What you're looking at
The Siemens SENTRON 5SU9346-1CR40 is a 4-pole RCD-operated circuit breaker — an RCBO — combining overcurrent protection and residual current detection in one 7-MU-wide DIN-rail module. It's rated 40 A with a C tripping curve and 6 kA breaking capacity per EN 60898, handling AC fault currents only. That C-curve means it's sized for motor and transformer inrush: holds through 5-10x rated current for a short window, then trips fast on a hard short. The 6 kA SCCR at 400 V AC is enough for most distribution boards downstream of a larger upstream breaker; if your panel's prospective fault current exceeds that, you'll need a current-limiting upstream device to keep the 5SU9 in its coordination zone.
Derate as the cabinet warms: 37.2 A at 40 °C, 35.6 A at 45 °C, 34.8 A at 50 °C, 34 A at 55 °C. If your panel runs hot — say, a packed enclosure with drives — the 40 A nameplate effectively becomes a 35 A breaker at 45 °C. The 6 kA breaking capacity is identical under both EN 60898 and IEC 60947-2, so it's rated for both residential and industrial coordination studies. Terminal capacity accepts 0.75 to 35 mm² solid or stranded, which covers everything from 1.5 mm² lighting circuits up to 10 mm² feeder tails.
The 7 width units (126 mm) mean it occupies seven 18 mm slots — check your rail budget if the enclosure is tight. Supply can land on either top or bottom terminals; no mandatory orientation. Touch protection is built into the terminal design, so no additional finger-duct covers needed inside the distribution board.