What this RCBO is and where it fits
The Siemens 5SU1356-1KK20 is a SENTRON RCBO — a combined residual-current and miniature circuit breaker in one 2-module-width (36 mm) package. It protects a single-phase final subcircuit against overload, short-circuit, and earth leakage. The 1P+N construction switches the line conductor and breaks the neutral, with one protected pole. This is the instantaneous (non-time-delayed) version, so it trips on residual current without intentional delay — standard for general-purpose socket and lighting circuits where no upstream selectivity is needed. Rated 20 A at 30 °C with a C-curve magnetic trip (5–10× In), it handles the inrush of small motors, transformers, and fluorescent lighting banks common in commercial and light-industrial panels. The breaking capacity is 6 kA per EN 60898 for domestic-type coordination and 15 kA per IEC 60947-2 for industrial installation — meaning it can clear a fault up to 15 kA at the supply terminals without welding its contacts. That dual rating gives the specifying engineer flexibility: the 6 kA figure satisfies most residential and commercial boards; the 15 kA figure covers industrial distribution where fault current is higher.
Temperature derating — the real current you get
The 20 A rating is only valid at 30 °C ambient inside the panel. At 40 °C it derates to 18.8 A; at 50 °C to 17.4 A; at 60 °C to 16.2 A; and at 70 °C to 14.6 A. If the distribution board runs hot — say, a packed enclosure with multiple RCBOs side by side — the actual continuous current the device can carry without nuisance tripping is lower than the label says. The derating curve is published; factor it into the load calculation before committing the BOM line.
Mounting and integration
Snaps onto standard 35 mm DIN rail. Two modular width units (36 mm wide) — occupies the same footprint as a 2-pole MCB. Depth is 77 mm, installation depth 70 mm. Supply can enter from top or bottom; no preferred orientation. Mounting position is any, so it works in horizontal or vertical busbar arrangements. IP20 only when installed in a distribution board with conductors connected — it is not a standalone outdoor device.