What it is and what it does
The Siemens 3RV1421-4AA10 is a SIRIUS circuit breaker designed specifically for transformer protection — not a general-purpose motor-protective breaker, but one tuned for the inrush and overload profile of a transformer primary. It carries a continuous current rating of 16 A and a rated insulation voltage of 690 V. The CLASS 10 trip characteristic means it will open within 10 seconds at 7.2× rated current, which is fast enough to protect a transformer winding from sustained overload without nuisance-tripping on magnetizing inrush.
Mounting and wiring
Snaps onto 35 mm DIN rail per DIN EN 50022 — screw and snap-on, so it stays put under vibration. The 45 mm width eats one standard module slot. Screw-type terminals on both main and auxiliary circuits; accepts solid or stranded wire up to 6 mm², with two conductors per clamp. No backwards clearance needed, so you can butt it against the left side of the enclosure. Mount in any position.
Protection and diagnostics
Phase failure detection is built in — if one phase drops out, the breaker trips. No ground-fault detection, so that needs a separate module if required. The front is IP20 finger-safe. Surge voltage resistance is 6 kV. Power dissipation runs 9.25 W total, 3.1 W per pole.