Siemens SIRIUS 3RT1025-1KB40-1AA0 — 24 VDC Power Contactor, Size S0
The Siemens SIRIUS 3RT1025-1KB40-1AA0 is a size S0 power contactor with a 24 V DC coil, built for switching motor loads in control panels. It mounts via screw or snap-on onto 35 mm DIN rail per EN 50022, so it integrates into standard panel layouts without custom bracketry. The front face carries an IP20 rating, meaning the live terminals behind the enclosure door are protected against accidental finger contact — a standard requirement for operator-access panels. The terminal area itself is IP00, as expected for screw-clamp connections inside a closed cabinet.
Coil and Switching Performance
The 24 V DC coil picks up at 0.7 times the rated voltage (about 16.8 V) and drops out at 1.25 times (30 V), so it holds in during brownout conditions down to roughly 70 % of nominal. Closing and holding power are both 4.2 W DC — a steady-state draw that a 24 VDC power supply can handle without oversizing. The built-in varistor surge suppressor clips the inductive kickback from the coil, which saves you from having to add an external flyback diode across the coil terminals.
Motor Ratings and Duty Cycles
For motor switching, the key numbers are the AC-3 and AC-4 ratings. At 400 V AC, the contactor is rated 7.5 kW in AC-3 duty (starting and stopping a running motor) and 15.5 A in AC-4 duty (plugging or inching). At 690 V, it handles 11 kW in AC-3. The AC-1 resistive rating at 400 V is 3 A. These figures tell you the contactor is sized for a 7.5 kW motor at 400 V under normal starting, and it can handle the higher thermal stress of reversing or jogging at 15.5 A. The mechanical life is 10 million operations typical, so it outlasts the machine in most cycling applications.
Wiring and Coordination
Main terminals accept solid conductors from 2x 0.5 to 2x 4 mm² and stranded from 2x 1 to 2x 10 mm². That covers most panel wiring from 18 AWG to 8 AWG. For Type 2 coordination (no damage to the contactor under 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. Side-by-side mounting is allowed with 6 mm clearance at the side, which keeps the panel width tight when you gang multiple contactors on the same rail.