What it is and where it fits
The Siemens SIRIUS 3RT1025-3XF40-0LA2 is a power contactor in the S0 frame size, designed for switching loads in industrial control panels. It mounts on 35 mm DIN rail per DIN EN 50022, with screw and snap-on fastening, and occupies a 45 mm width on the rail. The coil is rated 110 VDC and includes a built-in varistor surge suppressor, which suppresses the inductive kickback from the coil — no external suppression diode needed for DC switching.
DC switching capacity — what the ratings mean
For DC loads, the contactor's switching capacity depends on the operating voltage. At 24 VDC it handles 10 A; at 60 VDC it handles 2 A; at 110 VDC it handles 1 A; at 220 VDC it handles 0.3 A. These are the maximum continuous currents the main contacts can make and break at those voltages — the arc-quenching limits for DC, which is inherently harder than AC.
Panel integration and wiring
Main circuit terminals are screw-type, accepting solid or stranded conductors: 2x (0.25–2.5 mm²), 2x (1–2.5 mm²), 2x (2.5–6 mm²), and up to 2x 10 mm². Side-by-side mounting is permitted, so multiple contactors can pack tightly on the DIN rail without derating for heat buildup. Clearance distances: 10 mm upwards, forwards, downwards, and at the side — keep these free for arc flash and cooling.