SIRIUS Size S0 power contactor, 110 VDC coil
The Siemens 3RT1024-3BF40 is a SIRIUS power contactor in the Size S0 frame, designed for switching motor loads and resistive circuits in industrial control panels. The 110 VDC coil draws 5.4 W for both closing and holding, so the control transformer or DC supply needs to sustain that steady-state draw without sag — no inrush spike to worry about on DC, but the continuous 5.4 W means a 24 VA AC supply would be undersized if you're rectifying. Screw-type terminals on the main circuit accept up to 2x 10 mm² or 1x 8 AWG, and the auxiliary side handles 2x (0.25...2.5 mm²) solid.
Motor ratings and coordination
AC-2 duty at 400 V delivers 5.5 kW — that's the rating for slip-ring motor starting where the contactor makes and breaks under load. AC-4 at the same voltage is 12.5 A, covering plugging and inching duty for squirrel-cage motors. At 500 V and 690 V the contactor is rated for 7.5 kW, which is the same power at higher line voltages — useful where a 480 V or 690 V distribution is standard. Type 2 coordination requires a 25 A gL/gG fuse; Type 1 allows a 63 A fuse upstream. That selectivity window matters when you're coordinating with a motor protection breaker or downstream short-circuit device.
Mounting and integration
Snap-on mounting onto 35 mm DIN rail per EN 50022, or screw-fixed to a backplate. The 45 mm width (one standard modular unit) and 85 mm height fit a compact panel layout. Side-by-side mounting is permitted — no derating gap required between units in the same enclosure, though the IP20 front and IP00 terminal area mean you keep live parts behind a panel door or shrouding. Operating temperature spans -25 to +60 °C, so it handles unventilated cabinets in warm machine rooms. Altitude limit is 2 000 m without derating.