Transformer Protection Circuit Breaker, 1.25 A, Class 10
The Siemens SIRIUS 3RV1421-0KA10-ZX95 is a 3-pole circuit breaker designed specifically for transformer protection, rated at 1.25 A continuous current with a Class 10 trip characteristic. This means it coordinates with transformer inrush — the magnetic trip element allows the brief high magnetizing current on energization while the thermal bimetal protects against sustained overloads. That figure holds across the full voltage range up to 690 V AC, so this breaker can be placed upstream in a high-fault-current sub-distribution without worrying about cascading failure. Zero clearance needed at the sides or rear — 0 mm backwards and 0 mm at the side — so it packs tight in a crowded enclosure. The IP20 finger-safe front protection means you can reach into the panel without de-energizing the bus for routine visual checks.
Main circuit terminals are screw-type, accepting 2x (1... 2.5 mm²) solid or 2x (2.5... 6 mm²) solid, and the same ranges for stranded wire. For AWG, that translates to 2x (14... 10) on the main contacts. The auxiliary and control circuit uses the same screw-terminal style, so you don't need a second tool or separate terminal block for the aux switch wiring. Power dissipation is 7.25 W total in hot operating state, or 2.4 W per pole. That's low enough to skip forced ventilation in a standard IP20 enclosure as long as the ambient stays under 60 °C.