The Siemens 5SY4405-8CV is a 4-pole miniature circuit breaker (MCB) from the 5SY series, rated for 0.5 A with a D tripping characteristic.
The 4-pole footprint takes four module widths (roughly 72 mm) in the enclosure — factor that into your panel fill when laying out a distribution board or sub-feed. D-curve characteristic means the magnetic trip threshold is 10–20× In (5–10 A for this 0.5 A unit), so it's sized for loads with high inrush: small transformers, solenoid banks, or control-circuit power supplies where a B-curve would nuisance-trip on start-up.