The Siemens 5SU9604-1KK40 is a 1P+N RCD-operated circuit breaker (RCBO) from the SENTRON 5SU9 family, combining overcurrent and residual-current protection in a single 18 mm-wide module. It's rated 40 A at 30 °C with a C tripping characteristic, meaning it handles moderate inrush from motor or lighting loads without nuisance tripping, and it clears faults up to 10 kA per EN 60898 and IEC 60947-2. The 1P+N configuration protects one live conductor and switches the neutral, so it fits single-phase final subcircuits in distribution boards.
The 40 A rating is at 30 °C ambient; at 40 °C it's 37.6 A, at 45 °C it's 36.4 A, and at 55 °C it's 32.4 A — so if your panel runs warm, you need to account for the derating. The 18 mm width (1 width unit) keeps the rail density high — 18 modules per 360 mm DIN rail.