The Siemens 3RV2421-1FA10 is a SIRIUS-brand circuit breaker explicitly designed for transformer protection — not a general-purpose motor protective breaker, but one whose trip curve and breaking capacity are matched to the inrush and fault characteristics of a transformer primary. That distinction matters when you're coordinating a panel: a standard motor-protective breaker might nuisance-trip on transformer magnetizing inrush, whereas this unit's design accounts for that transient. The fuse coordination values are given as gL/gG 32 A at 400 V and 500 V, stepping down to 25 A at 690 V; these are the maximum upstream fuse ratings that preserve selectivity.
The 45 mm width occupies two 22.5 mm module spaces on the DIN rail, so plan your rail layout accordingly. Depth is 97 mm, height 97 mm; the unit sits flush with no forward projection beyond the front face (: 0 mm forwards). Clearance requirements: 50 mm upwards, 50 mm downwards, 30 mm at the side — these are the minimum free air gaps for arc venting and heat dissipation.
The main contacts use M4 screw terminals, accepting 2×(1–2.5 mm²) solid or stranded, or 2×(2.5–10 mm²) — typical for a transformer primary feed up to around 7.5 kVA at 400 V. Maximum switching frequency in AC-3 duty is 15 operations per hour, which is adequate for transformer switching but not for frequent motor starting cycles.