The Siemens 5SU1353-7KL40 is a SENTRON 5SU1 RCBO (residual current circuit breaker with overcurrent protection) combining a 40 A rated current, type C tripping characteristic, and a 4.5 kA breaking capacity per EN 60898 into a single 1P+N unit. It detects type A fault currents — pulsating DC superimposed on AC — which covers most modern electronic loads like switched-mode power supplies and variable-speed drives.
Ratings and what they mean for the panel
The 40 A rating is the continuous current at 30 °C ambient. Above that, the device derates: 37.6 A at 40 °C, 36.4 A at 45 °C, 34.8 A at 50 °C, 32.4 A at 55 °C, 32.4 A at 60 °C, 30.8 A at 65 °C, and 29.2 A at 70 °C. If the panel ambient runs at 50 °C, the effective limit is 34.8 A — not 40 A. The type C magnetic trip (5–10× In) handles moderate inrush from motor starters and transformers without nuisance tripping, while the 4.5 kA SCCR at 230/400 V AC is adequate for most final subcircuits in a building distribution board. Overvoltage category III (distribution-level) and energy limiting class 3 mean it limits let-through energy enough to protect downstream wiring and loads under short-circuit conditions. The test button requires a minimum of 195 V to operate correctly — verify line voltage before commissioning.
Mounting and wiring
The 36 mm wide (2 width units) housing snaps onto a standard DIN rail. Installation depth is 70 mm; overall depth is 77 mm. Accepts solid or stranded conductors from 0.75 mm² up to 35 mm² — enough for a 40 A circuit on 6–10 mm² copper. Supply can enter from either top or bottom, which simplifies busbar routing in a crowded enclosure. IP20 rated once the distribution board cover is installed and conductors are connected.