The Siemens 3RT2645-1AB05 is a SIRIUS capacitor contactor — Size S3 — purpose-built for switching capacitor banks in power factor correction (PFC) duty. Unlike a standard motor contactor, it handles the inrush surge a discharged capacitor presents when it hits the line.
Rated 6 A at 230 V AC and 3 A at 400 V AC, this contactor is sized for the capacitor currents you'll see on a 50 Hz line. At 690 V AC the rating drops to 0 A — it's not intended for 690 V capacitor switching. The DC switching ratings (6 A at 24 V, 2 A at 60 V, 1 A at 110 V, 0.9 A at 125 V, 0.3 A at 220 V) cover control-circuit or small resistive DC loads, not the capacitor bank itself. The arcing time runs 10 to 20 ms, and the switching rate tops out at 200 operations per hour at 230 V — that's fine for daily PFC cycling but not for a rapid-pulse application.
The power terminals accept solid wire up to 2x 4 mm² or stranded up to 2x 70 mm² (1x 70 mm²). At 40 °C ambient you can land a single 50 mm²; at 60 °C derate to 2x 35 mm². The auxiliary contact block (attachable, one slot) uses smaller terminals: 2x 0.5 to 1.5 mm² solid, 2x 0.75 to 2.5 mm² stranded. That covers most control-circuit wiring without needing a reducer.