What this RCBO is and what the ratings mean for fit
The Siemens 5SU1653-7KK40 is a SENTRON RCBO — a combined residual-current device and miniature circuit breaker in a single 2-module (36 mm wide) package, 1P+N, rated 40 A at 30 °C with a C tripping characteristic. The C-curve means it holds up to 5–10× rated current momentarily, sized for motor or lighting loads that draw a moderate inrush without nuisance tripping. Type A fault-current detection catches both sinusoidal AC and pulsating DC residual currents — the standard choice for circuits with single-phase rectifiers, switched-mode supplies, or VFDs feeding the load side. Breaking capacity is 4.5 kA per EN 60898, which covers most final subcircuits in a domestic or light commercial panel; if the prospective fault current at the installation point exceeds that, you need an upstream backup fuse or a higher-rated device.
Thermal derating — the real current rating depends on your panel temperature
The 40 A rating holds only at 30 °C ambient. At 40 °C it derates to 37.6 A; at 50 °C to 34.8 A; at 60 °C to 32.4 A; at 70 °C to 29.2 A. If the distribution board runs hot — say, a packed enclosure with multiple RCBOs side by side — the continuous load must stay under the derated figure to avoid thermal trip. The maximum power dissipation is 15.6 W, which adds to the enclosure's internal heat rise; factor that into the thermal calculation when grouping several units.
Mounting and wiring constraints
Width is 36 mm (2 width units), depth 77 mm, installation depth 70 mm, height 90 mm. Accepts solid or stranded conductors from 0.75 up to 35 mm² — covers everything from lighting final circuits to a 40 A submains feed. Mounting position is any, so it fits vertical or horizontal DIN-rail layouts. Supply can enter from top or bottom; the IP20 rating applies only when the distribution board is installed and conductors are connected — exposed terminals are not touch-safe. Overvoltage category III, so it's rated for fixed installation downstream of the utility meter, not for outlet-level use.