SIRIUS 3RU2126-4PB1 — what this overload relay does
The Siemens 3RU2126-4PB1 is a SIRIUS thermal overload relay designed to protect motors against overcurrent and phase-loss conditions. That's the standard for standard-duty motor starting — it lets the motor accelerate under load without nuisance tripping but still clears a locked-rotor condition fast enough to protect the winding insulation.
At 400 V rated voltage, this relay handles 18.5 kW motor load; at 500 V it's rated 22 kW, and at 690 V it goes to 30 kW. At 60 V it's only 0.3 A, so watch that if you're running a 60 VDC control bus. Power dissipation per pole is 3.2 W. In a densely packed panel with multiple relays side by side, that adds up — budget the thermal rise in the enclosure, especially if the ambient hits the upper end of the -40 to +70 °C operating range. Temperature compensation is active from -40 to +60 °C, meaning the trip characteristic stays accurate across that band without manual adjustment. Outside that range the thermal model drifts — but the storage and transport limits are wider at -55 to +80 °C.
The screw terminal requires a Pozidriv PZ 2 tip with a 5–6 mm shaft diameter — standard electrician's screwdriver, nothing special.
No official successor has been published; the 3RU2 series is the active SIRIUS overload relay line and the 3RU2126-4PB1 remains the correct order code for S0-frame CLASS 10 applications at these power ratings.