Transformer-protection motor starter protector
The Siemens 3RV1421-1EA10-ZX95 is a SIRIUS-brand circuit breaker specifically designed for transformer protection — not a general-purpose motor circuit breaker. Rated 4 A continuous at up to 690 V, it carries a 100 kA breaking capacity at 400 V AC — that's a serious fault-current rating for a 45 mm wide package, enough to handle most distribution-panel SCCR requirements without cascading upstream breakers.
The CLASS 10 curve is the headline here: it's steeper than a CLASS 20 motor-protector curve, so it trips faster on sustained overload. That makes it a match for transformer primaries where the inrush decays in a few cycles — but if you're tempted to swap it into a motor circuit with a long acceleration time, the nuisance-tripping risk goes up. Rated 100 kA at 240 V, 400 V, and 500 V AC, then drops to 6 kA at 690 V AC. That 690 V figure is the weak point: if your fault current at 690 V exceeds 6 kA, this breaker isn't your choice — you need a higher-rated unit or a current-limiting fuse upstream.
The terminals accept 2× solid or stranded conductors from 1 to 2.5 mm² and 2× from 2.5 to 6 mm² — plenty of capacity for the 4 A rating. At 45 mm wide × 97 mm high × 96 mm deep, it's a compact 3-pole unit. Zero clearance needed at the sides or back for cooling, so you can pack it tight on the rail. IP20 on the front with finger-safe touch protection — safe for open-panel installation where operators might brush against the front face. The auxiliary switch option is available, so you can wire a remote status signal back to the PLC.