The Siemens 5SU1353-7KK40 is a SENTRON RCBO — a combined residual-current circuit breaker and miniature circuit breaker in a single 2-module-wide (36 mm) package. It protects a 1P+N circuit against overload, short-circuit, and earth leakage in one device, saving a DIN-rail position compared to separate RCD and MCB units. Rated 40 A with a C tripping characteristic, it handles the inrush of motor starters, banks of LED drivers, or small transformer loads without nuisance tripping. The 30 mA Type A residual-current element catches both sinusoidal AC faults and pulsating DC fault currents — the standard for circuits supplying single-phase electronics, VFDs, or EV chargers. Breaking capacity is 4.5 kA per EN 60898, adequate for most sub-distribution boards in commercial and light industrial installations where the prospective short-circuit current at the board is known to stay under that threshold. The 77 mm depth fits standard consumer units and distribution boards with a 70 mm installation depth.
Current derating and ambient temperature
The 40 A rating holds at 30 °C ambient. At 40 °C it derates to 37.6 A, at 50 °C to 34.8 A, and at 60 °C to 32.4 A. At 70 °C the continuous rating drops to 29.2 A. If the board runs warm — say, a packed enclosure near a boiler or in a plant air handling unit — use the 50 °C or 60 °C column to size the load. The device is rated for ambient down to -40 °C and up to 75 °C, covering outdoor cabinets in cold climates and hot machine compartments.
Mounting and integration
Snaps onto 35 mm DIN rail (EN 60715). Two modular width units (36 mm) — check the board's spare way count. Supply can enter from top or bottom; the neutral is switched internally (1P+N). Pollution degree 2, overvoltage category III, so it's rated for fixed-installation distribution boards in residential, commercial, and industrial buildings. IP20 with the distribution board installed and conductors connected.