What it is and what it does
The Siemens 5SU1656-0KV20 is a combined residual-current device and miniature circuit breaker — an FI/LS protector — in a single 1+N-pole module. It combines Type AC residual-current detection at 300 mA with a B-curve thermal-magnetic overcurrent trip set at 20 A, all rated for a 6 kA breaking capacity. That means one unit handles both earth-leakage and short-circuit protection in a final subcircuit, saving a DIN-rail slot over separate RCD and MCB devices.
Where it goes
This is a DIN-rail-mounted protector sized for distribution boards and sub-distribution panels in residential, commercial, or light-industrial installations. The 70 mm depth (D=70MM) fits standard enclosure depths without crowding the gland plate. The Type AC detection handles sinusoidal AC residual currents — the common case for lighting, socket outlets, and general-purpose circuits without electronic loads that generate pulsed or DC fault currents.
Ratings and what they mean for your panel
The 300 mA residual-current threshold is a fire-protection level, not a personnel-protection level. It is specified where the circuit serves fixed equipment with some natural leakage.