What this RCBO is and what it does
The Siemens 5SU1353-1KK08 is a SENTRON RCBO — a combined residual-current and overcurrent protective device — rated 8 A with a C-curve tripping characteristic. It protects a single-phase final subcircuit (1P+N, 2 MW wide) against overload, short-circuit, and earth leakage. The C-curve means it holds up to 5–10× rated current momentarily, so it is the standard choice for inductive loads like small motors, contactor coils, and fluorescent lighting where inrush exceeds a B-curve's tolerance. Breaking capacity is rated 4.5 kA per EN 60898 and 15 kA per IEC 60947-2. The higher IEC figure applies when the device is used as a circuit breaker in an industrial distribution board; the EN figure governs domestic/commercial installations. Either way, the 15 kA SCCR gives headroom for most panelboard fault levels without upstream fusing. The residual-current element detects AC fault currents only (Type AC). That covers sinusoidal earth leakage from live-to-ground faults in standard resistive and inductive loads. For pulsed DC or smooth DC fault currents — common on VFD-fed circuits, EV chargers, or solar inverters — you would need a Type A or Type B RCBO instead.
Thermal derating — what the numbers mean for panel loading
The 8 A rating holds at 30 °C ambient. Above that, the bimetallic strip trips earlier to protect the device and wiring. At 40 °C it is 7.52 A; at 50 °C it drops to 6.96 A; at 60 °C it is 6.48 A; at 70 °C it is 5.84 A. If the panel ambient runs hot — say 50 °C inside a sealed enclosure — the effective continuous load limit is 6.96 A, not 8 A. Factor this into the load schedule before committing the BOM line.
Mounting and integration
The 5SU1353-1KK08 snaps onto a 35 mm DIN rail per EN 60715. Width is 36 mm (2 MW), depth 77 mm, height 90 mm. Installation depth behind the panel is 70 mm. Mounting position is any orientation — no derating needed for horizontal or inverted fit. Supply can enter from top or bottom, which simplifies busbar routing in a crowded enclosure. Terminals accept 0.75 mm² to 25 mm² — solid or stranded. The larger cross-section is useful for looping through downstream devices without a separate distribution block. IP20 rating applies when installed in a distribution board with conductors connected; the device itself is not sealed against dust ingress beyond finger-safe touch protection.