What this RCCB does and where it fits
The Siemens 5SM3445-6 is a 4-pole residual current circuit breaker (RCCB) from the SENTRON family, rated 125 A at 50 Hz with a 100 mA Type A residual current trip. It is an instantaneous device — no short-time delay — so it trips as soon as the fault current exceeds the threshold. The 10 kA short-circuit current rating (SCCR) means it can safely interrupt a fault up to that level without upstream devices needing to clear it first, which simplifies coordination in a distribution board. Type A detection covers sinusoidal AC residual currents plus pulsating DC residual currents up to 6 mA smooth DC — the standard choice for circuits with single-phase rectifiers, switched-mode power supplies, or electronic loads that produce pulsed DC faults. The 100 mA trip threshold suits sub-main or feeder protection where nuisance tripping from leakage on long cable runs or multiple SMPS loads needs to be avoided. Mounts on DIN rail (REG) in any position, which gives panel builders flexibility in crowded enclosures. The 4-modular-width (72 mm) body and 77 mm depth fit standard distribution board cutouts. Supply can enter from top or bottom, so it works in both feed-through and reverse-fed configurations common in retrofit work.
Thermal derating — the real-world current limit
The 125 A rating holds at 40 °C and 45 °C ambient. Above that, the device derates linearly: 117.5 A at 50 °C, 110 A at 55 °C, 102.5 A at 60 °C, 95 A at 65 °C, and 87.5 A at 70 °C. If the panel ambient runs above 45 °C — common in sealed, sun-exposed, or high-density enclosures — the load must be capped at the derated figure. The storage range (-40 °C to 75 °C) is wider than the operating range (-25 °C to 45 °C) because storage limits govern handling, not running.
Standards and compliance
Rated breaking capacity per EN 60898 is 1.25 kA; per IEC 61008-1 it is also 1.25 kA. Overvoltage category III suits fixed installation downstream of the main distribution board. The device is silicon-free, which matters in potting or conformal-coating environments where silicone outgassing can cause contact or adhesion failures. IP20 applies when installed in a distribution board with conductors connected — the enclosure provides the ingress protection.