What it is and what it does
The Siemens 5SM3745-0 is a 4-pole residual current circuit breaker (RCCB) from the SENTRON family, rated 125 A with a 500 mA trip sensitivity at 400 V AC, 50 Hz. It's an instantaneous (type AC) device — no intentional delay, so it trips on sinusoidal AC fault currents as soon as they exceed the threshold. The 10 kA short-circuit current rating (SCCR) means it can safely interrupt a fault up to that level without welding contacts or cascading upstream.
Where it goes and how it mounts
Snaps onto DIN rail (REG profile), 4 modular width units wide (72 mm). Mounting position is unrestricted — horizontal, vertical, upside-down, doesn't matter. Depth is 77 mm, installation depth 70 mm. The IP20 rating applies once the distribution board is installed with conductors connected; it's a panel-internal device, not a standalone outdoor unit.
What the ratings mean for your panel
The 500 mA sensitivity is a high-threshold trip — it's for equipment protection or circuits with high natural leakage (long cable runs, industrial heaters), not personnel protection (which typically uses 30 mA). The 125 A rating and 400 V AC make it a main-switch or feeder RCCB for a sub-distribution board. Power loss per pole at rated current is 8.9 W — four poles means about 35.6 W total heat to dissipate inside the enclosure; factor that into your thermal derating if the panel is densely packed.