What This Relay Does on the Line
The Siemens 3RU2126-4FJ0 is a SIRIUS thermal overload relay, Size S0, designed to protect motors against overload and phase failure. It's Trip Class 10, meaning it will trip within 10 seconds at 7.2× the set current — fast enough to protect standard induction motors during a stalled-rotor condition. Rated for main circuits up to 690 V, it handles 18.5 kW at 400 V, 22 kW at 500 V, and 37 kW at 690 V. The 480 V and 600 V ratings both list 40 A, so it covers North American 480Y/277 V and 600 V delta systems cleanly. The built-in auxiliary switch provides status feedback — a tripped message is signaled on the auxiliary contact set, which is what the "Note for message 'Tripped'" spec refers to. This lets a PLC or annunciator panel see the relay state without extra hardware.
Mounting and Wiring in the Panel
Mounts directly onto a contactor (contactor mounting), in any position. The S0 footprint is 45 mm wide by 85 mm deep — fits standard DIN-rail enclosures without overhang. Main circuit connections use ring cable lugs with M4 screws; auxiliary connections use Pozidriv PZ2 screwdriver tips, 5–6 mm shaft diameter. Temperature compensation is active from -40 to +60 °C, so the trip curve stays accurate even in hot enclosures or cold outdoor installations. Operating range is -40 to +70 °C; storage and transport handle -55 to +80 °C.
Auxiliary Contact Ratings — What They Mean for Your Control Circuit
The integrated auxiliary switch is rated for both AC and DC control circuits. At 24 V DC it switches 2 A; at 120 V AC it switches 3 A; at 230 V AC it switches 2 A; at 400 V AC it handles 1 A. For DC applications at higher voltages, derate to 0.3 A at 60 V, 0.22 A at 110/125 V, and 0.11 A at 220 V. These are the maximum breaking currents — your PLC output card or relay coil must stay within these limits. Per-pole power dissipation is 3.2 W. In a dense panel with multiple relays, factor this into the thermal budget — the S0 frame dissipates heat through the contactor base, so keep airflow around the mounting rail.