The Siemens 5SU9304-1KK40 is a SENTRON-series RCBO — an RCD-operated circuit breaker that combines overcurrent protection and residual current detection in a single 18 mm-wide module. It is a 1P+N device (protecting one phase and switching the neutral), rated 40 A at 30 °C with a Type A tripping characteristic (C-curve for overcurrent). The breaking capacity is 10 kA per both EN 60898 and IEC 60947-2, which covers most commercial and light industrial panelboard applications.
Thermal derating and mounting flexibility
The 40 A rating is 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 panel ambient runs warm, size the upstream protection accordingly. The device can be mounted in any position, and the supply can enter from either top or bottom — useful when retrofitting into an existing distribution board without re-routing conductors.
Integration into a panel
At 18 mm wide (one modular unit), this RCBO snaps onto a standard DIN rail. The installation depth is 70 mm, and the overall depth is 77 mm — check that the enclosure or distribution board has clearance for that depth plus wiring space. The IP20 rating applies when installed in a distribution board with connected conductors; the device itself is not rated for exposed or washdown locations.