What It Is and Where It Fits
The Siemens 3RU2126-4DB0-ZW96 is a SIRIUS thermal overload relay in Size S0, designed to protect motors against overload and phase failure. It mounts directly to a contactor (fastening method: contactor mounting) and uses a thermal bimetallic release to track the motor's thermal state. The integrated auxiliary switch includes a dedicated "Tripped" message contact, giving the PLC or HMI a hardwired signal that the relay has operated — no separate status module needed.
Trip Class 10 — What That Means for Your Motor
Trip Class 10 means the relay must trip within 10 seconds at 7.2× the current setting. That's the standard for standard-duty induction motors driving pumps, fans, and conveyors — fast enough to prevent winding damage under locked-rotor conditions, but not so fast that it nuisance-trips on normal starting current. For high-inertia loads (flywheels, crushers) you'd typically step up to Class 20; for submersible or hermetically sealed motors, Class 5. This relay is sized for the middle of the curve.
Auxiliary Contact Ratings — Sizing the Signal Side
The integrated auxiliary switch carries different current ratings depending on the control voltage used. At 24 V it's rated 2 A; at 120 V it's 3 A; at 230 V it's 2 A; at 400 V it drops to 1 A. The 60 V and 110–125 V ratings are lower (0.3 A and 0.22 A respectively). This matters when you're feeding a PLC input or a contactor coil — make sure the inrush doesn't exceed the rating at your control voltage. The switch is integrated, so no separate auxiliary block to order, but also no option to upgrade contact material.
Mounting and Integration — Contactor Direct
This relay mounts directly to the contactor — no separate DIN-rail bracket needed, though it can be panel-mounted if the contactor is. The Size S0 footprint is 45 mm wide by 85 mm deep by 85 mm high. Screw terminals on the main circuit accept 2× (0.5…1.5 mm²) or 2× (0.75…2.5 mm²) solid or stranded wire. Pozidriv PZ 2 screwdriver tip with a 5–6 mm shaft diameter is what the panel builder will grab. Mounting position is any, which helps in tight cabinets.
Environmental and Compliance
Operating temperature range is -40 to +70 °C; storage and transport range is -55 to +80 °C. Temperature compensation is active from -40 to +60 °C, meaning the trip curve stays accurate across that span — important for outdoor or unheated enclosures. The substance prohibition date of 10/01/2009 aligns with RoHS compliance; the part carries the SIRIUS brand and is designed to Siemens quality standards. MTTF with high demand rate is 2,280 years — a reliability figure that's more about the semiconductor-level failure rate than field experience, but it passes the bar for most safety-related applications.