What it is and where it fits
The Siemens 3RT1024-3BB40 is a SIRIUS S0 power contactor with a 24 VDC coil, three normally-open main contacts, and screw-type terminals. It is rated for 5.5 kW motor switching at 400 V in AC-3 duty and 12.5 A in AC-4 at 400 V. The contactor snaps onto a 35 mm DIN rail per DIN EN 50022 and occupies 45 mm width — standard S0 footprint for panel integration.
What the ratings mean for your panel
The 5.5 kW AC-3 rating at 400 V is the motor-switching figure that governs real-world conveyor, pump, and fan loads — not the resistive AC-1 number. AC-3 means switching starting currents up to 6–8× rated without welding the contacts. AC-4 at 12.5 A covers reversing or inching duty where the contactor breaks full motor current at standstill; if your application cycles under load, size on the AC-4 curve. The 24 VDC coil draws 5.4 W during pick-up and holds at the same 5.4 W — a linear solenoid, not a power-saver. Verify your 24 VDC supply can sustain the inrush without droop; a 5.4 W continuous load on a shared bus adds up across multiple contactors. Type 2 coordination calls for a 25 A gL/gG fuse upstream; Type 1 allows a 63 A fuse. Type 2 is the standard for IEC panels that must survive a short-circuit without welding the contactor — spec the 25 A fuse unless your panel design explicitly accepts Type 1 damage.