What it is and what it does
The Siemens 5SU9304-1KK25 is a 1P+N RCBO from the SENTRON 5SU9 series — an RCD-operated circuit breaker that combines overcurrent and residual current protection in a single 18 mm wide module. It's rated 25 A with a C tripping characteristic and a 10 kA breaking capacity per EN 60898 and IEC 60947-2, making it a fit for final subcircuits in residential and light commercial distribution boards where you need both short-circuit and earth-fault protection in one DIN-rail slot. The C-curve (tripping characteristic class C) means it holds through 5–10× rated current before instant tripping — sized for mixed resistive and moderate inductive loads like lighting, small motors, and socket outlets. The 10 kA rated short-circuit capacity (Icn) at 230/240 V AC tells you it can safely interrupt a fault up to that level without welding contacts or cascading upstream.
Thermal derating — the real-world current rating
The 25 A rating holds at 30 °C ambient. At 40 °C it derates to 23.5 A, at 45 °C to 22.75 A, at 50 °C to 21.75 A, and at 55 °C to 20.25 A. If your panel runs hot — say 50 °C inside the enclosure — this RCBO effectively becomes a 22 A device. That's the number to use for your load calc, not the 25 A label.
Panel fit and installation
The 5SU9304-1KK25 snaps onto a standard DIN rail (EN 60715). At 18 mm wide (1 width unit), it takes one modular slot in a distribution board. Depth is 77 mm (70 mm installation depth behind the panel surface), height 125 mm. It can be mounted in any position, and the supply can enter from either top or bottom — no orientation restrictions. Rated IP20 with the distribution board installed and conductors connected.
Compliance and approvals
Designed to EN 60898 and IEC 60947-2 for the 10 kA breaking capacity. Overvoltage category III, pollution degree 2. Halogen-free and silicon-free construction. Touch protection is provided on the terminals. The device is classified as energy limiting class 3, which means it limits let-through energy in a fault — relevant for downstream component coordination.