What it is and what the ratings mean
The Siemens 5SU1654-6KV20 is a combined residual-current device and miniature circuit breaker (RCBO) — one unit does both fault-current and overcurrent protection for a single-phase final subcircuit. The Type A designation means it detects AC residual currents plus pulsating DC, which covers most modern electronics and rectifier loads. The 300 mA IΔn is the earth-leakage trip threshold; at that level it's typically specified for circuits where a lower 30 mA threshold would cause nuisance tripping — think distribution boards feeding multiple socket outlets or fixed equipment with natural leakage. The 10 kA breaking capacity (IEC 60898) tells you it can safely interrupt a short-circuit fault up to that level, which suits standard domestic and light commercial panels. The B-curve 20 A overcurrent characteristic trips magnetically between 3 and 5 times rated current, so it's a good match for resistive and general-purpose loads without high inrush.
Deployment context
This RCBO snaps onto a standard DIN rail inside a distribution board or sub-distribution enclosure. The 1+N-pole format (switched phase plus switched neutral) means it isolates both conductors on fault, which is required in many national wiring regulations for final circuits. The 70 mm width is a common module size for RCBOs in European-style consumer units.