What it is and what it does
The Siemens 5SY5125-7 is a 1-pole C-curve miniature circuit breaker from the SENTRON family, rated for 10 kA breaking capacity per IEC/EN 60898-2 and also listed to UL1077. It snaps onto a DIN rail in any mounting position and takes up one modular width (18 mm). The C-curve means it trips between 5 and 10 times rated current — the standard choice for motor circuits, transformers, and general inductive loads where you need to ride out inrush without nuisance trips.
Where it fits
At 18 mm wide and 76 mm deep, it fits a standard DIN-rail enclosure with 70 mm installation depth — no special backbox needed. The quick-assembly fastening system lets you clip it on or off without tools, and the combined top and bottom terminals accept both busbar and conductor connections. Sealable covers are available for panel builders who need to lock out adjustments.