The Siemens SENTRON 5SY4120-5 is a 1-pole miniature circuit breaker rated 20 A at AC with tripping characteristic A, designed for mechanical engineering and industrial panel applications. It breaks 10 kA per EN 60898 and 20 kA per IEC 60947-2, with a DC breaking capacity of 15 kA per IEC 60947-2. Overvoltage category III and pollution degree 3 suit it for fixed-installation distribution boards where fault currents can reach those levels without upstream cascading.
What the ratings mean for fit
Characteristic A trips between 2 and 3 times rated current — faster than B or C curves — making it the right choice for protecting semiconductor loads, control transformers, or long cable runs where inrush is minimal and you need tight overcurrent protection. At 20 A it covers a 4 kW resistive load on a 230 V single-phase line. The 10 kA breaking capacity per EN 60898 is the standard for domestic/commercial final circuits; the 20 kA per IEC 60947-2 applies when the breaker is used in industrial distribution where prospective fault current is higher. The 15 kA DC rating per IEC 60947-2 covers DC control circuits up to that fault level — verify polarity and arc-chamber orientation for DC use. Temperature derating is built in: at 40 °C ambient the breaker carries 18.78 A, at 50 °C it drops to 17.48 A, and at 60 °C to 16.08 A. If your panel runs hot — say next to a drive or transformer — size the load to the derated figure, not the 20 A nameplate. The 18 mm width (1 modular unit) fits standard DIN-rail enclosures. Depth of 76 mm and installation depth of 70 mm mean it clears most shallow gland plates. IP20 with connected conductors — protected against finger contact but not washdown; keep it inside the enclosure.