The Siemens 5SU9404-0KK20 is a SENTRON RCBO — an RCD-operated circuit breaker combining overcurrent protection and residual current detection in a single 18 mm-wide module. It is rated 20 A with a B tripping characteristic and a 10 kA breaking capacity per EN 60898 (also 10 kA per IEC 60947-2). The 1P+N configuration switches the line and monitors the neutral, with Type AC fault current detection for sinusoidal AC residual currents.
The 20 A rated current at AC and B-curve define the magnetic trip threshold: B-curve trips at 3–5× rated current, so 60–100 A for a short-circuit, making it suitable for resistive or general-purpose loads with modest inrush — lighting, small control transformers, or office socket circuits. The 1P+N poles with one protected pole mean the line is switched and protected, while the neutral is switched but not overcurrent-protected — typical for single-phase final circuits. Ambient temperature range of -25 to +45 °C covers most indoor panel environments, but derate the continuous current above 30 °C: the evidence shows 20 A at 30 °C, 18.8 A at 40 °C, 18.2 A at 45 °C (–). The 18 mm width occupies one modular unit on a DIN rail, matching the standard SENTRON footprint. IP20 protection applies only when the distribution board is installed with connected conductors — this is a panel-internal device, not for wet or washdown locations.
Panel integration
The 18 mm width and 77 mm depth fit a standard DIN-rail slot in any SENTRON distribution board. Installation depth is 70 mm. Mounting position is any orientation. Supply can enter from either top or bottom. Conductor cross-section accepts 0.75–35 mm² solid or stranded. The touch-protection feature is built in.