What it is and where it fits
The Siemens 3RT1023-3BM40 is a SIRIUS power contactor in the S0 frame size, designed for switching motor loads in control panels. It mounts via screw or snap-on onto a 35 mm DIN rail per EN 50022, making it a direct fit for standard industrial enclosures. The contactor carries a 24 VDC coil with a closing and holding power of 5.4 W each, so the control transformer or DC supply must be sized for that inrush — not just the holding current. Rated for AC-2 duty at 4 kW at 400 V, this contactor handles slip-ring motor starting and reversing loads. For AC-4 (plugging/inching) at 400 V, the rated operational current is 8.5 A — the figure that governs frequent jogging applications.
Termination and coordination
Main circuit terminals are screw-type, accepting 2x (0.25...2.5 mm²) solid or 2x (1...2.5 mm²) stranded, with a maximum of 2x 10 mm². AWG equivalents: 2x (16...12) solid, 2x (14...10) stranded, or 1x 8. 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), use gL/gG 63 A. Side-by-side mounting is permitted without derating, with a 6 mm clearance at the side. The front face carries an IP20 rating; the terminal area is IP00, so the panel must provide the overall enclosure protection.
Environmental and mechanical limits
Operating temperature range is -25 to +60 °C, with storage and transport limits extending beyond that. Maximum operating altitude is 2 000 m without derating. Pollution degree 3 (conductive pollution or dry non-conductive that becomes conductive) applies — typical for industrial control rooms. Mechanical endurance is rated at 10 million operations typical, so this contactor is suited for high-cycle applications like conveyor or packaging lines where the coil cycles frequently.