What it is and what the ratings mean
The Siemens 5SU1356-1KK25 is a SENTRON RCBO — a combined residual-current and miniature circuit breaker in a single 2-module-width (36 mm) housing, 1P+N pole design with one protected pole. It is rated 25 A at 30 °C with a Type C tripping characteristic (magnetic release at 5–10× In), so it handles motor inrush and moderate inductive loads without nuisance tripping. The breaking capacity is 6 kA per EN 60898 for domestic/commercial coordination, and 15 kA per IEC 60947-2 for industrial panel duty — the higher figure is the one to use when sizing for fault levels in a distribution board with upstream transformer.
Thermal derating and ambient limits
The 25 A rating holds only at 30 °C ambient. At 40 °C it derates to 23.5 A, at 50 °C to 21.75 A, and at 70 °C to 18.25 A. The operating range spans -40 °C to 75 °C, which covers unheated enclosures in cold climates and warm panel interiors near transformers. The derating curve is continuous — no step change — so you can interpolate for intermediate temperatures.
Mounting and integration
Snap-on DIN rail mount, 2 MW (36 mm) wide, installation depth 70 mm. Supply can enter from top or bottom — no orientation restriction. The IP20 rating applies only when installed in a distribution board with conductors connected; the device itself is not sealed for washdown. Pollution degree 2 and overvoltage category III suit fixed-installation panels. Halogen-free and silicon-free construction matters for clean-room or high-reliability environments where outgassing must be minimized.