What it is and what it does
The Siemens 3RU2126-4FC0 is a SIRIUS thermal overload relay — the thing that sits between your contactor and motor, watching current and tripping on a sustained overload before the winding cooks. CLASS 10 trip class means it opens in under 10 seconds at 7.2× rated current, which is the standard match for standard induction motors starting across the line. S0 frame size, so it mates directly with the S0 contactor range via the contactor-mount fastening method — no separate base or wiring between them. Rated 690 V, with power ratings at 400 V (18.5 kW), 500 V (22 kW), and 690 V (37 kW) in AC-3e duty. The auxiliary switch is integrated — one less add-on to spec. Spring-loaded terminals on the main circuit, accepting 2× (0.5 to 2.5 mm²) solid or stranded. Temperature compensation active from -40 to +60 °C, so it doesn't drift when the panel warms up next to the drive.
What the ratings mean for fit
CLASS 10 is the key selection parameter here. If your motor starts against a high-inertia load that takes longer than 10 seconds to accelerate, you need CLASS 20 or 30 — this relay will nuisance-trip. For a standard pump, fan, or conveyor motor starting unloaded, CLASS 10 is the right call. The 690 V rated value covers 400 V, 480 V, 600 V, and 690 V systems; just watch that the 480 V and 600 V entries show 40 A rather than a kW figure, so you're sizing by motor FLA at those voltages. The note 'for message Tripped' on the line item is a Siemens catalog flag — it means this variant includes the remote trip-indication contact (the auxiliary switch) wired to signal a tripped state back to the PLC or annunciator. Not all 3RU2 variants carry that; if your panel needs a hardwired 'motor tripped' feedback, this is the one.
Integration notes
Mounts directly onto the S0 contactor — no DIN rail needed for the relay itself, though the contactor likely sits on one. Any mounting position allowed. Depth 84 mm, width 45 mm, height 102 mm — fits standard 45 mm-wide contactor footprint. Screwdriver shaft diameter 3 mm, tip 3.0 × 0.5 mm for the spring-loaded terminals. Operating frequency 50 to 60 Hz, so it's fine on either 50 Hz or 60 Hz supply. Storage and transport range -55 to +80 °C; operating range -40 to +70 °C. MTTF at high demand rate 2,280 years — that's a reliability number for safety-related applications, not a warranty. Substance prohibitance date 10/01/2009 aligns with RoHS compliance scope.