Siemens 5SM3745-6 SENTRON RCCB — 4-pole, 125 A, Type A, instantaneous
The Siemens 5SM3745-6 is a 4-pole SENTRON residual current circuit breaker (RCCB) rated for 125 A at 400 V AC, 50 Hz. It trips on a 500 mA residual current and is designed as an instantaneous (short-time-delayed: No) Type A device, meaning it detects sinusoidal AC and pulsating DC fault currents — the standard for mixed-load panels with electronics, VFDs, or rectified supplies. The 4-module-width body (72 mm) snaps onto DIN rail (REG) and can be mounted in any orientation. IP20 with conductors connected — suitable for enclosed distribution boards, not for wet-area standalone use.
Rated current and thermal derating
Full 125 A rating holds up to 45 °C ambient. Above that, derate linearly: 120 A at 50 °C, 115 A at 55 °C, 110 A at 60 °C, 105 A at 65 °C, 100 A at 70 °C. If the panel ambient runs near 50 °C, the 125 A frame still carries 120 A continuous — no need to oversize for a warm cabinet unless you exceed 70 °C.
Fault current withstand and selectivity
Short-circuit current rating (SCCR) is 10 kA per IEC 61008-1, with a let-through current of 8 000 A and a permissible I²t of 94 000 A²·s. For coordination with an upstream MCB, the conditional rating per EN 60898 is 1.25 kA — this is the value to use when the RCCB is backed by a 10 kA MCB in a standard final-distribution board. Surge-current resistance is 1 kA (8/20 µs), so it holds against lightning-induced transients without nuisance tripping.