What it is and what it does
The Siemens SENTRON 5SU9404-1KK20 is a 2-pole RCD-operated circuit breaker (RCBO) combining overcurrent and residual current protection in a single 18 mm-wide module. Rated 20 A at AC with a C-curve trip characteristic, it clears faults up to 10 kA per EN 60898 and IEC 60947-2. The residual current element trips at 100 mA, Type AC, meaning it detects sinusoidal AC earth faults — the common domestic and light industrial waveform. The 20 A rating is the nominal current at 30 °C ambient. At elevated panel temperatures the unit derates: 18.8 A at 40 °C, 18.2 A at 45 °C, 17.4 A at 50 °C, and 16.2 A at 55 °C. For a BOM line feeding a 16 A continuous load in a 50 °C enclosure, this still holds margin; a 20 A load at that temperature would nuisance-trip.
Panel fit and mounting
Width is 18 mm (1 MW), depth 77 mm, height 125 mm. Installation depth is 70 mm. Mounts in any position on standard DIN rail. Supply can enter from top or bottom. Touch protection is built in. IP20 rating applies when installed in a distribution board with conductors connected — not a standalone outdoor device.