What it is and what it does
The Siemens 5SM2627-8 is a SENTRON residual current unit (RC unit) designed for use with the 5SP4 series. It's a 2-pole, type A, selective (time-delayed) device rated at 100 A with a 300 mA tripping fault current, for 230 V AC 50/60 Hz circuits. The 'selective' designation means it's built to coordinate with downstream RCDs — it holds during a fault long enough for the downstream device to clear, so only the faulted branch drops out, not the whole submain. That's the difference between a nuisance blackout and a targeted trip.
What the ratings mean for fit
Rated 100 A at 40 °C ambient, but the thermal derating curve is published: 93 A at 45 °C, 90 A at 50 °C, 87 A at 55 °C, 85 A at 60 °C, 83 A at 65 °C, and 80 A at 70 °C. If your panel runs hot — say a packed enclosure near a furnace line — the 80 A floor at 70 °C is the number to design to, not the 100 A headline. Insulation voltage is 460 V, overvoltage category III, so it's rated for fixed-installation mains distribution, not just downstream subcircuits. Mounts on DIN rail (REG), any position, 4 modular width units (approx 71 mm wide). Depth is 70 mm — that's the same as a standard MCB, so it'll sit flush in a shallow enclosure without a deep gland plate. IP20 with conductors connected, which is normal for a distribution board interior; not rated for wet or dusty environments.