What it is and what it does
The Siemens 5SU9304-0KK20 is a combined residual-current-operated circuit breaker (RCBO) from the SENTRON family — one device that handles both overcurrent protection and earth-leakage detection in a single 1-module-wide (18 mm) package. It protects a single-phase final subcircuit: pole 1 is switched and protected, neutral is switched only. The B tripping curve means the magnetic release trips at 3 to 5 times rated current, suited for resistive and general-purpose loads where inrush is modest. The residual-current element is type AC, detecting sinusoidal AC fault currents at the 30 mA threshold — the standard for personal protection in residential and light commercial panels. Rated 20 A at 30 °C ambient, the device derates to 18.8 A at 40 °C, 18.2 A at 45 °C, 17.4 A at 50 °C, and 16.2 A at 55 °C. In a hot control room or a panel near a kiln, that derating curve is the number that decides whether the circuit holds — not the 20 A nameplate. The 10 kA breaking capacity per EN 60898 means it can safely interrupt a bolted fault up to that level, which covers most domestic and light commercial service-entrance scenarios.
Integration — DIN rail and panel fit
Snaps onto standard 35 mm DIN rail, one modular width (18 mm). Depth is 77 mm (70 mm installation depth behind the rail). Height 125 mm. The supply can enter from top or bottom. With IP20 protection only when the distribution board is installed and conductors connected, this is strictly an enclosed-panel device — not for open mounting. The halogen-free and silicon-free construction matters for environments where outgassing contaminates sensitive electronics or process atmospheres.
Environmental and compliance
Rated for ambient storage from -40 °C to 75 °C, operating range is the same — the device lives where it works. Overvoltage category III, pollution degree 2, surge voltage resistance 4 kV. These ratings tell the specifying engineer it is suited for fixed-installation downstream of the main distribution board, not for outdoor or wet locations.