The Siemens 3RV1421-0JA10-ZW97 is a SIRIUS circuit breaker specifically designed for transformer protection — not a general motor-protective breaker. Its A-release (overload) is set at 0.7...1 A, while the N-release (instantaneous short-circuit trip) fires at 21 A, sized to handle transformer inrush without nuisance tripping on energization. This is a size S0 frame with screw terminals, standard switching capacity, and ships in a multi-unit pack of 14 pieces — a line-item quantity for panel builders who burn through these on transformer-fed sub-distribution boards.
No auxiliary contacts are built in (0 NO, 0 NC, 0 CO), so if you need status feedback you'll need the separate auxiliary switch block that clips onto the side. Power dissipation runs 7.25 W total at rated load in hot state, 2.4 W per pole. In a densely packed enclosure that heat adds up — account for it in the thermal budget if you're stacking several of these side-by-side.