The Siemens 5SY8408-8 is a SENTRON 5SY8 miniature circuit breaker, 4-pole with D tripping characteristic, rated 8 A at 400 V AC. This is the current-production variant — lifecycle status is active, so it's the part you spec into a new BOM or order as a direct replacement for an existing 5SY8 install.
At 8 A rated current, it's a common feeder for a small motor group or a control transformer. If you're specifying for a UL 508A panel, the UL 1077/CSA C22.2 No. 235 rating is 5 kA — a different standard, so verify the SCCR requirement against your assembly rating. Overvoltage category III means it's rated for fixed-installation distribution boards, not just downstream equipment. The 72 mm width (4 width units at 18 mm each) is the standard DIN-rail footprint for a 4-pole MCB — it occupies four adjacent positions on the rail.
Any mounting position is acceptable. Humidity derating applies: max 95% RH at 55 °C, 55% at 70 °C, 35% at 75 °C. The terminals accept solid or stranded copper from 0.75 mm² up to 35 mm², with a tightening torque of 2.5 to 3.5 N·m. Sealable (padlockable) per the spec, which matters for lockout/tagout compliance.
Rated per IEC 60947-2 (40 kA at 400 V AC, 15 kA at 72 V DC) and UL 1077 / CSA C22.2 No. 235 (5 kA). Halogen-free and silicon-free construction. The product designation per DIN EN 61346-2 and DIN EN 81346-2 is function code F (protective device). Touch protection is provided on the terminals.