What it is and where it fits
The Siemens 5SU9354-7KK40 is a SENTRON RCD-operated circuit breaker — an RCBO that combines overcurrent protection and residual-current detection in a single 2-module-wide (36 mm) package. It protects a single-phase final subcircuit against overload, short-circuit, and earth leakage, with a Type C tripping characteristic for moderate inrush loads like small motors or lighting banks. Rated 40 A at 30 °C ambient, it derates to 37.6 A at 40 °C and 32.4 A at 55 °C — the thermal curve matters when the panel runs hot. The 10 kA breaking capacity per EN 60898 (and IEC 60947-2) means it can safely interrupt a fault current up to that level without upstream damage, suitable for domestic and light commercial distribution boards. Mounts on DIN rail in any position, with supply feed from top or bottom. The IP20 rating applies once installed in a distribution board with connected conductors — not for open-panel or wet-area use.
Key ratings and what they mean for the panel
The 1P+N design protects the phase conductor only; the neutral is switched but not protected — standard for single-phase RCBOs in TN or TT systems. The 2 MW width (36 mm) occupies two 18 mm DIN spaces, so factor that into your enclosure fill count. Overvoltage category III and pollution degree 2 suit it for fixed-installation distribution boards. The 4 kV surge voltage resistance covers typical mains transients. The 1 kA surge current withstand (8/20 µs) handles lightning-induced spikes on the line side. Halogen-free and silicon-free construction matters for environments where outgassing during a fault could damage sensitive electronics or pose toxicity risks.