What it is and what it does
The Siemens 5SM2625-8 is a 2-pole residual current unit (RC unit) from the SENTRON family, designed as a selective (time-delayed) companion for a 5SY miniature circuit breaker. It combines overcurrent and residual current protection in a compact, DIN-rail-mounted assembly rated at 63 A with a 300 mA tripping fault current, Type A sensitivity, for 230 V AC 50 Hz circuits. Type A residual current detection means it responds to sinusoidal AC fault currents and also to pulsating DC fault currents — the kind produced by rectifiers, switching power supplies, or variable-speed drives downstream. Selective (short-time delayed) design lets it ride through transient earth faults and coordinate with downstream RCDs, so only the faulted branch trips and the rest of the distribution board stays live.
Key ratings and what they mean for fit
The headline 63 A rating (at AC) is the continuous current the RC unit can carry in a 40 °C ambient. At higher enclosure temperatures the unit derates: 58.6 A at 40 °C, 58.59 A at 45 °C, 56.7 A at 50 °C, 54.81 A at 55 °C, 53.55 A at 60 °C, 52.29 A at 65 °C, and 50.4 A at 70 °C. If your panel runs hot, size the upstream breaker to the derated figure, not the 63 A label. The 300 mA rated tripping fault current is a high-threshold setting, typical for selective (S-type) RCDs. It avoids nuisance tripping from leakage currents in long cable runs or equipment with filters, while still providing earth-fault protection. The surge current resistance of 5 kA means it withstands lightning-induced or switching transients without damage. Rated insulation voltage is 460 V, overvoltage category III, so it's suitable for fixed-installation distribution boards fed from the mains supply. The 2-pole form factor occupies 2 modular width units (MW) per DIN 43880, which is 36 mm on a standard DIN rail. Depth is 70 mm, matching the 5SY MCB it couples with.
Mounting and integration
Snaps onto DIN rail (REG) in any mounting position. The RC unit is designed as a companion for the 5SY series MCB — it clips alongside or feeds from the same busbar. Supply can enter from top or bottom. IP20 with connected conductors in an installed distribution board; the enclosure itself provides the finger-safe coverage. Mechanical service life is 10 000 operating cycles typical. Silicon-free construction (product feature) matters for applications where silicone outgassing can contaminate contacts or optics — automotive paint shops or clean-room adjacent panels, for instance.