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 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.
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.