The Siemens 5SY4303-8CC is a 3-pole SENTRON miniature circuit breaker rated 3 A with a D tripping characteristic, designed for 400 V AC circuits. It snaps onto a DIN rail (quick assembly system) and accepts conductors from 0.75 to 35 mm² solid or stranded, so it handles both control wiring and power feeds in the same terminal.
The 3 A rating at 400 V AC is the continuous current it carries without tripping; the D characteristic (10–20× In magnetic trip) lets it ride through the start-up surge of a small motor or a bank of contactor coils. For UL 1077 / CSA C22.2 No. 235 applications, the interrupting rating is 5 kA, which is typical for supplementary protectors in North American control panels. Ambient temperature range is -25 to +55 °C for normal operation, with storage from -40 to +75 °C. The influence of surrounding temperature on derating is specified: max 95% RH at 55 °C, tapering to 35% RH at 75 °C. This matters if the breaker lives inside a hot, non-ventilated enclosure near other heat sources.
Width is 54 mm (3 width units at 18 mm each), depth 76 mm, height 90 mm, with an installation depth of 70 mm behind the panel. Mounting position is any orientation. The combined terminal at top and bottom accepts both solid and stranded copper without ferrules, torqued to 2.5–3.5 N·m.