The Siemens SENTRON 5SM3745-8 is a selective (time-delayed) residual current circuit breaker (RCCB) — 4-pole, rated 125 A at 400 V AC, 50 Hz, with a 500 mA residual trip threshold, Type A. Selective means it delays tripping to coordinate with downstream RCCBs, so a fault on a branch doesn't take out the whole submain. That 500 mA threshold is higher than the standard 30 mA or 300 mA, deliberately — it's sized for the main incomer or a feeder where nuisance tripping from leakage on multiple circuits needs to be avoided.
Breaking capacity and standards compliance
Short-circuit withstand is 10 kA per IEC 61008-1 (1.25 kA per EN 60898). The 1.25 kA figure under EN 60898 is the conditional rating for the RCCB when backed by an upstream MCB or fuse; the 10 kA under IEC 61008-1 is the unconditional rating for the RCCB alone. Either way, if your panel's fault current exceeds 10 kA, you need a current-limiting device ahead of it.
Temperature derating — the real usable current
Above that, derating kicks in: 120 A at 50 °C, 115 A at 55 °C, 110 A at 60 °C, 105 A at 65 °C, and 100 A at 70 °C. The permissible I²t is 94 000 A²·s; let-through current is 8 000 A — both figures matter for coordination with the downstream MCB.
Snap-on DIN rail (REG) mount, any orientation. That 77 mm depth means it needs clearance behind the DIN rail — standard 80 mm deep enclosures are tight; 100 mm or deeper is safer. IP20 with conductors connected — fine inside a distribution board, not for wet or dusty locations. Silicon-free construction, so it won't outgas contaminants in sensitive environments.