The Siemens 5SY4125-8CV is a 1-pole miniature circuit breaker (MCB) with a D tripping characteristic, rated 25 A and a breaking capacity (Icu) of 15 kA at 230/400 V AC. The D-curve means it tolerates high inrush currents — typically 10–20× rated current — making it the right choice for motor, transformer, or lighting ballast circuits where the start-up surge would nuisance-trip a B- or C-curve breaker. The 15 kA Icu at 400 V tells you it can safely interrupt a fault current up to that level without welding contacts or rupturing the case, which is essential for panel SCCR compliance.
This MCB snaps onto a standard 35 mm DIN rail per EN 60715, occupying a single modular width (roughly 18 mm per pole). The tunnel terminals accept up to 25 mm² conductor. In a crowded panel, the D-curve and 15 kA rating let you coordinate downstream with a higher-rated upstream device — typical for a motor feeder branch where the breaker sits after a 63 A or 100 A molded-case switch.