The Siemens 3RT2028-1BP40 is a SIRIUS power contactor in the Size S0 frame, designed for switching motor loads and resistive loads in industrial control panels. It mounts via screw or snap-on onto 35 mm DIN rail per DIN EN 60715, fitting the standard panel footprint with a 45 mm width. The coil is 230 V AC; an auxiliary switch is built in, rated for DC switching up to 10 A at 24 V.
DC auxiliary contact ratings — the number that matters for PLC interfaces
The built-in auxiliary switch is rated for DC loads at several voltage points: 10 A at 24 V, 2 A at 48 V and 60 V, 1 A at 110 V, 0.9 A at 125 V, and 0.3 A at 220 V. These numbers govern whether the contactor can directly switch a PLC input or relay coil without an interposing relay. At 24 V the full 10 A covers most solenoid loads; above 48 V the current drops sharply due to DC arc extinction limits — plan for that drop if driving a 125 VDC valve bank.
Switching frequency by duty class
Maximum switching rates are specified per IEC duty class: 1 000 cycles/hour at AC-1 (resistive), 750 cycles/hour at AC-2 and AC-3 (slip-ring and squirrel-cage motors), and 250 cycles/hour at AC-4 (plugging/inching). The AC-3e rating also allows 750 cycles/hour. For a conveyor line running AC-3 duty at 750 cycles/hour, the contactor can cycle every 4.8 seconds — adequate for moderate-speed start/stop profiles but not for high-speed indexing where you'd need a larger frame or a contactor with a higher mechanical life.
Mounting and clearances
The mounting position allows ±180° rotation on a vertical surface and can be tilted forward/backward by ±22.5°. Minimum clearances to adjacent components: 10 mm upward, 10 mm forward, 10 mm downward, 6 mm at the side. Those spacings are required for heat dissipation and arc flash containment — crowding the contactor against other devices will shorten coil life. The screw-type terminals accept solid or stranded wire from 1 to 10 mm², and two conductors per terminal from 0.5 to 2.5 mm².