The Siemens 3RV5431-4FA10 is a SIRIUS circuit breaker specifically designed for transformer protection, carrying a 40 A rated operational current at 400 V AC-3 and a 690 V rated insulation voltage. This is the part you spec when a motor-starting curve is too slow and a standard thermal-magnetic would trip on the magnetizing inrush.
The S2 frame (55 mm wide, 140 mm tall, 149 mm deep) needs 50 mm clearance above and below, 140 mm forward, and 30 mm to the side — no clearance needed behind the rail. Box terminals accept 2x 0.75–25 mm² solid/stranded or 1x up to 35 mm²; torque to 3–4.5 N·m.
Operates from -20 to +60 °C with 10–95 % relative humidity; survives transport down to -50 °C. Rated for installation up to 2000 m above sea level and withstands 25 g / 11 ms shock per IEC 60068-2-27. The substance prohibitance date of 07/01/2006 indicates RoHS compliance ahead of the EU directive.
The 40 A rating at AC-3 (400 V) governs motor and transformer switching duty — that is the current it can make and break under load. Power loss is 20 W typical, which matters for thermal management inside a sealed enclosure. The thermal overload release handles sustained overloads; the magnetic short-circuit trip clears faults fast. Operating frequency is 50–60 Hz; at AC-3 duty the maximum switching rate is 15 operations per hour.