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). That 10 kA rating is the fault current the device can interrupt without welding contacts or failing catastrophically — typical for a domestic or light commercial distribution board where the prospective fault current at the main switch is under 10 kA. 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
The 125 A rating holds at 40 °C and 45 °C. 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. If your panel ambient runs above 45 °C, size the upstream overcurrent device for the derated value, not the 125 A nameplate. The permissible I²t is 94 000 A²·s; let-through current is 8 000 A — both figures matter for coordination with the downstream MCB.
Mounting and panel fit
Snap-on DIN rail (REG) mount, any orientation. Occupies 4 modular width units (72 mm wide). Installation depth is 70 mm; overall depth is 77 mm. 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.