What it is and what it does
The Siemens SENTRON 5SU1656-7KK40 is a combined residual current circuit breaker with overcurrent protection (RCBO) — one device that handles both earth-leakage and overload/short-circuit protection in a single 2-MW (36 mm) wide package. It's rated 40 A with a Type C tripping characteristic and a 6 kA breaking capacity per EN 60898, making it a fit for general-purpose distribution boards feeding motor or lighting circuits where inrush isn't extreme. The fault-current sensing is Type A, so it catches pulsating DC residual currents from rectified loads like switched-mode supplies or variable-speed drives.
Ratings that matter for fit
The 40 A rating is at 30 °C ambient. The thermal derating curve is published: 37.6 A at 40 °C, 36.4 A at 45 °C, 34.8 A at 50 °C, 32.4 A at 55 °C (the upper operating limit), and 29.2 A at 70 °C. If the panel ambient runs hot — say a crowded enclosure with drives — the effective continuous current drops. The power dissipation is 15.6 W at rated load, which adds to the enclosure's internal heat rise; factor that into the thermal coordination for the distribution board.
Panel integration and wiring
Mounts on DIN rail, any position. The 2 MW width (36 mm) and 77 mm depth plus 70 mm installation depth mean it fits standard SENTRON distribution boards without crowding adjacent ways. Terminals accept solid or stranded conductors from 0.75 mm² up to 35 mm² — covers most submain and final-circuit sizes. Halogen-free and silicon-free construction matters for clean-room or high-reliability environments where outgassing or contamination is a concern. Supply can enter from top or bottom, which simplifies busbar routing.