What it is and where it lands
The Siemens 3VA1225-4FF42-0AA0 is a SENTRON molded case circuit breaker (MCCB) rated for 125 A continuous, with a 4-pole configuration and a TM240 thermal-magnetic release. It's designed for line protection — meaning it sits at the incoming feed of a distribution panel, not downstream on a motor branch. The fixed 125 A rating means you don't adjust it; you size the feeder for that number and the breaker holds it.
Breaking capacity — what the curve tells you
This MCCB delivers 121 kA interrupting capacity at 240 V, dropping to 75.6 kA at 415 V, 52.5 kA at 440 V, 17 kA at 500 V, and 11.9 kA at 690 V. That's a steep derating as voltage climbs — common for a thermal-magnetic breaker. At 240 V it handles high fault currents typical of North American 120/240 V split-phase services; at 400 V class it still clears 75.6 kA, which covers most industrial distribution transformers up to about 2 MVA. If your system runs at 690 V, the 11.9 kA figure is the one to coordinate against.
Thermal derating — don't ignore the ambient curve
The breaker holds its full 125 A from 40 °C up to 50 °C. At 55 °C it derates to 243 A, then 237 A at 60 °C, 230 A at 65 °C, and 223 A at 70 °C. The operating ambient range is -25 °C to 70 °C. If the panel sits in a hot mezzanine or near a furnace line, that 223 A at 70 °C is your real limit — not the nameplate 125 A. The TM240 release's thermal element tracks ambient, so you don't need a separate derating factor; it self-compensates.
Physical fit and panel integration
Dimensions are 140 mm wide, 158 mm high, 70 mm deep. The 4-pole body is wider than a 3-pole — check your DIN-rail or mounting-plate space before you cut the backpanel. Front IP40 protection means it's sealed against tools and fingers but not washdown; keep it inside the enclosure. Rated insulation voltage is 800 V, which covers 690 V systems with margin. Power loss at full load is 57 W — factor that into your enclosure thermal calculation if you're stacking several breakers.