What it is and what it does
The Siemens 3RU2126-1GC0 is a SIRIUS thermal overload relay — a Class 10 device sized S0, rated for main circuits up to 690 V (AC-3e) and designed to protect motors against overload and phase loss. The thermal bimetallic release tracks motor heating; a Class 10 trip curve means it will open within 10 seconds at 7.2× the set current, which matches the acceleration time of most standard induction motors driving pumps, fans, and conveyors. The integrated auxiliary switch provides a normally-closed and normally-open contact for the control circuit — the NC leg breaks the contactor coil on a trip, the NO leg signals the PLC. Spring-loaded terminals accept 2× (0.5 to 2.5 mm²) solid or stranded wire, so no ferrules are required for stranded conductors up to that size.
Mounting and panel fit
This relay mounts directly onto a SIRIUS contactor (contactor mounting) — no separate DIN-rail footprint. The S0 frame measures 45 mm wide, 102 mm high, and 84 mm deep, so it adds 84 mm of depth behind the panel door. Mounting position is any, which simplifies layout in crowded enclosures. The spring-loaded terminals face forward, making wiring accessible even with the relay snapped onto the contactor. Temperature compensation is active from -40 to +60 °C, so the trip point stays accurate across the operating range of the enclosure.
Auxiliary contact ratings — what the numbers mean
The integrated auxiliary switch is rated for several control voltages. At 24 V it switches 2 A; at 120 V it switches 3 A; at 230 V it switches 2 A; at 400 V it switches 1 A. These are the make/break ratings for the NC and NO contacts — the 3 A at 120 V is the highest current across the listed voltages, so it handles typical 120 VAC control circuits without derating. At 60 V the rating drops to 0.3 A, which is still adequate for low-voltage DC pilot circuits. Each pole dissipates 2.2 W, so the total heat load in the panel is the sum across all three poles — about 6.6 W when the relay is carrying current.