What it is and what it does
The Siemens 5SU1656-0KK25 is a 2-pole (1P+N) RCBO from the SENTRON series, combining overcurrent protection and residual current detection in a single 2-module-width unit. It's rated 25 A at 30 °C with a B tripping characteristic, meaning it trips instantaneously at 3–5× rated current — standard for protecting residential and light commercial circuits feeding resistive or low-inrush loads like lighting and socket outlets. The residual current element is Type AC, so it detects sinusoidal AC fault currents only; not suitable for pulsating DC or smooth DC faults you'd get from VFDs or LED drivers. Breaking capacity is 6 kA per EN 60898 (the MCB standard for household-type devices) and 15 kA per IEC 60947-2 (the industrial standard for circuit-breakers). That 15 kA rating gives you headroom in sub-distribution boards where the prospective short-circuit current might exceed 6 kA — common in commercial or light industrial panels fed by a transformer. The part carries overvoltage category III and pollution degree 2, so it's rated for fixed installation in the main distribution board, not for portable equipment.
Thermal derating and real-world current
The 25 A rating is at 30 °C ambient. In a warm panel — say 40 °C — the continuous current drops to 23.5 A; at 50 °C it's 21.75 A; at 60 °C it's 20.25 A; at 70 °C it's 18.25 A. If your panel runs hot (enclosed DIN-rail enclosures in summer can easily hit 50–60 °C), size the upstream protection and load accordingly. The part is halogen-free and silicon-free, which matters for fire-load and outgassing specs in transit or data-center applications.
Panel integration
Snaps onto DIN rail, 2 modular width units (36 mm wide). Installation depth is 70 mm; overall depth is 77 mm. Mounting position is any — no restriction on upright or sideways. Supply cord can enter from top or bottom. The IP20 rating applies only when the distribution board is installed with connected conductors — so it's not for open-panel or washdown environments.