Transformer-protection circuit breaker, SIRIUS series
The Siemens 3RV2421-1EA10 is a SIRIUS circuit breaker explicitly designed for transformer protection, not general motor or feeder duty.
Zero clearance needed forward or backward, so it can sit flush against a gland plate or back panel. Screw-type terminals on the main circuit accept 2× (1 to 2.5 mm²) or 2× (2.5 to 10 mm²) solid or stranded copper. M4 terminal screws — torque to the manufacturer spec for the wire size.
Coordination and backup protection
For installations requiring a backup fuse upstream — common when the breaker is installed downstream of a transformer with high fault current — the 3RV2421-1EA10 coordinates with gL/gG fuses rated 32 A at 400 V and 500 V, and 25 A at 690 V. That fuse size is the maximum that still allows the breaker to clear its rated fault current without the fuse blowing first. If your upstream fuse is larger, the breaker's interrupting capacity may be reduced. Maximum switching frequency under AC-3 load is 15 operations per hour — fine for transformer protection where switching is infrequent, but not rated for frequent motor starting cycles.