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.
Occupies 4 width units (72 mm) on a DIN rail, with a depth of 77 mm and an installation depth of 70 mm.