What it is and what the ratings mean
The Siemens 5SU1354-7KK40 is a SENTRON RCBO — a combined overcurrent and residual-current device in a 2-module-wide (36 mm) package. It protects a single-phase final subcircuit (1P+N, two poles, one protected) against overload, short-circuit, and earth leakage. The 40 A rated current at 30 °C and 30 mA residual sensitivity cover most socket-outlet and lighting circuits in commercial and light-industrial panels. The 10 kA breaking capacity per EN 60898 means it can safely interrupt a fault current up to that level — adequate for most distribution boards with a service fuse or upstream MCCB sized to limit the prospective fault current. The Type A residual-current characteristic detects pulsating DC fault currents (rectified half-wave) as well as sinusoidal AC — a requirement for circuits feeding electronic loads like switched-mode power supplies, VFDs, or LED drivers that can generate smooth DC leakage. The C tripping characteristic (5-10× In magnetic trip) handles moderate inrush from motorized loads and small transformers without nuisance trips.
Thermal derating — the real-world current limit
The 40 A rating holds only at 30 °C ambient inside the panel. At 40 °C the RCBO must be derated to 37.6 A; at 45 °C to 36.4 A; at 50 °C to 34.8 A; at 55 °C to 32.4 A; at 60 °C to 32.4 A; at 65 °C to 30.8 A; and at 70 °C to 29.2 A. If the panel runs warm — say, a packed enclosure with multiple devices — the actual continuous current the device can carry without tripping is lower than the nameplate 40 A. The 77 mm depth and 70 mm installation depth fit standard distribution boards, but the derating curve is what governs real-world circuit loading.
Mounting and panel integration
Snap-on mounting to a 35 mm DIN rail; the 36 mm width (2 modular units) and 77 mm depth fit standard distribution-board cutouts. The supply can enter from either top or bottom, and the device can be mounted in any orientation. IP20 applies only when the distribution board is installed and conductors are connected — meaning the RCBO itself is finger-safe inside the enclosure but not rated for exposed or wet locations. Halogen-free and silicon-free construction matters for panels in environments sensitive to outgassing (clean rooms, food processing).