What it is and what it does
The Siemens SIRIUS 3RT5055-6AC36 is a size S6 power contactor built for switching three-phase motor loads up to 75 kW in AC-3 duty at 400 V. The 36 V AC/DC coil lets it pull in on either supply type, which simplifies panel wiring when you've got a mix of control transformers and DC power supplies on the same rail. The 3-pole main circuit handles the motor feed; the built-in 2 NO + 2 NC auxiliary contacts give you feedback and interlock wiring without bolting on a separate block. The 75 kW AC-3 rating at 400 V is the number that decides the fit for motor starting — it covers a 100 hp (roughly 75 kW) induction motor on a 400 V line, with the contactor sized to handle the inrush and break the locked-rotor current. The bar connections on the main terminals handle the 185 A continuous current at 40 °C ambient, and the screw-type auxiliary terminals take up to 2.5 mm² stranded wire.
Where it goes in the panel
Size S6 means a 120 mm wide footprint on the mounting plate, fastened with screws (not DIN-rail snap-on at this frame size). The bar connections accept up to 95 mm² copper at 40 °C, dropping to 70 mm² at 60 °C ambient — derate the main conductors if the panel runs hot. The 1 000 V rated insulation voltage gives headroom for 690 V motor circuits common in heavy industrial lines. The contactor can be mounted with the vertical surface rotated up to 90° or tilted 22.5° front-to-back, which helps when you're squeezing it into a tight enclosure alongside the motor starter and overload relay. Side-by-side mounting is allowed, so multiple contactors can share a common bus bar without a gap.