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 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.
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. Ambient temperature range -25 to +45 °C covers most indoor industrial conditions.
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. Sealable, with touch protection on the terminals.