400 A MCCB with ETU850 — what the ratings mean for your panel
The Siemens 3VA2340-7KP42-0AA0 is a SENTRON molded case circuit breaker (MCCB) rated for 400 A continuous current at 40 °C, with a 4-pole configuration and an ETU850 electronic trip unit. This is the line-protection version — no ground-fault monitoring built in, so if you need GF protection you'd add an external module or look at the GF variant. The interrupting ratings are the headline here: 330 kA at 240 V, 242 kA at 415 V and 440 V, 187 kA at 500 V, and 7.5 kA at 690 V. That 330 kA at 240 V is an enormous fault-clearing capacity — this breaker is sized for high-available-fault-current main or feeder positions where a standard MCCB would weld shut. The 7.5 kA at 690 V is the practical ceiling; above that voltage, the arc extinction physics limit what a single air-break frame can do, so coordination studies need to respect that knee.
Thermal derating — the real current you can run
The 400 A rating holds flat from 40 °C through 50 °C. At 55 °C it derates to 375 A, at 60 °C to 350 A, at 65 °C to 325 A, and at 70 °C to 300 A. That's a linear-ish drop of about 5 A per degree above 50 °C. If your panel ambient runs at 55 °C — common in a sealed, sun-loaded enclosure — you lose 25 A of headroom. Plan the conductor sizing and the upstream protection for the derated value, not the nameplate 400 A. The maximum power loss is 96 W, which is moderate for a 400 A frame; that heat has to leave the enclosure, so factor it into your thermal budget.
ETU850 trip unit — what it gives you
The ETU850 is an electronic trip unit with communication capability and other measurement functions — so it can report load current and event data over the SENTRON communication bus. The adjustable response time (tr) ranges from 0.5 s to 17 s, which gives you selectivity coordination flexibility downstream. The N-conductor protection is adjustable: OFF or 20 % to 100 % of the phase setting. That matters for 4-wire systems where you need to protect the neutral against harmonic currents. The latching endurance is rated at 20,000 operations — that's the mechanical life; the electrical life under load will be lower, depending on the fault current magnitude.
Physical fit and panel integration
The 3VA2340-7KP42-0AA0 measures 248 mm high, 184 mm wide, and 110 mm deep. That's a 4-pole frame — wider than a 3-pole by roughly one pole pitch. The front face carries an IP40 protection rating, so it's protected against tools and wires >1 mm but not against water ingress; keep it inside a sealed enclosure if washdown is nearby. The depth of 110 mm means it will fit a standard 200 mm deep enclosure with room for rear busbars, but check the busbar bending radius if you're running 400 A through the line side. The reference code per DIN EN 81346-2 is Q, which is the standard designator for switching devices in Siemens schematics.