What this MCCB is and what it does
The Siemens SENTRON 3VA1225-5FF42-0AJ0 is a 4-pole molded case circuit breaker (MCCB) rated for 125 A continuous current, built for line protection in industrial distribution panels. The TM240 thermal-magnetic release handles overload and short-circuit protection without external power — it's a self-powered trip unit, so no auxiliary supply is needed on the breaker itself. The interrupting capacity runs 187 kA at 240 V AC, 121 kA at 415 V, 75.6 kA at 440 V, 30 kA at 500 V, and 17 kA at 690 V — that's enough headroom for most low-voltage switchgear applications where fault currents can spike. Rated insulation voltage is 800 V, so it's suitable for 690 V line-to-line systems. This is a line-protection design (not motor protection), meaning it's intended for feeder circuits, busbars, and distribution — not for direct motor overload duty where a separate overload relay would be needed.
Current derating and thermal limits
The breaker carries 125 A continuously up to 50 °C ambient without derating. Above that, it steps down: 243 A at 55 °C, 237 A at 60 °C, 230 A at 65 °C, and 223 A at 70 °C. If your panel runs hot — say near a row of drives or in a non-climate-controlled enclosure — the 55 °C figure is the one to watch. The operating temperature range is -25 °C to 70 °C; storage range is -40 °C to 80 °C. Power loss at full load is 57 W, which matters for enclosure heat budgeting.
Physical fit and auxiliary wiring
Dimensions are 70 mm deep, 140 mm wide, 158 mm tall — standard 4-pole MCCB footprint for panel mounting on a backplate or DIN rail adapter. The breaker ships with 2 auxiliary switches plus 1 trip alarm switch (HP type), so you get status feedback for the open/closed position and a separate contact that closes only on a trip event. That alarm contact is useful for remote fault annunciation without tying up a PLC input on the main contactor.