The Siemens SIRIUS 3RU2126-4FB1 is a size S0 thermal overload relay designed for motor protection in control panels. It's rated for Class 10 trip characteristics, meaning it will trip within 10 seconds at 7.2x the current setting — standard for protecting standard induction motors during startup. The relay is rated for 18.5 kW at 400 V, and can handle up to 690 V rated insulation voltage. It uses screw-type terminals for the main current circuit, sized for M4 screws, and accepts 2x (0.5 to 1.5 mm²) or 2x (0.75 to 2.5 mm²) solid or stranded wire. The auxiliary switch is integrated, with a note for message "Tripped" — that's the NC contact that changes state when the bimetals warp, giving you a remote trip indication.
What the ratings mean for fit
Class 10 is the key selection parameter here. It's designed to coordinate with contactors and circuit breakers in a motor starter assembly — the overload relay handles the thermal protection, not the short-circuit interruption. The 18.5 kW rating at 400 V is for AC-3e duty (squirrel-cage motors: starting, switching off during running). At 480 V it's rated for 40 A, and at 690 V for 37 kW. The temperature compensation range of -40 to +60 °C means the bimetal curve stays accurate even if the panel ambient drifts — critical when the relay sits next to a hot contactor. Operating temperature range is -40 to +70 °C, storage and transport down to -55 °C.
Mounting and integration
Mounting position is any orientation, and it's designed for stand-alone installation — not a modular snap-on to a contactor. That means you need to plan for DIN rail or direct panel mounting, and wire it separately from the contactor. The size S0 frame is compact: 45 mm wide, 97 mm high, 95 mm deep. For the auxiliary contacts, the screw terminals use a Pozidriv PZ 2 tip, with a screwdriver shaft diameter of 5 to 6 mm. The MTTF with high demand rate is listed at 2,280 years — a reliability figure for safety-related applications, though this is a standard thermal relay, not a safety-rated device.