The Siemens 5SU1636-6FP20 is a 3-pole RCBO from the SENTRON portfolio — a combined residual-current and overcurrent protective device that fits a standard DIN-rail enclosure. It is designed for 50 Hz supply, rated 20 A at 30 °C with a B-curve tripping characteristic, meaning it trips instantaneously between 3 and 5 times rated current — typical for protecting residential or light commercial final circuits where inrush is low (lighting, socket outlets, small appliances). The 3-pole design (3P) protects three-phase loads; supply can enter from either top or bottom, simplifying panel wiring.
The 20 A rating holds only at 30 °C ambient. At 40 °C the device is rated 19.5 A; at 50 °C it drops to 18.9 A; at 60 °C to 18.4 A; and at 70 °C to 17.8 A. If your distribution board runs warm — common in densely packed enclosures — use the derated figure for sizing, not the 30 °C label.