What it is and what it does
The Siemens 5SU1324-7KX25 is a SENTRON RCBO — a combined residual-current circuit breaker and overcurrent protection device in a single 3-module-width unit (54 mm wide). It protects a 2-pole circuit against overload, short-circuit, and earth leakage in one package, saving a DIN-rail slot compared to separate MCB + RCCB. Rated 25 A at 30 °C with a C tripping curve (magnetic trip between 5× and 10× In, so it handles motor inrush and transformer energisation without nuisance trips) and a 30 mA Type A residual-current element that detects sinusoidal AC and pulsating DC fault currents. Breaking capacity is 10 kA per EN 60898, adequate for most residential and light commercial distribution boards.
Thermal derating — what the numbers mean for panel loading
The 25 A rating holds at 30 °C ambient. Above that, the RCBO derates: 23.5 A at 40 °C, 22.75 A at 45 °C, 21.75 A at 50 °C, 20.25 A at 55 °C and 60 °C, 19.25 A at 65 °C, and 18.25 A at 70 °C. If the panel runs warm — say 50 °C inside a sealed enclosure — you need to size the circuit for 21.75 A continuous, not 25 A. The mounting position is unrestricted, which helps airflow in tight cabinets.
Panel fit and wiring
The RCBO occupies 3 modular width units (54 mm) on a DIN rail, with a depth of 77 mm and an installation depth of 70 mm. Supply can enter from either top or bottom. The IP20 rating applies once installed in a distribution board with connected conductors — not for standalone outdoor use. Halogen-free and silicon-free construction suits environments where outgassing matters, such as clean rooms or food processing areas.