The Siemens 5SU9304-1KK20 is a SENTRON RCBO — an RCD-operated circuit breaker combining overcurrent and residual-current protection in a single 1 MW (18 mm) module. Rated 20 A at 30 °C with a C tripping curve, it breaks 10 kA per EN 60898 and IEC 60947-2, covering most residential and light commercial final circuits. The 1P+N design protects the live conductor only; the neutral is switched but not protected.
The 20 A rating at 30 °C derates to 18.8 A at 40 °C and 16.2 A at 55 °C — if the distribution board sits in a hot attic or near a boiler, size the upstream MCB accordingly. The C-curve (magnetic trip at 5–10× In) suits motor and lighting loads with moderate inrush; for purely resistive circuits a B-curve would trip faster. The IP20 rating applies only when the distribution board is installed and conductors are connected — not for open-panel use.
The 1 MW width (18 mm) means it occupies one slot in a standard modular enclosure. The test button requires a minimum 115 V to function; verify the test circuit voltage before commissioning.