What this MCB covers
The Siemens SENTRON 5SY4123-7XX02 is a 1-pole miniature circuit breaker rated 3.5 A at 400 V AC, with a C tripping characteristic — meaning it handles moderate inrush from motor starters and small transformers without nuisance tripping, while still protecting branch circuits against sustained overloads and short circuits. The 10 kA breaking capacity per EN 60898 covers standard industrial panel duty; the 35 kA rating per IEC 60947-2 gives headroom for higher-fault installations where the upstream transformer is close or the SCCR requirement is stiffer. The built-in auxiliary switch (1 NO + 1 NC) saves a DIN slot versus a separate accessory, which matters when you're packing 1-pole breakers into a tight sub-panel.
Panel integration details
Mounts on DIN rail via the quick assembly system — snaps on and off without tools. At 27 mm wide (1 width unit), it fits standard 18 mm pitch enclosures. The combined terminal at top and bottom accepts solid or stranded wire from 0.75 mm² up to 35 mm², with a tightening torque range of 2.5 to 3.5 N·m. Installation depth is 70 mm, overall depth 76 mm. Rated IP20 with connected conductors, so it's fine inside a closed panel but not for wet environments. Ambient temperature range -25 to +45 °C covers most indoor industrial conditions.
Approvals and compliance
Rated breaking capacity under UL 1077 and CSA C22.2 No. 235 is 5 kA, so it's accepted in North American supplementary protector applications. Overvoltage category III, energy limiting class 3. Halogen-free and silicon-free construction — relevant for environments where outgassing or contamination is a concern. Sealable, with touch protection on the terminals.