The Siemens 5SU1654-0KV40 is a combined residual-current and miniature circuit breaker — an FI/LS protector — in a 1+N-pole form factor. The 70 mm width means it occupies four standard 17.5 mm modular DIN-rail slots in a distribution board.
The 300 mA residual-current threshold (type AC, detecting sinusoidal AC earth faults only) suits general socket-outlet and lighting circuits where pulsing DC fault currents are not expected. The B-curve (3–5× In magnetic trip) protects cable runs and loads with limited inrush, typical of resistive and general-purpose circuits.