The Siemens 5SM2642-0 is a SENTRON residual-current device (RCD-unit) — a 4-pole, instantaneous-trip unit rated 40 A at 50 Hz AC, with a 300 mA fault-current threshold. It's designed for AC residual-current type (sinusoidal AC only), overvoltage category III, pollution degree 2, and carries a 460 V insulation voltage with 4 kV surge withstand.
The 40 A nominal rating drops as ambient temperature rises: 37.2 A at 40 °C and 45 °C, 36 A at 50 °C, 34.8 A at 55 °C, 34 A at 60 °C, 33.2 A at 65 °C, and 32 A at 70 °C. The unit mounts in any position, fastens via REG (regular) method, and takes 3 modular width units (roughly 52.5 mm) on the DIN rail. Depth is 77 mm, width 124 mm, height 90 mm, installation depth 70 mm. Supply can enter top or bottom.