What it is and what the ratings mean
The Siemens 5SU9356-1KN40 is a SENTRON RCBO — a combined RCD and MCB in one 2-module-wide (36 mm) package, saving a DIN rail position over separate devices. The 40 A rating at 30 °C (37.6 A at 40 °C, 34.8 A at 50 °C) covers most residential and light commercial final circuits; the 30 mA residual-current trip protects against lethal shock in AC systems. Tripping characteristic C means it holds up to 5–10× rated current for short overloads, so it won't nuisance-trip on motor or capacitor inrush but clears fast on a hard fault. The 6 kA breaking capacity per EN 60898 covers domestic service-entrance duty; the 10 kA per IEC 60947-2 gives headroom for industrial panelboards with higher fault current.
Panel fit and environment
Occupies 2 MW (36 mm) on a DIN rail, with an installation depth of 70 mm and overall depth of 77 mm. Mounting position is any orientation. Rated for overvoltage category III and pollution degree 2 — standard for fixed-installation distribution boards. IP20 with conductors connected, so it's enclosed inside the panel. Halogen-free and silicon-free construction matters for clean-room or corrosive-environment installs where outgassing is a concern. Supply can enter from top or bottom, which simplifies wiring in tight enclosures.
Derating and selectivity
The 40 A rating is at 30 °C ambient. At 40 °C it's 37.6 A, at 50 °C it's 34.8 A, and at 55 °C it's 32.4 A — so in a warm panel or next to other heat-generating devices, size up or account for the drop. Energy limitation class 3 means it lets through less let-through energy on a fault, which helps with downstream coordination. Touch protection is built in on the terminals.