What it is and what it does
The Siemens 5SU1324-6KX25 is a SENTRON RCBO — a combined residual-current circuit breaker and miniature circuit breaker in one 3-module-wide unit (54 mm wide, 77 mm deep, 90 mm high). It protects a final circuit against both overload/short-circuit and earth leakage, saving a DIN-rail position versus separate devices. Rated 25 A with a B tripping characteristic (instantaneous trip between 3 and 5 times rated current), it's sized for resistive and light inductive loads — lighting circuits, socket outlets, small motor feeds — where inrush isn't severe. The residual-current element is Type A, 30 mA, covering sinusoidal AC and pulsating DC fault currents up to 6 mA smooth DC. Breaking capacity is 10 kA per EN 60898, sufficient for most domestic and light commercial distribution boards where the prospective short-circuit current stays under that threshold. Insulation voltage is rated 264 V.
Thermal derating — the real current rating
The 25 A rating holds at 30 °C ambient. At 40 °C it's 23.5 A; at 50 °C it's 21.75 A; at 60 °C it's 20.25 A; at 70 °C it's 18.25 A. If the distribution board runs warm — and most do, especially with high-density RCBOs — size the load to the derated figure, not the nameplate 25 A.
Mounting and integration
Snaps onto standard DIN rail, any mounting position. Occupies 3 modular width units (54 mm). Supply can enter from top or bottom. Installation depth is 70 mm plus the device depth of 77 mm — check the enclosure depth before panel layout. IP20 when installed in a distribution board with conductors connected; not for wet locations without an outer enclosure.