The Siemens SIRIUS 3RU2126-1HB0 is a thermal overload relay — the part that sits between a contactor and a motor, watching the current draw and tripping if the motor starts pulling too hard for too long. That's fast enough to protect the motor windings but slow enough to let starting inrush pass without nuisance trips. It's rated for a maximum of 690 V, and the switching currents vary by voltage: 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. For motor protection, it's sized to handle 3 kW at 400 V, 4 kW at 500 V, and 5.5 kW at 690 V. The AC-3e rated maximum is 690 V, so it's built for three-phase motor duty.
How it mounts and wires in
This relay is S0 frame size — that's the smallest of the SIRIUS overload relay frames, measuring 85 mm tall, 45 mm wide, and 85 mm deep. It fastens directly to the contactor via the contactor mounting method, so it snaps onto the bottom of a matching SIRIUS contactor without extra hardware. The main current circuit uses screw-type terminals, and the screwdriver tip needed is Pozidriv PZ 2 with a shaft diameter of 5 to 6 mm. Wire range is 2x (0.5 to 1.5 mm²) solid or stranded, or 2x (0.75 to 2.5 mm²). The auxiliary switch is integrated right into the body, so there's no separate add-on to buy for the trip-indication signal. Mounting position is any, which helps in tight panels where you're cramming gear together. Temperature compensation is active from -40 to +60 °C, meaning the trip curve stays accurate even if the panel's sitting next to a hot motor or a steam line. That's a wide thermal envelope — it'll survive a summer in a non-climate-controlled enclosure or a winter on a loading dock.
The note reads for message Tripped — the auxiliary switch signals the PLC or indicator when the relay has tripped. Substance prohibitance date is 10/01/2009.