Transformer-protection breaker, S0 frame — 25 kA at 400 V
The Siemens 3RV2821-4BD10 is a SIRIUS circuit breaker in the S0 frame size, designed specifically for transformer protection and listed to UL 489 / CSA C22.2 No.5. The headline breaking capacity is 25 kA at 400 V AC (Icu per IEC 60947-2), with a 55 kA version at the same voltage — the higher figure likely reflects the Ics service rating. At 240 V AC it handles 100 kA. For fuse backup coordination, the spec calls for gG 63 A at 400 V, gG 50 A at 500 V, and gG 50 A at 690 V — meaning if you need higher interrupting capacity than the breaker alone provides, those are the upstream fuse sizes to coordinate with.
Mounting and clearance — fits a standard S0 footprint
Dimensions are 45 mm wide, 144 mm high, 97 mm deep — the S0 frame standard. That means you can butt it against a backplane or enclosure wall on the depth axis, but need breathing room vertically for arc quenching and heat dissipation.
Terminals and wiring — screw-type, M4 main contacts
Main contact screws are M4. Screwdriver shaft diameter 5 to 6 mm, Pozidriv size 2 tip. That's a standard Pozidriv #2 — common in European panel shops, but if your crew carries only Phillips, add the correct bit to the BOM.
Environmental and operational limits
Rated operational voltage range is 20 to 690 V. No ground fault detection, no phase failure detection built in. If those functions are required for your transformer circuit, they need to be handled externally by a separate monitoring relay or electronic overload. Maximum switching frequency at AC-3 and AC-3e duty is 15 operations per hour — fine for transformer inrush switching or infrequent motor starting, not for high-cycle applications like a rapid-reversing conveyor.
Lifecycle stage is current — this is an active, in-production part from Siemens.