What it is and where it fits
The Siemens 3RU2126-4DB0-ZX95 is a SIRIUS thermal overload relay in the S0 frame size, designed to protect motors against sustained overcurrent by tracking the thermal load on the winding. It mounts directly onto a contactor (fastening method: contactor mounting) and is rated for a main circuit voltage of 690 V, with a CLASS 10 trip characteristic — meaning it will trip within 10 seconds at 7.2× the set current, fast enough to protect standard induction motors during a locked-rotor start. The relay includes an integrated auxiliary switch that signals a tripped state (the note reads "for message "Tripped"") for remote annunciation or PLC input.
Key ratings and what they mean for your panel
The S0 frame size keeps the footprint compact — 45 mm wide, 85 mm deep, 85 mm tall — so it fits standard DIN-rail enclosures without crowding adjacent devices. The auxiliary contact ratings span common control voltages: 2 A at 24 V DC, 3 A at 120 V AC, 2 A at 230 V AC, and 1 A at 400 V AC, which covers most PLC input and relay coil circuits in industrial panels. Power dissipation per pole is 2.7 W, so factor that into enclosure thermal calculations when grouping several relays.
Environmental and compliance notes
The relay operates across -40 to +70 °C and includes temperature compensation from -40 to +60 °C, so it maintains accurate trip timing in outdoor cabinets or near heat sources. Storage and transport range is -55 to +80 °C. The substance prohibitance date of 10/01/2009 indicates RoHS compliance as of that date.
Wiring and installation
Main circuit terminals are screw-type, accepting 2x (0.5 to 1.5 mm²) or 2x (0.75 to 2.5 mm²) solid or stranded wire. Use a Pozidriv PZ 2 screwdriver with a 5 to 6 mm shaft diameter. The relay can be mounted in any position, which simplifies layout in tight panels.