What you're looking at
The Siemens 5SY4125-5 is a 1-pole miniature circuit breaker from the SENTRON family, rated 25 A with a tripping characteristic A. That characteristic A means it trips between 2 and 3 times rated current — tight, fast — so it's the go-to for protecting semiconductor circuits, control transformers, and sensitive electronic loads where a standard B or C curve would let through too much energy before opening. Breaking capacity is 10 kA per EN 60898 and 20 kA per IEC 60947-2 — the higher figure under the industrial standard means it can sit in a panel with a 20 kA prospective fault current at the bus, no problem. Overvoltage category III and pollution degree 3 confirm it's rated for fixed-installation distribution boards, not just clean-room final circuits.
Thermal derating — the real-world current
The 25 A rating holds at 35 °C ambient. At 40 °C it's 23.8 A; at 50 °C it's 22.55 A; at 60 °C it's 21.2 A. If your panel runs hot — say a packed DIN rail with drives above — size the load to the derated figure, not the label. Humidity derating also applies: max 95% RH up to 55 °C, then drops to 55% at 70 °C and 35% at 75 °C.
Panel fit and mounting
Width is 18 mm (1 modular width), depth 76 mm, installation depth 70 mm. Snap-on quick assembly system to DIN rail, any mounting position. Combined terminals top and bottom accept the same conductor range — no special feed-through lug needed. IP20 with conductors connected; touch protection built in.