What it is and what it does
The Siemens 5SU9406-0KK20 is a SENTRON-series RCD-operated circuit breaker (RCBO) that combines overcurrent protection and residual current protection in a single 18 mm wide module. It is rated for 20 A at 30 °C with a B tripping characteristic, meaning it trips at 3–5 times the rated current for fast clearing of short circuits while tolerating moderate inrush from resistive loads. The breaking capacity is 6 kA per both EN 60898 and IEC 60947-2, which certifies its ability to safely interrupt faults up to that level in a 230/400 V AC distribution panel. This is a 1P+N device — it switches the live conductor and monitors the neutral for residual current, but does not switch the neutral. The fault current type is AC (sinusoidal), and the design is instantaneous (no intentional time delay), so it responds immediately to earth leakage. It is intended for final circuits in residential or light commercial distribution boards where combined overcurrent and earth-fault protection is required in a single DIN-rail slot.
Current derating and thermal limits
The 20 A rating applies at 30 °C ambient. At 40 °C the continuous current must be derated to 18.8 A; at 45 °C to 18.2 A; at 50 °C to 17.4 A; and at 55 °C to 16.2 A. The operating ambient range is -25 to +45 °C, so the 45 °C derating figure is the relevant maximum for a panel running near its thermal limit. Storage temperature spans -40 to +75 °C. Power dissipation is 0.75 W at rated load, which is low enough that grouping multiple units in an enclosure does not require special ventilation beyond standard panel design. The energy limiting class is 3, indicating good let-through energy limitation for downstream equipment protection.
Mounting and wiring
The unit occupies one width unit (18 mm) on a DIN rail, with an installation depth of 70 mm and overall depth of 77 mm. Height is 125 mm. Mounting position is unrestricted — any orientation is acceptable. Supply can enter from either the top or bottom terminals, simplifying busbar routing in a crowded panel. Terminals accept solid conductors from 0.75 to 35 mm² and stranded conductors from 0.75 to 35 mm². The housing is halogen-free and silicon-free, which matters for panels in environments sensitive to outgassing (e.g., clean rooms, medical facilities). Touch protection is provided per the product feature listing.
Standards and approvals
The 5SU9406-0KK20 is designed to EN 60898 (for household and similar installations) and IEC 60947-2 (for low-voltage switchgear and controlgear), with a 6 kA rated short-circuit capacity under both standards. Overvoltage category is III, suitable for fixed installations downstream of the main distribution board. The IP20 rating applies when installed in a distribution board with connected conductors.