What it is and what it does
The Siemens 3VA1125-6EF32-0KH0 is a SENTRON molded case circuit breaker (MCCB) for line protection. It is a 3-pole unit rated 25 A at 40 °C, with a trip indicator and a shunt trip release (STL) plus two auxiliary switches and one trip alarm switch (HQ). The rated insulation voltage is 800 V. Breaking capacity is the headline here: 220 kA at 240 VAC, 154 kA at 415 V, 121 kA at 440 V, and still 17 kA at 690 V. That's a high-interrupting rating — it clears severe fault currents without cascading upstream, which matters when the transformer is big or the fault current is high.
Thermal derating and panel fit
The 25 A rating holds flat from 40 °C to 50 °C, then drops to 24 A at 55–60 °C, and 23 A at 65–70 °C. That's a mild derating curve — you don't lose much headroom in a warm enclosure. Operating range is -25 °C to 70 °C; storage from -40 °C to 80 °C. Footprint: 76.2 mm wide (3 in), 130 mm tall (5.12 in), 70 mm deep (2.76 in). That's a standard 3-pole MCCB width — it snaps onto a DIN rail or mounts with screws. The shunt trip and auxiliary switches are factory-installed, so no field wiring of add-on kits.