The Siemens 5SY4308-8CV is a 3-pole miniature circuit breaker from the SENTRON 5SY4 series, rated D8 (8 A, D-curve) with a short-circuit breaking capacity Icu of 15 kA at 400 V AC.
What the D-curve and 15 kA mean for fit
The 15 kA Icu at 400 V AC gives headroom for high-fault installations; it matches the typical SCCR requirement for industrial panel feeders. Three-pole construction suits 3-phase circuits; the DIN-rail mount clips into standard 18 mm per-pole spacing.