The Siemens 3RU2126-4EB1 is a SIRIUS thermal overload relay, size S0, with a CLASS 10 trip characteristic. It protects motors against overload and phase failure by monitoring the current draw and tripping the associated contactor when the thermal model reaches the trip threshold. The CLASS 10 rating means it will trip within 10 seconds at 7.2x the set current — fast enough for most standard induction motor starts, but not for high-inertia loads that need a longer acceleration ramp.
Ratings and what they mean for fit
Rated insulation voltage is 690 V, with power ratings of 15 kW at 400 V, 18.5 kW at 500 V, and 30 kW at 690 V. The AC-3e maximum voltage is also 690 V, so it's suited for 400 V and 690 V motor circuits common in European and export panels. The auxiliary switch is integrated, saving a separate aux block on the contactor. The thermal overload release is ambient-compensated from -40 to +60 °C, so the trip curve stays stable across the panel's internal temperature range. Terminals accept 2x (0.5 to 1.5 mm²) or 2x (0.75 to 2.5 mm²) solid or stranded wire, with M4 main contact screws. The screwdriver spec is Pozidriv PZ2 with a 5 to 6 mm shaft diameter — standard for panel builders. Mounting position is any, and the relay can be installed stand-alone or directly onto a SIRIUS contactor.
Deployment context
This relay mounts in a motor control center or panel enclosure, typically on a DIN rail or directly coupled to a SIRIUS contactor. The S0 frame size is a common footprint across the SIRIUS family, so panel cutouts and wiring layouts for the contactor side are standard. The integrated auxiliary switch eliminates one wiring step during panel assembly.