Part identity and what it does
The Siemens 3VA1225-5FF42-0HH0 is a SENTRON molded case circuit breaker (MCCB) rated for 125 A continuous current across four poles, built for line protection in distribution panels and switchgear. Its 187 kA interrupting capacity at 240 V handles high-fault utility feeds without cascading upstream — that's the spec that decides whether this breaker clears a bolted fault before the transformer secondary protection trips. Insulation voltage is rated 800 V, so it's comfortable on 480 V and 600 V class systems.
Breaking capacity across voltages
Interrupting rating drops as voltage rises: 187 kA at 240 V, 121 kA at 415 V, 75.6 kA at 440 V, 30 kA at 500 V, and 17 kA at 690 V. That 17 kA at 690 V is the number to check if you're on a 690 V mining or marine distribution bus — most 125 A frames don't carry that high a rating at that voltage.
Thermal derating and panel fit
Rated 250 A at 40 °C ambient, but that's the frame thermal rating — the breaker's actual continuous current is 125 A. Derating starts above 50 °C: 243 A at 55 °C, 237 A at 60 °C, 230 A at 65 °C, 223 A at 70 °C. The 70 mm depth and 140 mm width mean it fits standard MCCB panel cutouts; the 158 mm height is typical for this frame size. Power loss is 57 W maximum — factor that into enclosure ventilation if you're packing several breakers in a row.
Auxiliary and release configuration
This variant ships with a shunt trip (STL) release and two auxiliary switches plus one trip alarm switch (HQ configuration). No undervoltage release, no ground-fault monitoring, no communication module. The shunt trip lets a remote PLC or E-stop circuit trip the breaker — common for emergency-off chains in conveyor or pump panels. Trip indicator and voltage trigger are present on the mechanism.