starts — pumps, fans, compressors that don't need a longer acceleration curve. For a motor that takes longer to spin up, you'd step to CLASS 20 or 30; this one is for normal across-the-line starts. Rated insulation voltage is 690 V, with motor power ratings of 18.5 kW at 400 V, 22 kW at 500 V, and 30 kW at 690 V. The auxiliary switch is integrated — no separate add-on block needed for the trip/alarm signal. Terminals are spring-loaded, accepting 2× (0.5 to 2.5 mm²) solid or stranded, which speeds up wiring compared to screw-clamp on the same footprint.
The auxiliary contact ratings cover a range of control voltages: 2 A at 24 V, 3 A at 120 V, 2 A at 230 V, 1 A at 400 V. At 60 V and 125 V the rating drops to 0.3 A and 0.22 A respectively, which matters if you're switching a DC control circuit at those levels. Temperature compensation is active from -40 to +60 °C, so the trip curve stays accurate across the panel's ambient range. The MTTF at high demand rate is 2,280 years — a reliability figure that reflects the thermal bimetal design with no electronics to fail. Dimensions are 45 mm wide, 102 mm high, 84 mm deep — the S0 footprint. That's the same width as the matching S0 contactor, so the stack takes up one 45 mm slot in the panel. Mounting position is any, which helps when the panel layout is tight.
The substance prohibitance date is October 1, 2009, which aligns with RoHS compliance — the part meets the EU RoHS directive for hazardous substances. The SIRIUS series carries CE, UL, CSA, and other international approvals as standard for Siemens industrial controls, though the specific cert marks aren't listed in this spec set. For a compliance binder, the manufacturer's declaration and the product label are the traceable documents.