What this part is and what it does
The Siemens 5SM3646-8LA is a SENTRON selective RCCB — residual current operated circuit breaker — built for 4-pole, 400 V AC, 50 Hz applications where you need fault discrimination downstream. It's Type A, so it catches pulsating DC fault currents on top of standard AC sine-wave leakage, and selective means it's time-delayed to coordinate with downstream RCCBs: a ground fault on a branch trips the branch device first, not this main isolator. Rated at 63 A continuous and 300 mA residual trip, it's sized for a submain or feeder in a commercial or industrial panel. (cites:,).
Mounting and integration
Snaps onto standard DIN rail (REG profile), occupies 4 width units (about 72 mm), and needs 70 mm installation depth behind the panel cover. Mounting position is any — vertical, horizontal, upside down — which helps when you're squeezing it into a crowded enclosure. Terminals accept 1.5 to 25 mm² solid or stranded copper, with a tightening torque of 2.5 to 3 N·m. That's a solid lug range: big enough for 25 mm² feeder tails, small enough for 1.5 mm² control wiring if you're looping through. (cites:,).
Where it lives in the panel
Rated IP20 with the distribution board installed and conductors connected — that's the standard for enclosed panel gear, not for open washdown areas. Finger and back-of-hand safe per the protection class, which is what you expect for a DIN-rail device behind a locked door. Ambient range is -25 to +45 °C operating, with a wider storage band of -40 to +75 °C for warehousing. Overvoltage category III means it's rated for fixed-installation mains-level transients. (cites:,).