What it is and where it fits
The Siemens 3RT1025-1BP44 is a SIRIUS power contactor in frame size S0, designed for switching motor loads and resistive loads in control panels. It mounts on a 35 mm DIN rail per DIN EN 50022, with screw and snap-on fixing, and the 45 mm width means it slots into a standard S0 footprint on the rail. The coil is 230 V AC, with a closing and holding power of 5.4 W DC, so the control transformer needs to cover that draw.
Motor switching and contact ratings
For motor duty, the AC-2 rating at 400 V is 7.5 kW, which covers standard slip-ring or wound-rotor motor switching. The AC-4 rating at 400 V is 15.5 A, relevant for reversing or inching duty where the contactor makes and breaks under load. At 500 V it handles 10 kW and at 690 V it handles 11 kW, so it can serve 690 V line applications up to that power. The auxiliary contact block carries two instantaneous contacts rated at 10 A at 24 V, 6 A at 230 V, and 0.3 A at 220 V DC. That covers signalling and interlocking in the control circuit.
Short-circuit coordination and environment
With type 2 coordination, the required fuse is gL/gG 25 A; with type 1 coordination it's 63 A. That dictates the upstream protection selection for the motor branch circuit. Pollution degree 3 means it's rated for industrial environments with conductive pollution, and the operating temperature range is -25 to +60 °C, so it handles a warm panel. Maximum operating altitude is 2 000 m. The front is IP20, the terminal area is IP00, so it needs to live inside an enclosure. Side-by-side mounting is permitted, which saves rail space when ganging multiple contactors.