The Siemens 3RU6126-4BB1 is a SIRIUS-brand thermal overload relay for three-phase motor protection. The adjustable current range is 14 to 20 A, set via a dial on the front face. It's built for the S0 contactor frame size, so it mates directly with the matching SIRIUS contactor (like the 3RT1036) for a combined motor starter package. The overload release is purely thermal — a bimetallic strip heats up with motor current and mechanically trips the auxiliary contacts. No electronics to fail, no power supply needed on the load side. It accepts screw-type terminals for both the main and auxiliary circuits, and the wiring range covers 2x (0.5 to 1.5 mm²) solid or stranded, or 2x (0.75 to 2.5 mm²) with ferrules. For AWG, that's 2x (16 to 12) or 2x (14 to 8) on the main contacts.
This relay is designed for stand-alone installation — you can mount it on its own DIN rail or directly onto the S0 contactor. It needs 6 mm of clearance to grounded parts on either side, so plan your DIN rail spacing accordingly. The tightening torque for the main screw terminals is 2 to 2.5 N·m, which is a common range for this class of relay. The auxiliary and control circuit terminals use the same screw-type connection. Connections for the main circuit are at the top and bottom, so wiring in and out is straightforward in a typical panel layout.
Environmental and electrical ratings
The insulation voltage at pollution degree 3 is also 690 V AC — that's the standard for industrial control panels where conductive dust or humidity might be present. Relative humidity during operation can go from 0 to 90%, non-condensing. Installation altitude is limited to 2000 m above sea level. Per pole, that's 2.7 W. The auxiliary contact ratings are 2 A at 24 V, 0.22 A at 110 V, and 2 A at 230 V — enough for signaling back to a PLC input or a contactor coil.