What it is and where it fits
The Siemens 5SU9356-7KK20 is a SENTRON RCBO — a combined residual-current device and miniature circuit breaker in one 2-module-wide (36 mm) package. It protects a single-phase final subcircuit against overload, short-circuit, and earth leakage in one go, saving a DIN-rail slot over separate MCB + RCCD. Rated 20 A at 30 °C with a C tripping curve, it handles the inrush of motor starters and lighting ballasts common on a lube-pump or conveyor circuit. The 30 mA residual sensitivity (Type A) catches pulsating DC fault currents from electronics and VFDs — the sort of leakage you get from a variable-speed drive feeding a bearing-lube pump out here in the grease. Breaking capacity is 6 kA per EN 60898 and 10 kA per IEC 60947-2, so it clears faults up to that level without welding contacts. Overvoltage category III and pollution degree 2 suit it for fixed-installation distribution boards — the standard panel environment.
What the ratings mean for fit
The 20 A rating is at 30 °C ambient. At 40 °C it derates 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. If your panel runs hot near a drive cabinet or a motor junction box, size the load to the derated number — don't count on the full 20 A above 30 °C. The C-curve trips at 5 to 10 times rated current, which means it holds through the starting surge of a small motor or a bank of LED drivers without nuisance tripping. That's the right choice for a mixed load with some inductive kick. The design is instantaneous (no intentional delay), so it clears earth faults fast — important for personnel protection on a 30 mA RCD. The energy limitation class 3 keeps let-through energy low, protecting downstream wiring and connected gear.
Mounting and integration
Snaps onto a standard 35 mm DIN rail. Two modular width units (36 mm) — check your panel fill factor; it takes the space of two single-pole MCBs. Installation depth is 70 mm behind the panel face; overall depth is 77 mm including the front. Supply can come from either top or bottom, which saves rework when you're wiring a crowded gland plate. Mounting position is any — no derating for horizontal or inverted orientation. IP20 with the distribution board installed and conductors connected; that's the standard for enclosed panels, not for open washdown areas. Halogen-free and silicon-free construction matters if the panel is in a clean room or a food plant where outgassing can't happen.