SENTRON 5SY4123-7 — 1-pole C-curve MCB, 3.5 A
The Siemens SENTRON 5SY4123-7 is a 1-pole miniature circuit breaker rated 3.5 A at AC with a C-curve tripping characteristic, designed for branch-circuit protection in distribution boards and control panels. Its 18 mm width (1 modular unit) snaps onto standard DIN rail, and the 76 mm depth leaves clearance for wiring in shallow enclosures. Breaking capacity is 10 kA per IEC/EN 60898-1 for domestic-type coordination, and 35 kA per IEC 60947-2 / UL1077 for industrial panel applications — the higher figure matters when the prospective fault current at the panel exceeds 10 kA, which is common in transformer-fed switchgear. The C-curve (magnetic trip between 5× and 10× In) suits moderate inrush loads like small motor starters, contactor coils, and groups of LED drivers — it avoids nuisance trips on capacitor charging or transformer magnetizing inrush that a B-curve would see as a fault.
Thermal derating and ambient conditions
At 35 °C ambient the breaker carries its full 3.5 A; at 50 °C it derates to 3.06 A, and at 60 °C to 2.81 A. If the panel runs hot — say, a packed enclosure with drives — the derated value governs the load you can assign to this circuit. The operating range spans -40 °C to 75 °C, and the IP20 rating (with conductors connected) is standard for dry indoor panel environments.