What it is and what the ratings mean
The Siemens 5SU1656-1KV16 is an FI/LS combination unit — a residual-current circuit breaker (RCD) and miniature circuit breaker (MCB) in one 70 mm wide module. That means a single device provides both earth-leakage protection and overcurrent/short-circuit protection for a final subcircuit, saving a DIN-rail position compared to separate RCD and MCB units. The 16 A C-curve thermal-magnetic trip handles motor inrush and general-purpose loads; the 300 mA Type AC residual-current element catches sinusoidal AC fault currents to ground. The 6 kA breaking capacity at the rated voltage covers typical domestic and light commercial panel fault levels. On a DIN-rail panel, the 70 mm width is the footprint to plan for.