What it is and what the ratings mean
The Siemens 5SU1654-0KV25 is a combined residual-current device and miniature circuit breaker (RCBO) — an FI/LS protector in Siemens' terminology — in a single 1+N-pole module. The B-curve 25A overcurrent trip protects cable and loads against short-circuit and overload up to that rating, while the 300 mA Type AC residual-current element detects sinusoidal AC earth faults. The 10 kA breaking capacity at the rated voltage means it can safely interrupt a fault current up to that level without welding contacts or cascading damage upstream in the panel. Type AC detection covers sinusoidal AC residual currents only — it is not rated for pulsating DC or smooth DC faults, so the application must be purely AC loads without electronic rectification that could mask a fault.
Deployment context
The 70 mm width (4 module spaces on a DIN rail) fits standard Siemens distribution boards and enclosures. The 1+N-pole format switches the line conductor and switches the neutral, so it is suited for single-phase final subcircuits in TN or TT earthing systems. In a panel, treat it like any DIN-rail mounted RCBO — leave air gap for heat dissipation at 25 A continuous load and verify the neutral bar arrangement matches the switched neutral output.