Transformer-protection circuit breaker, SIRIUS series
The Siemens 3RV2421-4DA10 is a SIRIUS-branded circuit breaker specifically designed for transformer protection, not general motor or feeder duty. At 240 V the interrupting rating rises to 100 kA, reflecting the higher available fault current typical in North American 240 V distribution.
Depth is 97 mm; forwards clearance is 0 mm. Screw-type terminals on the main current circuit accept 2×(1…2.5 mm²) or 2×(2.5…10 mm²) solid or stranded copper — sized for transformer primary wiring up to about 10 mm².
Rated ultimate short-circuit breaking capacity varies by voltage: 100 kA at 240 V, 25 kA at 400 V, 5 kA at 500 V, and 2 kA at 690 V. The 25 kA at 400 V is the figure most relevant for European 400 V three-phase panels; the 2 kA at 690 V means it is not intended for 690 V industrial mains without upstream current-limiting fuses. Back-up fuse recommendations are listed: gL/gG 63 A at 400 V, 50 A at 500 V and 690 V.