The Siemens 5SU9304-0KK40 is a SENTRON RCBO — an RCD-operated circuit breaker combining overcurrent and residual-current protection in one 18 mm-wide module. It's rated 40 A with a B-curve trip characteristic and 30 mA Type AC residual sensitivity, breaking capacity 10 kA per EN 60898 and IEC 60947-2. The 1P+N design protects one live pole while switching the neutral, and the 600 mm pigtail with solid neutral simplifies wiring in distribution boards.
The B-curve (3-5 In magnetic trip) is the default for general-purpose socket circuits and lighting; it avoids nuisance tripping from inrush on resistive loads while still clearing fast on hard shorts. At 18 mm width (1 modular unit), it occupies one DIN-rail position — critical when backfilling an existing board where every slot counts. The 30 mA Type AC residual sensitivity catches sinusoidal AC earth faults — the most common type in domestic and office wiring. It's not suitable for pulsating DC or smooth DC fault currents (Type A or F would be needed for EV chargers, VFD-fed circuits, or three-phase rectifier loads).
Mounts in any position on standard DIN rail. The 1 MW width occupies one position. The -40 °C to +75 °C ambient range covers unheated utility rooms and outdoor enclosures in temperate climates.