Thermal Overload Relay for Motor Protection
The Siemens 3RU2126-4BB1 is a SIRIUS thermal overload relay in the S0 frame size, designed to protect three-phase motors against overload and phase failure. It is a standalone-mount unit with screw-type terminals for the main circuit, accepting 2x (0.5...1.5 mm²) or 2x (0.75...2.5 mm²) solid or stranded wire. The relay carries a Trip Class 10 rating, 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 without nuisance tripping on normal starting transients. Rated voltage is 690 V, with corresponding motor power ratings of 7.5 kW at 400 V, 11 kW at 500 V, and 15 kW at 690 V. At 480 V and 600 V the current rating is 20 A. The auxiliary switch is integrated and carries the following contact ratings: 2 A at 24 V, 3 A at 120 V, 2 A at 230 V, 1 A at 400 V, and 0.75 A at 690 V — these are the values you'd use for the trip-indication circuit back to the PLC or annunciator.
Mounting and Environmental Fit
The relay measures 97 mm high, 45 mm wide, and 95 mm deep — the S0 footprint that drops into a standard motor starter assembly or stands alone on a panel plate. Mounting position is any orientation, which helps when you're squeezing it into a crowded enclosure. Operating temperature range is -40 to +70 °C, with temperature compensation active from -40 to +60 °C, so it stays accurate in a hot mill environment or a cold outdoor cabinet. Storage and transport range is -55 to +80 °C. Main circuit terminals accept M4 screws; the screwdriver spec is Pozidriv PZ2 with a 5–6 mm shaft diameter. Power dissipation per pole is 2.7 W — factor that into your enclosure thermal budget if you're running multiple relays in a sealed box.