Transformer-protection circuit breaker from the SIRIUS family
The Siemens 3RV2421-4AA15 is a SIRIUS-brand circuit breaker specifically designed for transformer protection, not general motor or feeder duty. It carries a Trip Class 10 rating and includes phase failure detection, which means it will trip on loss of a phase — a common failure mode in transformer-fed loads that can cause single-phasing damage downstream. Breaking capacity is 25 kA at 400 V, with a rated operational voltage range of 20 to 690 V.
Coordination and selectivity notes
Rated for gL/gG fuse backup at 63 A (400 V), 50 A (500 V), and 40 A (690 V). If you are coordinating this breaker downstream of a larger fuse or MCCB, those are the maximum upstream fuse ratings that maintain selectivity under short-circuit conditions. The 100 kA rated short-circuit capacity at 240 V applies only with the specified upstream fuse — without it, the 25 kA at 400 V is the standalone interrupting rating.
Main contact terminals accept M4 screws. Wire capacity is 2x (0.5 to 1.5 mm²) or 2x (0.75 to 2.5 mm²) solid or stranded.