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.
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. 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. 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. Touch protection is built in, so live terminals are shielded when the breaker is in place.
Panel integration
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.