The Schneider Electric A9N21399 is a DT40 miniature circuit-breaker from the ProDis range, rated 32 A with 3 protected poles and a D tripping curve. The D curve means magnetic trip occurs at 10 to 14 times the rated current (10–14 x In), so it handles high inrush loads like motor starters and small transformers without nuisance tripping — that is the main reason to pick D over B or C in a distribution board feeding inductive equipment. Breaking capacity is 6000 A Icn at 400 V AC per EN/IEC 60898-1, and 10 kA Icu at 400–415 V AC per IEC 60947-2. The 78 mm depth and 81 mm height leave clearance for standard enclosure depths.
Clip-on DIN rail mount means it snaps directly into a 35 mm rail inside a distribution board or control panel. IP20 finger protection (IEC 60529) is standard for enclosed panels — no special sealing needed.