What it is and what it does
The Siemens 3VA1225-4GF42-0AE0 is a SENTRON molded case circuit breaker (MCCB) with 4 poles, rated for a continuous current of 250 A at 40 °C and fitted with a TM240 thermal-magnetic overcurrent release. It is designed for line protection — the primary feed breaker in a distribution panel, not a motor-protective device. The TM240 release means the thermal element is calibrated for 240 A frame, adjustable down to cover the 250 A rating; you size the breaker to the conductor ampacity, not the load. Breaking capacity is the headline: 121 kA at 240 V AC, 75.6 kA at 415 V, 25 kA at 440 V, and 11.9 kA at 690 V. At 240 V this breaker clears a 121 kA fault — that's high enough for most transformer-secondaries and large service entrances without cascading an upstream fuse. The 415 V figure (75.6 kA) covers the common 400 V industrial distribution level. These are the values that determine whether the breaker coordinates with the available fault current on your bus.
Thermal derating and panel fit
The 250 A rating holds flat from 40 °C to 50 °C. Above that it derates: 243.3 A at 55 °C, 236.5 A at 60 °C, 229 A at 65 °C, and 223 A at 70 °C. If the panel ambient runs hot — say next to a transformer or in a non-climate-controlled enclosure — the effective ampacity drops. Size your conductors and load for the derated value at your actual ambient, not the 40 °C sticker. Dimensions are 158 mm high, 140 mm wide, 70 mm deep. Front protection is IP40 — splash-resistant from the front but not sealed. The 4 auxiliary switches (HQ type) are built in, giving status feedback without an add-on module. No undervoltage release, no shunt trip, no communication module on this variant — it's a straight thermal-magnetic breaker with aux contacts.