Transformer-protection breaker, 12.5 A, CLASS 10 — what you're getting
The Siemens 3RV1421-1KA15 is a SIRIUS-brand circuit breaker designed specifically for transformer protection — that means its trip curve is shaped to handle the inrush of an unloaded transformer without nuisance tripping, while still clearing a fault. Rated 12.5 A continuous current with a CLASS 10 trip class, it's meant for the primary side of small to medium control transformers up to that current draw. Mounts on a 35 mm DIN rail per DIN EN 50022 — screw or snap-on, any position. At 45 mm wide and 97 mm tall, it fits a standard MCCB or motor-protective breaker footprint in the panel. Depth is 96 mm, so factor that into your gland-plate clearance if the breaker sits near the enclosure back wall.
SCCR and interrupting capacity — the real numbers for your panel label
This breaker carries a 100 kA short-circuit current rating at 240 V and 400 V AC, dropping to 42 kA at 500 V and 6 kA at 690 V. That 100 kA at 400 V is the figure you'll use for most industrial panel SCCR calculations — it means you can install this breaker downstream of a high-capacity transformer or bus without needing a current-limiting fuse ahead of it, as long as the available fault current stays under that mark. Rated operational voltage tops out at 690 V AC, with a maximum operating altitude of 2 000 m. Ambient operating range is -20 to +60 °C, storage and transport from -50 to +80 °C. Shock-rated to 25g for 11 ms — it'll hold up on a machine that sees occasional jolts.
Terminals and wiring
Main current circuit uses screw-type terminals. Solid wire accepts 2x (0.5 to 1.5 mm²) or 2x (0.75 to 2.5 mm²); stranded takes 2x (1 to 2.5 mm²) or 2x (2.5 to 6 mm²). That covers standard control-panel wiring from 18 AWG up to about 10 AWG. The auxiliary switch is a transverse design — included, with zero dedicated auxiliary contacts on the main block, so the internal switch handles signaling.