What it is and what it does
The Siemens 5SU1356-7KK20 is a SENTRON RCBO — a combined residual-current circuit breaker with overcurrent protection — in a 1P+N, 2-module-width (36 mm) package. It protects a single-phase final subcircuit against earth leakage and overload/short-circuit in one device, saving a DIN-rail position compared to separate RCCB and MCB. Rated 20 A at 30 °C ambient with a Type C tripping characteristic (magnetic release at 5–10× In), it handles moderate inrush from motorized loads like small pumps or compressors. The breaking capacity is 6 kA per EN 60898 for domestic/commercial panels and 15 kA per IEC 60947-2 for industrial distribution boards — the higher figure gives headroom for higher-fault installations.
Derating and installation constraints
The 20 A rating derates with rising ambient temperature: 18.8 A at 40 °C, 17.4 A at 50 °C, 15.4 A at 65 °C, and 14.6 A at 70 °C. In a warm enclosure, size the upstream protection and load accordingly — the 70 °C figure is the practical limit for a crowded panel. Mounting position is any, and supply can enter from top or bottom, which simplifies panel layout. The 77 mm depth (70 mm installation depth) fits standard distribution boards; the IP20 rating applies once the board is closed with conductors connected.
Compliance and documentation
The 5SU1356-7KK20 is halogen-free and silicon-free, which matters for clean-room or sensitive electronic environments where outgassing can cause contact corrosion. Overvoltage category III and pollution degree 2 suit fixed-installation distribution boards. The energy limitation class is 3, meaning it limits let-through energy under fault conditions.