The Siemens SIRIUS 3RU6126-4FB0 is a thermal overload relay sized for motor protection, with an adjustable current response range of 34 to 40 A. It is designed for direct mounting onto size S0 contactors, forming a compact motor starter combination for panel builds. This is the figure that governs whether the relay coordinates with your motor's acceleration time. Rated operational current is 40 A, operating voltage up to 690 V at 50/60 Hz, with a total typical power loss of 7.5 W (9.6 W at rated current in hot operating state).
Mounts directly onto the contactor via the fastening method (contactor mounting), no DIN rail needed for the relay itself — it clips onto the contactor and the assembly snaps onto the DIN rail. The S0 contactor size is the pairing constraint: this relay mates with S0-frame contactors like the 3RT1023 series. Side spacing to grounded parts requires 6 mm clearance. Main contacts accept 2×(0.5…1.5 mm²) solid/stranded or 2×(0.75…2.5 mm²), with AWG equivalents 2×(16…12) and 2×(14…8). Tightening torque is 2 to 2.5 N·m. Auxiliary contacts use smaller gauge: 2×(20…16) and 2×(18…14) AWG.
Relative humidity during operation is 0 to 90 %. Shock resistance is 8g / 11 ms per IEC 60068-2-27. Maximum installation altitude is 2000 m above sea level.