The Siemens 5SM1646-0 is a SENTRON 5SM1 series residual current circuit breaker (RCCB), 4-pole, type AC, rated 63 A at 400 V AC with a 300 mA residual trip threshold. It's an instantaneous device — no intentional delay — so it clears ground faults as fast as the waveform allows, which matters for personnel protection in circuits feeding portable tools or wet-area outlets where you need fast interruption.
Mounts on DIN rail (REG), any orientation, with a 55 mm installation depth and 4 width units. That's a standard 4-module footprint — drops into any SENTRON distribution board or generic enclosure that accepts DIN-rail devices. IP20 with conductors connected and the distribution board installed; finger and back-of-hand safe per the protection class.
For a 63 A RCCB in a typical sub-distribution board, 10 kA is adequate for most industrial and commercial installations where the upstream breaker or fuse limits prospective fault current.
Thermal derating is published: holds 63 A up to 45 °C ambient, then steps down to 60.48 A at 50 °C, 57.96 A at 55 °C, 55.44 A at 60 °C, 52.92 A at 65 °C, and 50.4 A at 70 °C. Mechanical endurance is 10,000 switching cycles typical — not a daily-switch device, but fine for backup isolation and infrequent manual testing.