The Siemens 5SY6640-8 is a SENTRON miniature circuit breaker rated 40 A at 400 V AC, with a D tripping characteristic and 3P+N poles. It interrupts faults at 6 kA per EN 60898 and 10 kA per IEC 60947-2, so it handles high inrush loads like motor starters, transformers, or welding equipment without nuisance tripping on startup. The 72 mm width takes 4 width units on the rail. Terminal blocks accept solid or stranded conductors from 0.75 up to 35 mm², and the combined top/bottom terminals let you feed through without extra busbars.
The 5 kA UL 1077 / CSA C22.2 No. 235 rating is for North American supplementary protection applications, not branch-circuit primary protection. Ambient temperature range is -25 to +55 °C, storage -40 to +75 °C. Relative humidity limits: 95% at 55 °C, 55% at 70 °C, 35% at 75 °C. If your panel runs hot or humid, derate per the manufacturer's thermal curve — the breaker's thermal trip element responds to ambient, so a hot enclosure can cause nuisance tripping below 40 A.