What this MCB does on the rail
The Siemens 5SY4104-7CV is a 1-pole miniature circuit breaker with C-curve tripping characteristic, rated 4 A at 230/400 V AC and breaking 15 kA per IEC 60947-2. The C-curve (5–10× In) handles motor inrush and transformer magnetizing currents without nuisance trips, while still protecting downstream wiring. Rated for 50/60 Hz supply, it fits standard 18 mm modular DIN-rail enclosures.
Temperature derating — don't ignore it
At 35 °C ambient the 5SY4104-7CV carries its full 4 A rating. Push the panel ambient to 50 °C and it derates to 3.4 A; at 55 °C it's 3.22 A. The thermal curve is published and repeatable, not a guess.
Width is one modular unit (18 mm), depth 76 mm, height 90 mm. Mounting position is any; the breaker works upright, sideways, or inverted. Top and bottom combined terminals accept the same wire gauge, so line/load orientation doesn't matter.