What this RCBO delivers for the panel schedule
The Siemens 5SU9406-1KK40 is a 1P+N RCD-operated circuit breaker (RCBO) from the SENTRON 5SU9 family, combining overcurrent and residual-current protection in a single 18 mm wide module. Rated 40 A at 30 °C with a C tripping characteristic, it handles the inrush of motor starters and lighting banks without nuisance trips. The 6 kA breaking capacity is certified to both EN 60898 and IEC 60947-2, so it clears faults up to that level in a domestic or light commercial distribution board. The 1P+N configuration protects the phase conductor and switches the neutral — the neutral pole is not protected, only switched. Ambient rating covers -25 to +45 °C, and the unit can be mounted in any position, which simplifies panel layout.
Current derating and conductor sizing
The 40 A rating holds at 30 °C ambient. At 40 °C it derates to 37.6 A, at 45 °C to 36.4 A, at 50 °C to 34.8 A, and at 55 °C to 32.4 A. If the distribution board runs warm — say above 40 °C — the continuous load must be capped accordingly. Terminals accept solid or stranded copper from 0.75 mm² up to 35 mm², covering most final-circuit cable sizes. Power dissipation is 0.75 W at rated current, negligible for thermal budgeting in a crowded enclosure.
Integration into the DIN-rail panel
Width is 18 mm (1 width unit), depth 77 mm, installation depth 70 mm, height 125 mm. It clips onto standard 35 mm DIN rail. Supply can enter from either top or bottom, which saves wiring time when the busbar is on the opposite side. Touch protection is built in (IP20 with connected conductors in a closed distribution board). Overvoltage category III suits fixed-installation mains connection. The unit is halogen-free and silicon-free — relevant for panels in clean-room or corrosive environments.