What this part is and where it lands
The Siemens 3RT1025-3KB40-ZX95 is a SIRIUS power contactor in size S0, designed for switching motor loads and resistive loads in industrial control panels. It snaps onto a 35 mm DIN rail per EN 50022, so it drops into any standard panel layout without special bracketing. The coil is 24 V DC with a built-in varistor surge suppressor, which means you don't need to add an external suppression module across the coil terminals. That saves a wiring step and a part on the BOM. Rated operating temperature is -25 to +60 °C during operation, with pollution degree 3, so it's suited for the typical dirty environment inside an industrial enclosure.
Key ratings and what they mean for fit
The contactor's main contacts are rated for motor switching: at AC-3 duty (standard squirrel-cage motors) at 400 V, it handles 5.5 kW; at AC-4 (plugging/reversing) at 400 V, it handles 15.5 A. The AC-3 figure is the one you size against for normal motor starting — the AC-4 figure matters if the application does frequent inching or reversing. For resistive loads (AC-1), the contactor is rated 10 A at 230 V and 6 A at 400 V. If you're switching heaters or lighting banks, use the AC-1 column, not the motor column. The coil draws 4.2 W holding power, same as pick-up, so the DC supply needs to deliver that continuously. That's a steady-state load on the 24 V bus — factor it into your power budget.
Panel fit and wiring
Width is 45 mm, depth 100 mm, height 88 mm. Side-by-side mounting is allowed with no derating gap. Main circuit terminals accept screw-type connections: 2x (1 to 2.5 mm²) solid/stranded, or 2x (2.5 to 6 mm²), or max 2x 10 mm². Coil terminals accept 2x (0.25 to 2.5 mm²) solid. That covers standard panel wire sizes up to 10 mm² on the power side. IP20 on the front, IP00 at the terminals — so the terminal area is not finger-safe. The contactor needs to be inside a locked enclosure or behind a panel door.