Siemens 5SY8403-8 — 4-pole D-curve MCB, 3 A, 50 kA
It is designed for 400 V AC systems and carries a 50 kA breaking capacity per IEC 60947-2, with 15 kA at DC per the same standard and 5 kA per UL 1077 / CSA C22.2 No.235. The D-curve (10–20× In) suits it for motor and transformer branch circuits where inrush currents are high — the 3 A rating means it holds through the startup surge of small inductive loads without nuisance tripping. The 4-pole, 4-width-unit footprint occupies four 18 mm modules on the rail. Terminals accept solid or stranded conductors from 0.75 to 35 mm², with a tightening torque of 2.5 to 3.5 N·m via screw-type terminals. Combined top and bottom terminals simplify busbar and daisy-chain wiring in the panel.
Humidity tolerance is specified: max 95% at 55 °C, max 55% at 70 °C, max 35% at 75 °C. Overvoltage category III. Sealable design supports lockout/tagout procedures. Touch protection is provided on the front face.