What this contactor is and where it fits
The Siemens 3RT1025-1BM44 is a SIRIUS S0-size power contactor with screw-type terminals, designed for switching motor loads in industrial control panels. It mounts on a 35 mm DIN rail per EN 50022 — screw and snap-on both work, so you can drop it into a new build or a retrofit without changing the rail layout. Rated at 7.5 kW in AC-2 duty at 400 V and 15.5 A in AC-4 at 400 V, this contactor handles the inrush and breaking demands of wound-rotor and squirrel-cage motors. AC-2 covers slip-ring motors; AC-4 covers reversing, plugging, and inching — the tougher duty cycle that generates more arc wear. If your application cycles the motor under load, the AC-4 rating is the one that governs contact life.
Mounting and wiring details
The 45 mm wide body (S0 frame) fits standard panel layouts. Side-by-side mounting is allowed — no gap needed between units — which saves rail space in dense panels. The front carries IP20 finger protection; the terminal area is IP00, so keep live parts out of reach when the panel door is open. Main circuit terminals accept 2x (1 to 2.5 mm²) stranded, 2x (2.5 to 6 mm²), or up to 2x 10 mm². Auxiliary/control terminals take 2x (0.5 to 1.5 mm²) solid or 2x (0.75 to 2.5 mm²) stranded, max 2x (0.75 to 4 mm²). That covers most panel wire sizes without needing adapters. The magnet coil draws 5.4 W on pickup and 5.4 W holding — DC holding, so no AC hum and consistent power draw.
Coordination and protection
For Type 2 coordination (no damage to the contactor after a short circuit), use a gL/gG fuse rated 25 A. For Type 1 coordination (contactor may need replacement after a fault), the fuse can go up to 63 A. This lets you balance protection level against fuse size — Type 2 keeps the contactor in service, Type 1 saves panel space with a larger fuse. Operating temperature range is -25 to +60 °C, with a maximum operating altitude of 2000 m. Pollution degree 3 means it's rated for industrial environments with conductive dust or occasional condensation — typical for unsealed control cabinets.