What it is and what it does
The Siemens 5SU9406-1KK20 is a 1P+N RCD-operated circuit breaker (RCBO) from the SENTRON 5SU9 series, combining overcurrent protection and residual current detection in a single 18 mm wide module. It carries a 20 A rated current at 30 °C with a Type C tripping characteristic, meaning it handles inrush currents from motor or lighting loads without nuisance tripping. The breaking capacity is 6 kA per both EN 60898 and IEC 60947-2, so it safely interrupts fault currents up to that level in a 230/400 V AC installation. The RCBO detects AC fault currents only (Type AC RCD), and the instantaneous design means no intentional time delay on the residual current element — it trips as soon as the leakage exceeds the threshold. The 1P+N configuration switches the live pole and monitors the neutral, but only the live pole is protected against overcurrent.
Mounting and integration
At 18 mm wide (1 width unit), this RCBO snaps onto a standard DIN rail and accepts conductors from 0.75 mm² up to 35 mm² solid or stranded. The installation depth is 70 mm, with an overall depth of 77 mm — check your enclosure depth if you are retrofitting into a shallow panel. Mounting position is any orientation, and the supply cord can enter from either top or bottom, which simplifies busbar routing in a crowded distribution board. The IP20 rating applies only when the distribution board is installed with connected conductors — the front face is touch-protected, but the terminals are not sealed against dust ingress until the enclosure is closed.
Thermal derating and ambient limits
The 20 A rating holds at 30 °C ambient. At 40 °C it derates to 18.8 A, at 45 °C to 18.2 A, at 50 °C to 17.4 A, and at 55 °C to 16.2 A. If the panel runs warm — say, above 40 °C — size the circuit for the derated figure, not the 20 A label. The operating ambient range is -25 °C to +45 °C; storage range is -40 °C to +75 °C. Maximum power loss is 0.75 W, so heat buildup from a single unit is negligible, but in a dense row of RCBOs the cumulative dissipation can raise cabinet temperature — factor the derating curve into your thermal budget.