What it is and what it does
The Siemens 3RU2126-1FC0 is a SIRIUS thermal overload relay in Size S0, Trip Class 10, with spring-loaded terminals and contactor mounting.
Key ratings and what they mean for fit
The relay is rated for a maximum insulation voltage of 690 V, which covers 400 V and 480 V motor circuits with headroom. At 400 V the relay is rated for 1.5 kW motor load; at 500 V for 2.2 kW; at 690 V for 4 kW. The AC-3e rated maximum voltage of 690 V means the auxiliary contacts can switch motor loads at that voltage level. The operating frequency is 50 to 60 Hz, so it works on either standard line frequency without derating. The auxiliary contact ratings span the control voltages you'll find in a typical panel: 2 A at 24 V, 3 A at 120 V, 2 A at 230 V, and 1 A at 400 V. The 0.3 A at 60 V and 0.22 A at 110 V and 125 V cover the less common control voltages. The per-pole power dissipation is 2.2 W, which matters when you're stacking multiple relays in a confined enclosure — the cumulative heat needs to stay within the cabinet's thermal budget. The relay includes an integrated auxiliary switch (the N/C contact that signals the trip condition) and a thermal overload release mechanism. The note "for message 'Tripped'" on the listing refers to the auxiliary contact that changes state when the relay trips, providing a signal to the PLC or indicator lamp.
Where it fits in the panel
The 3RU2126-1FC0 is a Size S0 overload relay, which is the smallest frame in the SIRIUS modular system. It mounts directly onto a Siemens contactor of matching frame size — the fastening method is contactor mounting, so no DIN rail is required for the relay itself; it clips onto the contactor and the whole assembly snaps onto the DIN rail. The dimensions are 45 mm wide, 102 mm high, and 84 mm deep, which fits the standard S0 footprint. The relay can be mounted in any position, which simplifies layout in tight cabinets. The spring-loaded terminals accept 2× (0.5 to 2.5 mm²) solid or stranded wire, and the screwdriver requirement is a 3 mm shaft diameter with a 3.0 × 0.5 mm tip.
Environmental range and compliance
The relay operates from -40 to +70 °C, with temperature compensation from -40 to +60 °C. Storage and transport range is -55 to +80 °C. MTTF under high-demand rate is 2,280 years.