What it is and what it does
The Siemens 5SU9404-0KK40 is a SENTRON RCD-operated circuit breaker — an RCBO that combines overcurrent protection and residual current detection in one 18 mm-wide unit. Rated 40 A at AC with a B tripping characteristic, it clears faults up to 10 kA per both EN 60898 and IEC 60947-2. That 10 kA SCCR means it can safely interrupt a short-circuit fault up to that level without the arc escaping or damaging upstream gear — a solid fit for final subcircuits in commercial and light industrial panels.
What the ratings mean for your panel
The 40 A rating is at 30 °C ambient. At 40 °C it derates to 37.6 A; at 45 °C to 36.4 A; at 50 °C to 34.8 A; at 55 °C to 32.4 A. If your panel runs warm — say, a packed enclosure near a motor drive — use the derated figure, not the headline 40 A, to size the load. The B curve trips between 3 and 5 times rated current, so it's suited for resistive and general-purpose loads, not high-inrush motor starts. Overvoltage category III means it's rated for fixed-installation distribution boards, not plug-in portable gear. It's a 1P+N design — one protected pole plus a switched neutral. The neutral pole is not overcurrent-protected; it just breaks and makes with the phase. The residual current element is type AC, so it detects sinusoidal AC fault currents only. For pulsating DC or smooth DC faults you'd need a type A or B RCD module. The instantaneous design means no intentional time delay — it trips within the standard RCD response window. Wire range accepts 0.75 to 35 mm² both solid and stranded. That covers everything from 1.5 mm² lighting circuits up to 10 mm² feeder tails. Installation depth is 70 mm; the overall depth is 77 mm. At 18 mm wide (one width unit), it fits standard DIN-rail enclosures with standard busbar spacing. Mounting position is any — no derating for horizontal or upside-down install. Touch protection is built in, so live terminals are shielded when the breaker is in place. Halogen-free and silicon-free construction matters if this breaker sits in a clean room, medical facility, or any environment where outgassing from a fault could contaminate sensitive equipment. Ambient temperature range is -25 to +45 °C for operation; storage range is -40 to +75 °C.
Panel integration
Snaps onto standard 35 mm DIN rail. The 18 mm width (one modular unit) means it takes up one slot in a distribution board. Supply can come from either top or bottom — no mandatory orientation. The IP20 rating applies only when installed in a distribution board with conductors connected; it's not rated for wet or dusty environments on its own.