Transformer Protection Circuit Breaker — 12.5 A, CLASS 10
The Siemens 3RV1421-1KA10 is a SIRIUS series circuit breaker specifically designed for transformer protection, rated at 12.5 A continuous current with a CLASS 10 trip characteristic. That CLASS 10 rating means it trips within 10 seconds at 7.2× rated current — fast enough to protect a transformer winding from sustained overcurrent without nuisance-tripping on the inrush that saturates the core on energisation. It carries a 100 kA breaking capacity at 400 V AC, so it can interrupt a bolted fault at the transformer secondary without the upstream feeder breaker needing to clear it first. Three-pole construction, screw-type terminals for the main and auxiliary circuits, and it fits a 45 mm wide DIN-rail footprint.
Breaking Capacity Across Voltages
The interrupting rating drops as system voltage rises — 100 kA at 240 V and 400 V AC, 42 kA at 500 V, and 6 kA at 690 V AC. That 6 kA at 690 V is the figure to check if you are feeding a 690 V transformer secondary; a 6 kA fault current is modest, but if the available fault current at the panel exceeds that, this breaker is not the right choice for that bus. The 100 kA at 400 V covers most low-voltage distribution in North American and European industrial plants.
Phase Failure Detection and Mounting
Built-in phase failure detection means the breaker will trip if one phase drops out — critical for a transformer that would otherwise single-phase and overheat the remaining windings. Mounting is screw and snap-on onto a 35 mm DIN rail per DIN EN 50022, with zero clearance required at the back or sides, so you can pack breakers tight in a panel. The front is IP20 finger-safe; the terminals are touch-protected as well. Shock-rated to 25g for 11 ms, which holds it on the rail through a vibration event in a substation or on a machine skid.