What this RCBO is and what the ratings mean
The Siemens 5SU1644-1RC40 is a 40 A, 4-pole RCBO from the SENTRON family — a combined overcurrent and residual-current device that protects a three-phase circuit against overload, short-circuit, and earth leakage in one 7-module-wide unit. The C characteristic means the magnetic trip fires at 5 to 10 times rated current (200–400 A), so it handles motor inrush and transformer energisation without nuisance tripping. The Type AC 300 mA residual-current element detects sinusoidal AC fault currents down to 300 mA, which covers basic personnel protection and fire prevention on circuits without non-sinusoidal loads. Breaking capacity is 10 kA per EN 60898 and 15 kA per IEC 60947-2 — the higher figure under the industrial standard means it can interrupt a 15 kA prospective fault current on a 415 V AC supply without welding the contacts or cascading damage upstream. That 15 kA rating is the one to use for selectivity studies in an industrial panel. Rated insulation voltage (Ui) is 440 V, overvoltage category III, pollution degree 2 — standard for fixed-installation distribution boards in commercial and light-industrial buildings. Operating frequency is 50 Hz only; this part is not rated for 60 Hz networks.
Panel fit and integration
The 5SU1644-1RC40 snaps onto a DIN rail (EN 60715) and occupies 7 modular width units (MWU) — 126 mm of rail space. Depth is 70 mm behind the panel face, so it fits standard 80 mm deep enclosures. Supply can enter from top or bottom; mounting position is any orientation. IP20 protection applies only when the distribution board is installed and conductors are connected — the device itself is not sealed against dust or moisture ingress. Ambient temperature range is -40 °C to +75 °C, with max 95% humidity.