What this MCB does and where it fits
The Siemens SENTRON 5SY4303-7CC is a 3-pole miniature circuit breaker rated 3 A with a C tripping characteristic, designed to protect branch circuits in industrial control panels and mechanical engineering installations. Its 10 kA breaking capacity per EN 60898 means it can safely interrupt fault currents up to that level under standard conditions — adequate for most sub-distribution boards and machine supply circuits where the prospective short-circuit current is within that range. The C-curve trips between 5 and 10 times rated current, so it's sized for inductive loads like small contactors and relay coils that draw a moderate inrush without nuisance tripping on start-up. Mounting is on a standard DIN rail via the quick assembly system, and the breaker occupies 3 modular width units (54 mm). The mounting position is unrestricted — any orientation works — and the installation depth behind the panel is 70 mm. Pollution degree 3 and overvoltage category III ratings confirm it's suited for the industrial environment inside a cabinet where conductive dust and transient surges are expected.
Thermal derating and real-world current
The 3 A rating holds at 35 °C ambient. Above that, the thermal-magnetic trip element derates: 2.82 A at 40 °C, 2.73 A at 45 °C, 2.63 A at 50 °C, and 2.53 A at 55 °C. If the panel runs warm — common in a packed enclosure with drives or transformers — the actual continuous current the breaker can carry without tripping drops accordingly. The surrounding temperature influence spec also caps relative humidity: max 95 % up to 55 °C, then derating to 55 % at 70 °C and 35 % at 75 °C. That's a reminder that a sealed, unventilated cabinet can push internal ambient past the nominal rating point, and the breaker's load must be sized for the real temperature, not the label.