What it is and what it does
The Siemens SIRIUS 3RH1122-1BF40 is an auxiliary contactor — a relay block that follows the state of a main contactor or a control signal to switch additional circuits. It's built in the S00 size frame, the smallest in the SIRIUS family, and carries a 110 V DC coil. The 3RH1122-1BF40 provides two normally-open (NO) instantaneous contacts, meaning they close the instant the coil is energized with no intentional delay. With a rated surge voltage resistance of 6 kV and pollution degree 3, it's designed for industrial control panels where voltage spikes and dusty environments are normal. The coil draws 3.2 W both at pickup and holding, so the power supply doesn't need to handle an inrush spike — useful when sizing a 24 V or 110 V DC bus for a panel full of contactors.
Mounting and integration
Snaps onto a 35 mm DIN rail using the integrated clip, or can be screwed directly to a mounting plate. At 45 mm wide and 72 mm deep, it leaves room alongside a main contactor in a standard 60 mm deep enclosure. The mounting position is flexible — it can be rotated ±180° on a vertical surface, and tilted forward or backward by ±22.5°, which helps when wiring into a tight corner of the panel. Terminals accept solid conductors: 2x 0.5–1.5 mm², 2x 0.75–2.5 mm², or 2x 4 mm². That covers most control wiring from signal loops to short motor-circuit leads.
Contact ratings and switching capability
The contacts are rated for switching at various voltages: 10 A at 24 V, 6 A at 230 V, 3 A at 400 V, and 1 A at 690 V. At the coil voltage of 110 V DC, the rated switching current is 1 A — that's the figure to check when switching a DC load like a small solenoid or indicator lamp. Contact reliability is rated at one incorrect switching operation per 100 million operations at 17 V and 1 mA, which is typical for PLC-level signal switching. The mechanical life is 30 million operations.
Environmental and compliance
Operating temperature range is -25 to +60 °C, with storage and transport from -55 to +80 °C. Front protection is IP20, so it's intended for installation inside a closed panel or enclosure. Shock resistance is rated at 10g for 5 ms and 5g for 10 ms, which covers most industrial vibration environments like conveyor lines or pump skids.