What it is and where it fits
The Siemens 3RT2028-1BG40 is a SIRIUS S0 power contactor with a 24 VDC magnet coil terminated through screw-type terminals. It mounts on a 35 mm DIN rail per DIN EN 60715, using either the screw-fastened or snap-on method, and accepts solid or stranded conductors from 1 to 10 mm² on the main circuit. The S0 frame size (45 mm wide, 85 mm tall, 107 mm deep) is the standard building block for motor loads up to roughly 5.5 kW at 400 V in AC-3 duty. Clearance requirements are 10 mm upward, 10 mm forward, 10 mm downward, and 6 mm to the side, which keeps panel layout predictable.
Switching ratings and what they mean for your load
The main contactor is rated for 1 000 operations per hour in AC-1 (resistive) duty, 750/h in AC-2 and AC-3 (motor start/run), and 250/h in AC-4 (plugging/reversing). The AC-3e rating also sits at 750/h, matching the standard motor-switching tempo for conveyor, pump, and fan applications. The built-in auxiliary switch (1 N/O + 1 N/C, wired as 16-8 on the main contact block) carries 10 A at 24 VDC, 2 A at 48 VDC, 2 A at 60 VDC, 1 A at 110 VDC, 0.9 A at 125 VDC, 0.3 A at 220 VDC, and 3 A at 400 VAC. That DC curve matters if you are switching a PLC input or a DC brake coil — the 24 V rating is generous, but the drop-off above 60 V is steep.
Environmental and mounting flexibility
Operating temperature range is -25 to +60 °C, storage from -55 to +80 °C. The contactor can be mounted in any orientation — +/-180° rotation on a vertical surface, and tilted forward/backward by +/-22.5° — which simplifies panel layout in tight enclosures. Arcing time is 10 ms, and the DC dropout delay (coil off to contacts open) is 15-18 ms. That is fast enough for most safety circuits but worth noting if you are coordinating with a downstream contactor that has a longer dropout.