Compact starter for motor branch circuits
The Siemens 3RA6120-2CB33 is a SIRIUS compact starter combining a contactor and overload relay in a single 45 mm-wide package. It is a direct starter design rated for continuous operation per IEC 60947-6-2, meaning it handles sustained motor loads without derating for duty cycle. Control voltage is 24 V AC/DC, so it integrates cleanly into a 24 VDC control bus without an additional power supply tap. The coil draws 2.8 W AC or 2.9 W DC at maximum — low enough that a standard 24 VDC power supply can drive several of these in a panel without exceeding the supply budget.
Trip class and motor protection
Trip class is adjustable between Class 10 and Class 20. Class 20 allows 20 seconds at 7.2×, covering high-inertia loads like conveyors or centrifuges that need a longer start curve. The adjustability means one SKU covers both duty types. Rated operational current at 480 V is 4 A, and at 600 V it is also 4 A. This is the continuous current the main contacts carry in AC-3 motor-switching duty. The 1 500 W rating at 400 V gives a quick power-based sizing check for 400 V line-fed motors.
The 45 mm width and 165 mm depth fit standard 200 mm-deep enclosures, leaving room for wiring ducts and other components. Any mounting position is permitted, so panel layout is flexible. Wire termination accepts 2x (0.25 to 1.5 mm²) solid or stranded conductors. The main circuit uses plug-in terminals without screw clamps, which speeds replacement — pull the old unit, plug in the new one, no re-torquing. Two status LEDs give local indication of operating state and fault conditions, useful during commissioning and troubleshooting without needing a multimeter or HMI connection.
Vibration resistance covers 4 to 5.8 Hz at 15 mm amplitude and 5.8 to 500 Hz at 20 m/s² for 10 cycles. Shock resistance is 60 m/s² (6g) for 10 ms, three shocks per axis. This is suitable for installation on machinery with moderate vibration, like conveyor drives or pump skids.