What it is and what it does
The Siemens 5SU1354-1LB40 is a SENTRON RCBO — a combined residual-current circuit breaker and overcurrent miniature circuit breaker in one 2-module-wide (36 mm) package. It carries a 40 A rated current with a C tripping characteristic and a 10 kA breaking capacity per EN 60898. That 10 kA rating means it can safely interrupt a fault current up to 10,000 A at the rated voltage, which covers the vast majority of domestic and light commercial distribution boards. The 'C' curve means it trips between 5 and 10 times rated current — the standard choice for inductive loads like motors, pumps, and compressors where inrush is expected.
DIN-rail fit and panel integration
At 36 mm wide (2 width units) and 90 mm tall, this RCBO snaps onto a standard 35 mm DIN rail. The 77 mm depth plus 70 mm installation depth means it sits flush in a typical distribution board without protruding past the gland plate. Conductor range accepts 0.75 to 35 mm² solid or stranded — that's enough for a 40 A feed on the line side and daisy-chaining downstream loads. Mounting position is any, so no restriction on board orientation. IP20 when installed in a distribution board with conductors connected.
Thermal derating — what the numbers mean
The 40 A rating is at 30 °C ambient. At 40 °C it's 37.6 A; at 50 °C, 34.8 A; at 60 °C, 32.4 A; at 70 °C, 29.2 A. That's a roughly 27 % derating from 30 °C to 70 °C. If this RCBO sits in a crowded board next to other heat sources, the effective current capacity drops — plan the load accordingly. Power loss at rated current is 15.6 W, which adds to the thermal budget inside the enclosure.
Compliance and documentation
Rated per EN 60898 for the 10 kA breaking capacity. Overvoltage category III, energy limiting class 3. Halogen-free and silicon-free construction — relevant for clean-room or low-smoke installations. The design code is 5SU1; the function letter per DIN EN 61346-2 and DIN EN 81346-2 is F (protective device). Type of fault current is AC (sine-wave residual current detection).