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.
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. Overvoltage category III means it's rated for fixed-installation supply-side use, not just downstream equipment.