SIRIUS Size S0 power contactor — 24 VDC coil, screw terminals
The Siemens 3RT1025-3BD40-1AA0 is a SIRIUS power contactor in the Size S0 frame, designed for motor switching and resistive loads in industrial control panels. It mounts via screw or snap-on onto 35 mm DIN rail per EN 50022, occupying a 45 mm width on the rail — a standard slot that leaves room for adjacent overload relays or busbars. The 24 VDC coil draws 5.4 W both on closing and holding, so the control transformer or power supply must be sized for that continuous draw — no inrush spike beyond the steady-state value. The coil holds at 24 VDC nominal; verify the DC supply can deliver the 5.4 W without droop under load. Rated for motor duty at 10 kW at 500 V and 11 kW at 690 V in AC-3 category, and 7.5 kW at 400 V in AC-2. The AC-4 rating at 400 V is 15.5 A — that's the figure to check for reversing or inching applications where the contactor makes and breaks full load current.
Mounting and integration — DIN rail, side-by-side, clearance
Snap-on mounting onto 35 mm DIN rail per EN 50022. Side-by-side mounting is permitted with no derating for standard AC-3 duty, but the 6 mm clearance at the side must be maintained to adjacent metalwork or enclosures for heat dissipation. The IP20 front protection means the contactor is safe for finger contact from the front in a closed panel; the terminals themselves carry IP00, so live parts are exposed until the terminal shrouds are fitted. Screw-type terminals on the main circuit accept 2x (0.25 to 2.5 mm²) solid or 2x (1 to 2.5 mm²) stranded, with a maximum of 2x 10 mm². For AWG, that translates to 2x (16 to 12) solid or 2x (14 to 10) stranded, with a single 8 AWG maximum. Strip length and torque values follow the terminal marking on the contactor.
Coordination and protection — fuse sizing
For Type 2 coordination (no damage to the contactor under short circuit), the required upstream fuse is gL/gG 25 A. For Type 1 coordination (contactor may need replacement after fault), the fuse can be gL/gG 63 A. These values assume the standard assignment per IEC 60947-4-1. The pollution degree 3 rating means the contactor is suited for industrial environments with conductive pollution — typical for factory-floor panels without climate control.
Operating conditions and lifecycle
Rated for continuous operation from -25 °C to +60 °C ambient, with a maximum operating altitude of 2 000 m without derating. The mechanical endurance is 10 million operating cycles typical — that's the figure for the contactor mechanism itself, not the electrical life under load, which varies with switching current and category.