What it is and what the ratings mean
The Siemens 5SL6420-8CC is a 4-pole miniature circuit breaker (MCB) rated 400 V with a 6 kA breaking capacity, a D-trip curve, and a 20 A current rating — a standard DIN-rail mount device for protecting branch circuits in commercial and light industrial panels. The D-curve means it trips magnetically between 10 and 20 times rated current (200–400 A for this 20 A unit), making it the right choice for loads with high inrush like small motors, transformers, or solenoid banks — not for general lighting or resistive loads where a B or C curve would do. The 6 kA breaking capacity at 400 V tells you it can safely interrupt a fault current up to 6,000 A without welding its contacts or failing open — adequate for most sub-distribution boards fed from a transformer or a larger upstream breaker, but check your available fault current at the panel if you're close to the service entrance.
Lifecycle and sourcing reality
RoHS compliance is confirmed since June 2013, and REACH candidate-list information is available on request — both matter if your quality or compliance team needs documentation for the file. The product class is B: return restricted.
Where it fits in the panel
Snaps onto standard 35 mm DIN rail in a 4-module-wide footprint (each pole is one module at 18 mm). Leave a finger's gap above and below for the arc-chamber venting — don't bury it against the gland plate or a solid cable duct. Rated for 400 V line-to-line, so it covers 3-phase 4-wire systems (three phases plus neutral) up to 400/230 V or 400/277 V configurations. The D-curve handles the inrush of a 5–7.5 kW motor starter or a small control transformer on the same branch.