What this contactor is and what it does
The Siemens 3RT1015-2BD42 is a SIRIUS power contactor in the compact S00 frame size, designed for switching motor loads and resistive circuits in industrial control panels. It mounts via screw or snap-on onto 35 mm DIN rail per EN 50022, with a 45 mm width that fits standard panel layouts. The 42 VDC coil draws 3.3 W for both closing and holding, so the power supply sizing is straightforward — no inrush spike to budget for. The contactor is rated for AC-3 duty at 400 V (3 kW) and AC-4 at 400 V (6.5 A), covering standard three-phase induction motor switching up to that power. Spring-loaded terminals on the main circuit accept 2x (0.25 to 2.5 mm²) solid or stranded, or 2x (24 to 14 AWG). That's a common panel wire range; no special ferrules needed unless your local code requires them for stranded.
What the ratings mean for fit
The 10 A AC-12 maximum operating current (resistive load) sets the ceiling for general-purpose switching. For motor loads, the AC-3 rating at 400 V (3 kW) is the one that governs real use — that's a 4-pole induction motor up to about 6.5 A full-load current at 400 V. The AC-4 rating (6.5 A at 400 V) covers plugging and inching duty where the contactor makes and breaks stalled-rotor current. Switching capacity varies with coil voltage: at 24 V it's rated 10 A, at 60 V it's 2 A, at 110 V it's 1 A, at 220 V it's 0.3 A, at 230 V it's 6 A, at 400 V it's 3 A, at 500 V it's 3.5 kW, and at 690 V it's 4 kW. The 230 V and 400 V figures are the ones most relevant for European panel voltages. IP20 on the front and at the terminals means finger-safe against accidental contact, but the contactor is not sealed — it belongs inside an IP54 or higher enclosure if washdown or dust is present. Pollution degree 3 (conductive pollution or dry non-conductive that becomes conductive) is the rating, so it expects an enclosure in industrial environments. Operating temperature range is -25 to +60 °C, which covers most indoor panel conditions. Altitude is rated to 2 000 m without derating; above that, dielectric strength and cooling drop off. Side-by-side mounting is allowed, so you can gang multiple contactors without spacing.
Coordination and protection
For type 2 coordination (no damage to the contactor after a short circuit), Siemens specifies a gL/gG fuse rated 20 A. For type 1 coordination (contactor may need replacement after a fault), the fuse can go up to 35 A. These are the values to use when sizing the branch circuit protection. The mechanical life is rated at 30 million operating cycles typical, so this contactor is suited for high-duty-cycle applications like conveyor or pump cycling.