The ABB S203-D50 is a 50 A, 3-pole miniature circuit breaker with a Trip Curve D characteristic, rated for 480Y/277 VAC systems. It mounts on standard DIN rail and carries a 10 kA interrupting rating at 277 VAC.
Trip Curve D means the magnetic trip threshold is set at 10–20× rated current (500–1000 A for this 50 A breaker). That's deliberately high — it's meant for loads with a large inrush, like transformers, welding equipment, or motor starters with high magnetizing current. A standard Curve C would nuisance-trip on those starts; Curve D holds through them. On the DIN rail, it occupies three module widths (about 54 mm) and clips onto the same profile as the rest of the S200 series, so it rows neatly with adjacent breakers and accessories.