The Siemens SIRIUS 3RT1035-3AD20 is a size S2 power contactor with screw-type terminals for the main current circuit, rated for 18.5 kW at 400 V in AC-2 duty — that's the category for slip-ring motor starting, where the contactor makes and breaks the rotor circuit under load. The AC-3 rating (direct-on-line squirrel-cage motors) is typically higher for this frame; AC-2 is the limiting case for this variant. The enclosure front carries IP20; the terminal area is IP00 (open terminals), so keep live parts behind a panel door or cover. Rated operational current on the main contacts: 10 A at 24 V, 2 A at 60 V, 1 A at 110 V, 0.3 A at 220 V, 6 A at 230 V, 3 A at 400 V — these are the DC-1 / DC-3 load-break values for the auxiliary or main poles when switching DC loads.
Dimensions: 55 mm wide, 112 mm high, 115 mm deep. The depth matters when you're laying out a shallow enclosure — 115 mm is the body depth; add clearance for the screw terminals and wire bend radius. Accepts solid conductors 2x (0.25 to 2.5 mm²) and stranded 2x (0.75 to 25 mm²) for the main circuit, plus a separate 2x (0.75 to 16 mm²) range for auxiliary wiring. Use ferruled stranded for reliability under vibration. Coil operating range: 0.8 to 1.1 times rated voltage at 50 Hz, 0.85 to 1.1 at 60 Hz. Pick a control transformer that stays within that band under load. Coil pickup and dropout timing: arcing time 10 to 15 ms, with AC operate time 7 to 20 ms. That's fast enough for most interlocking but verify if your safety circuit requires a specific break time.
Environmental and operating limits
Mechanical endurance typical: 10 million operations. That's the no-load life; electrical life depends on the switching category and load current — AC-4 at 300 cycles/hour will wear the contacts faster than AC-1 at 1200.