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 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.
Deployment context
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.