What this RCCB does and where it fits
The Siemens 5SM3742-6 is a 4-pole residual current circuit breaker (RCCB) from the SENTRON 5SM3 series, designed for DIN-rail mounting in distribution boards. It protects against earth faults by detecting the imbalance between live and neutral currents — the Type A designation means it catches pulsating DC fault currents as well as sinusoidal AC, which is the standard for modern electronics-fed loads like VFDs, switched-mode power supplies, and LED drivers. Rated 25 A at 400 V AC, 50 Hz, with a short-circuit current rating of 10 kA, this unit handles the typical residential and light commercial supply. The 4-pole construction (3 phases + neutral) suits three-phase panelboards where all live conductors must be disconnected on a fault.
Thermal derating and conductor sizing
The 25 A rating holds up to 45 °C ambient; above that it derates linearly to 20 A at 70 °C. If your panel runs hot — near a transformer or in a non-ventilated enclosure — check the derating curve: at 55 °C you're down to 23 A, at 65 °C to 21 A. The terminal range accepts 1.5 to 25 mm² solid or stranded copper, which covers the full range of conductors for a 25 A circuit.
Physical integration notes
Occupies 4 width units (72 mm) on a DIN rail, with a depth of 77 mm and an installation depth of 70 mm. The IP20 rating applies once installed in a distribution board with conductors connected — it's not a standalone outdoor device. Mounting position is any, which simplifies layout in tight enclosures. The supply can enter from top or bottom, so no need to flip the rail for cable routing.