The Siemens 5SU9406-0KK40 is a SENTRON RCBO — an RCD-operated circuit breaker that combines overcurrent and residual-current protection in a single 18 mm wide module. It's a 1P+N design (switches the line, monitors both line and neutral) with B-curve tripping characteristic, rated 40 A at 30 °C ambient. The 4 kV surge voltage resistance and 264 V insulation voltage give it solid headroom for 230/240 V AC supplies at 50/60 Hz. Rated current derates with ambient temperature: 37.6 A at 40 °C, 36.4 A at 45 °C, 34.8 A at 50 °C, and 32.4 A at 55 °C. That matters if the board sits in a hot enclosure or near other heat-generating devices.
Snaps onto standard DIN rail, 18 mm wide (1 MW), 77 mm deep, 125 mm tall. Installation depth is 70 mm. IP20 rated once the distribution board is installed with conductors connected. Halogen-free and silicon-free construction, which matters for enclosed assemblies where off-gassing or outgassing could affect contacts or electronics.