What this MCB does on the line
The Siemens 5SY3125-6 is a 1-pole SENTRON miniature circuit breaker rated 25 A with a B tripping characteristic, meaning it trips at 3–5× rated current — the standard choice for residential and light commercial lighting and socket-outlet circuits where inrush is low. It breaks 4.5 kA per EN 60898 and 6 kA per IEC 60947-2, so it handles typical domestic and industrial fault levels up to that point without cascading upstream. The 400 V AC rating covers single-phase line-to-neutral or line-to-line applications; the 72 V DC maximum extends it to control circuits and DC auxiliary supplies.
IP20 with connected conductors; install in a cabinet that keeps the front accessible for the sealable toggle.
Breaking capacity across standards
This MCB carries three breaking-capacity ratings because it is certified to multiple standards. Per EN 60898 (the household standard) it is 4.5 kA at 400 V AC. For DC circuits per IEC 60947-2 it is 15 kA at 72 V. Under UL 1077 and CSA C22.2 No. 235 it is 5 kA at 400 V AC. The energy limiting class is 3, so it limits let-through energy well enough to protect downstream components in a selective coordination scheme.
Humidity tolerance: 95 % at 55 °C, derating to 35 % at 75 °C.