What it is and what it does
The Siemens SENTRON 5SM3742-0 is a 4-pole residual current circuit breaker (RCCB) rated for 25 A at 400 V AC, with a 500 mA trip threshold for AC fault currents. It's an instantaneous type — no intentional delay — so it trips as soon as the residual current exceeds that threshold, which matters for shock protection in circuits where you need fast disconnection. Rated short-circuit current withstand is 10 kA, and the I²t value is 70 000 A²·s, meaning it can handle a fault up to that level without rupturing. That 10 kA SCCR is typical for a distribution board downstream of a larger upstream breaker. Mounts on DIN rail (REG) and fits in any position — top or bottom feed — with an installation depth of 70 mm and overall depth of 77 mm. IP20 rated once installed in a distribution board with conductors connected.
Temperature derating and conductor sizing
The 25 A rating holds at 40 °C and 45 °C, then derates linearly: 24 A at 50 °C, 23 A at 55 °C, down to 20 A at 70 °C. If the panel runs hot, factor that in — at 60 °C you lose 3 A off the headline number. Terminal capacity accepts 1.5 mm² minimum up to 25 mm² — solid or stranded. That covers most distribution wiring up to 6 AWG. Silicon-free construction, so it's safe for use in environments sensitive to silicone outgassing.