What this contactor is and what it does
The Siemens 3RT2028-1BM40 is a SIRIUS power contactor in the S0 frame size, designed for switching motor loads and resistive loads in industrial control panels. It mounts on a 35 mm DIN rail per DIN EN 60715 via screw and snap-on fastening, fitting into a 45 mm wide slot with a 107 mm depth and 85 mm height. The coil is 24 VDC with screw-type terminals, and the contactor includes an auxiliary switch block.
Key ratings and what they mean for your circuit
The DC switching ratings tell you exactly what this contactor can handle across common control voltages: 10 A at 24 V, 2 A at 48 V and 60 V, 1 A at 110 V, 0.9 A at 125 V, 0.3 A at 220 V, and 3 A at 400 V. These are the maximum currents the main contacts can interrupt at those DC voltage levels — critical for sizing the contactor to your load, especially in battery or rectified-supply circuits where DC arc extinction is harder than AC. For AC motor duty, the contactor is rated 5 hp at 230 V, which covers typical small pump, fan, or conveyor motors in that power class. The operating frequency limits per duty cycle are: 1,000 operations per hour at AC-1 (resistive), 750/h at AC-2 and AC-3 (motor starting/running), and 250/h at AC-4 (plugging/reversing). The AC-3e rating also sits at 750/h. These numbers govern how often the contactor can cycle without overheating — a panel-builder consideration for high-speed production lines. Mechanical life is rated at 10,000,000 operations typical, meaning the contactor's mechanical structure will outlast most application cycles before wear-out. The operating temperature range is -25 to +60 °C, with storage from -55 to +80 °C, so it handles standard industrial enclosure environments without derating concerns.
Mounting and wiring details
The contactor accepts solid or stranded conductors from 1 to 10 mm² on the main circuit, and fine-stranded wiring from 2x (0.5 to 1.5 mm²) or 2x (0.75 to 2.5 mm²) for auxiliary circuits. The mounting position is flexible: +/-180° rotation on a vertical surface, and can be tilted forward/backward by +/-22.5° on a vertical surface. Clearance requirements are 10 mm upwards, 10 mm forwards, 10 mm downwards, and 6 mm at the side — useful for panel layout density calculations. The coil dropout time on DC supply is 15 to 18 ms, and the arcing time is 10 ms. These timing specs matter for coordination with upstream protective devices and for sequencing in safety circuits where contactor dropout delay affects stop times.