Transformer-protection circuit breaker, 1.25 A, CLASS 10
The Siemens 3RV1421-0KA15 is a SIRIUS brand circuit breaker specifically designed for transformer protection, carrying a continuous current rating of 1.25 A and a CLASS 10 trip characteristic. This means it handles the inrush of a transformer without nuisance tripping — CLASS 10 allows up to 10 seconds at 7.2x rated current, which covers magnetizing inrush on small to medium transformers. The 100 kA breaking capacity holds at 240 V, 400 V, 500 V, and 690 V AC, so this single part covers the full low-voltage range without derating on fault clearing. Phase failure detection is built in — if one line drops, the breaker trips, protecting the transformer from single-phasing. The auxiliary switch is a transverse design and the product extension allows adding auxiliary contacts later without replacing the entire unit.
Mounting and wiring — 45 mm wide, screw terminals, DIN rail
Mounts via screw and snap-on onto 35 mm standard mounting rail per DIN EN 50022, in any position. The 45 mm width matches the standard SIRIUS 3RV1 footprint, so it fits alongside contactors and overloads in a motor control center without extra spacing — 0 mm clearance required at the sides or backwards. Screw-type terminals on both the main and auxiliary/control circuits. Main circuit accepts 2x (0.5 to 1.5 mm²) solid or 2x (1 to 2.5 mm²) stranded, and 2x (0.75 to 2.5 mm²) solid or 2x (2.5 to 6 mm²) stranded — covers most panel wiring up to 6 mm². AWG equivalents are 2x (14 to 10) for the main contacts. Front protection is IP20, safe for finger-probe access inside an enclosure.
Environmental limits and power dissipation
Operates from -20 °C to +60 °C, stores and transports from -50 °C to +80 °C. Shock resistance is 25g for 11 ms. Power dissipation in hot operating state under AC conditions is 7.25 W — factor that into enclosure thermal calculations if mounting multiple units in a sealed panel.