SIRIUS thermal overload relay, Size S0 — 3RU2126-1DC0
The Siemens 3RU2126-1DC0 is a SIRIUS thermal overload relay in Size S0 with Trip Class 10. It mounts directly to a contactor and uses spring-loaded terminals for the main circuit wiring. Rated for AC-3e switching up to 690 V and 50/60 Hz, this relay handles motor loads in the 1.1 kW to 2.2 kW range depending on line voltage. The thermal bimetallic release compensates for ambient temperature shifts from -40 °C to +60 °C, so the protection curve stays accurate whether the relay is in a heated control room or a cold warehouse.
Mounting and wiring — panel integration
Fastens directly to the contactor (contactor mounting) — no DIN rail required, which saves rail space in a crowded panel. The Size S0 footprint fits alongside SIRIUS contactors of the same frame size. Spring-loaded terminals accept 2× (0.5 to 2.5 mm²) solid or stranded wire; a 3 mm diameter screwdriver tip is specified for the terminal screws. Mounting position is any orientation, so vertical or horizontal busbars are both fine. Depth of 84 mm, width 45 mm, height 102 mm — compact enough for shallow enclosures. The integrated auxiliary switch provides a normally closed contact for the "Tripped" message, which can be wired into a PLC input or a motor control center alarm circuit.
Coordination and protection
Trip Class 10 means the relay coordinates with standard motor starters and contactors like the 3RT2 series. The thermal release is self-powered — no external control voltage needed for the overload function. Power dissipation per pole is 1.9 W, which matters for thermal management in a multi-starter group. Auxiliary contact ratings are given for several control voltages: 2 A at 24 V, 3 A at 120 V, 2 A at 230 V, 1 A at 400 V. These are the switching capacities for the integrated auxiliary switch, so it can directly drive a PLC input or a small relay without an interposing device.