The Siemens 5SY8608-8 is a SENTRON miniature circuit breaker — a 4-pole (3P+N) unit with a D tripping characteristic, rated 8 A at 45 °C and 400 V AC.
Breaking capacity — the number that decides selectivity
At DC per IEC 60947-2 it's rated 15 kA. For UL 1077 / CSA C22.2 No. 235 supplemental protection, the rating is 5 kA at AC — a different standard, not a derating; UL 1077 recognizes a lower SCCR for supplementary protectors than the IEC main standard.
Thermal derating and environment
The 8 A rating holds at 45 °C ambient. At 50 °C it derates to 7.75 A; at 55 °C to 7.5 A. The breaker operates from -40 °C to 75 °C ambient, with humidity limits of 95% up to 55 °C, tapering to 35% at 75 °C. Vibration resistance per IEC 60068-2-6: ±1 mm at 5–25 Hz, 50 m/s² at 25–150 Hz.
IP20 with connected conductors — finger-safe but not washdown; keep it inside a sealed enclosure if the panel sees hose-down.