What it is and what it does
The Siemens 5SU9406-1KK25 is a 1P+N RCD-operated circuit breaker — an RCBO — from the SENTRON 5SU9 family. It combines overcurrent protection with residual current detection in a single 18 mm wide module. The C tripping characteristic means it handles moderate inrush currents typical of motor and lighting loads without nuisance tripping, while the 25 A rated current at 30 °C sets the continuous load ceiling for the protected circuit.
Ratings and what they mean for fit
Rated 25 A at 30 °C ambient, the 5SU9406-1KK25 derates to 23.5 A at 40 °C, 22.75 A at 45 °C, 21.75 A at 50 °C, and 20.25 A at 55 °C. If your panel runs warm — say 45 °C internal ambient — the usable continuous current drops to 22.75 A. That derating curve is the number that governs real-world sizing, not the headline 25 A. The breaking capacity is 6 kA per both EN 60898 and IEC 60947-2, which covers standard domestic and light commercial fault levels. Overvoltage category III means it's rated for fixed-installation supply-side use, not just downstream equipment.
Panel integration and wiring
Width is 18 mm (1 width unit), depth 77 mm with an installation depth of 70 mm — fits standard DIN-rail enclosures. Accepts solid or stranded conductors from 0.75 mm² up to 35 mm², so it handles both thin control wiring and full-size feeder tails. Mounting position is any orientation; supply can enter from top or bottom. IP20 with the distribution board installed and conductors connected — that's normal for enclosed panel use, not for wet locations. Halogen-free and silicon-free construction matters if the panel is in a plenum or clean-room environment.