The Siemens 3RT1015-1KB42 is a SIRIUS power contactor in frame size S00 — the smallest footprint in the SIRIUS family, at 45 mm wide, 57.5 mm tall, and 72 mm deep. That compact envelope fits densely packed control panels where every DIN-rail millimeter counts. The contactor is rated for motor switching: 3 kW at 400 V in AC-2 duty (slip-ring motors, wound rotor) and 6.5 A in AC-4 duty (plugging, inching, reversing) at the same voltage. For resistive or general-purpose loads, the AC-1 rating would be higher, but the AC-3 and AC-4 numbers are the ones that govern motor branch circuits. The screw-type terminals on both main and auxiliary circuits accept solid or stranded conductors up to 4 mm² (or 2x 2.5 mm²), which is typical for motor loads up to the 3 kW / 6.5 A range. The varistor surge suppressor is built in across the coil, so no external flyback diode or RC snubber is needed for the DC coil circuit.
The 3RT1015-1KB42 is the current design. If a buyer is looking for a functional equivalent from the same SIRIUS family, the closest sibling in the 3RT series would share the same S00 frame and 24 V DC coil but differ in auxiliary contact configuration or surge suppressor type — those are the variables to check on the nameplate.
Both screw-clamp and snap-on mounting are supported, so it can be clipped onto the rail without tools or screwed down for vibration resistance. Side-by-side mounting is permitted, meaning zero clearance between contactors on the rail, which is standard for S00 frame devices.