What it is and what it does
The Siemens 5SU9156-7VK20 is a SENTRON RCBO — an RCD-operated circuit breaker that combines overcurrent protection and residual current detection in a single 2-module (36 mm wide) unit. It is rated 20 A with a C tripping characteristic (magnetic release at 5–10× In, standard for motor and lighting circuits with moderate inrush) and a Type A fault-current sensor that detects pulsating DC residual currents as well as sinusoidal AC. Breaking capacity is 6 kA per EN 60898 for domestic-type installations and 10 kA per IEC 60947-2 for industrial panel duty.
Derating that matters for panel builders
The 5SU9156-7VK20 carries a full derating table from 30 °C to 55 °C: 20 A at 30 °C, 18.8 A at 40 °C, 18.2 A at 45 °C, 17.4 A at 50 °C, and 16.2 A at 55 °C. If the distribution board ambient sits at 45 °C — common in a sealed enclosure — the usable continuous current drops to 18.2 A, so a 20 A load would nuisance-trip on a warm day. Plan the branch circuit load at the derated figure, not the label.
Terminal capacity and wiring
Solid and stranded conductors from 0.75 mm² up to 35 mm² are accepted. That 35 mm² upper end is unusual for a 20 A RCBO — it means the terminals are oversized for the rating, making daisy-chaining or looping through larger feed conductors straightforward. Strip length and torque values follow the standard SENTRON terminal spec; no special crimp lugs needed for the full range.
Mounting and clearance notes
Mounting position is any orientation, which simplifies layout in a crowded panel. Installation depth is 70 mm; overall depth including the front projection is 77 mm. The unit occupies 2 width units (36 mm) on a DIN rail. IP20 applies when the distribution board is installed with conductors connected — exposed terminals are touch-protected per the product feature listing.
Environmental and compliance
Halogen-free and silicon-free construction, which matters for installations with sensitive electronics or where low-smoke, zero-halogen cabling is specified. Storage temperature range is -40 to +75 °C; operating ambient is -25 to +45 °C. Overvoltage category III (fixed installation, downstream of the main distribution board). Energy limiting class 3, meaning it limits let-through energy well in a fault condition.