What it is and where it fits
The Siemens 5SU9354-8KK40 is a SENTRON RCD-operated circuit breaker — an RCBO that combines overcurrent protection and residual-current detection in one 2-MW (36 mm wide) module. Rated 40 A at 30 °C with a D tripping curve, it handles high-inrush loads like motor starters, small compressors, or pump contactors where a B or C curve would nuisance-trip on startup. The Type A fault-current detection catches pulsating DC as well as sinusoidal AC earth faults, which matters on single-phase drives or switched-mode supplies that generate DC ripple on the line.
Breaking capacity and selectivity
Rated breaking capacity is 10 kA per both EN 60898 and IEC 60947-2, so it clears a bolted fault up to that level without welding the contacts or cascading upstream. On a 230/240 V AC supply with a 10 kA prospective fault current at the panel, this RCBO holds — no need for a current-limiting upstream MCB unless the SCCR of the downstream equipment demands one. Energy limiting class 3 means it lets through less let-through energy than a lower-class breaker, which helps protect the wiring and the load side.
Thermal derating — read the real current
The 40 A rating is at 30 °C ambient inside the enclosure. In a warm panel — say 45 °C — the continuous current drops to 36.4 A; at 55 °C it's 32.4 A. If the load is a 3.7 kW motor drawing about 7 A per phase, you have headroom. But if you're pushing a 36 A continuous resistive load in a hot cabinet, the D-curve won't nuisance-trip on inrush, but the thermal element will eventually open at the derated value. Mounting position is any, so vertical or horizontal DIN-rail orientation doesn't change the derating curve.
Integration notes
Two width units (36 mm) on standard DIN rail. Installation depth is 70 mm behind the panel face, plus the 77 mm overall depth leaves room for wiring behind a 100 mm deep enclosure. Supply can enter from top or bottom — no polarity restriction on the line/load side for this 1P+N design. The neutral pole is switched (not just solid), so it isolates both conductors on a fault. Touch protection is built in; pollution degree 2 covers normal industrial environments with non-conductive dust. Halogen-free and silicon-free construction matters if the panel is in a clean room or semiconductor fab where outgassing is a concern.