The Siemens SIRIUS 3RV1421-4AA38-1KT0 is a circuit breaker specifically designed for transformer protection — not a general-purpose motor-protective breaker, but one tuned to handle the inrush and thermal characteristics of transformers. The 45 mm width means it takes up a single modular space in a standard panel layout.
Rated at 16 A continuous current, this breaker handles transformer loads up to that steady-state value. The Class 10 trip characteristic means it will hold through the high inrush of an energizing transformer (typically 8–12x rated for 0.1–0.5 s) but still trip quickly on a hard fault — critical for protecting both the transformer and downstream wiring. Breaking capacity is the real selector here: 100 kA at 240 V AC, 50 kA at 400 V AC, 10 kA at 500 V AC, and 4 kA at 690 V AC. No ground-fault detection on this variant, so if that's required, you'd need an add-on module or a different base unit. The auxiliary switch is transverse-mounted, and its ratings are 1 A at 24 V, 0.15 A at 60 V, and 0.5 A at 230 V — enough for signaling to a PLC or indicator lamp.
Mounting and Environment
Zero clearance needed at the sides or back — 0 mm spacing — so you can pack modules tight in the enclosure. Shock-rated at 25g for 11 ms — fine for most industrial environments including machine-mounted panels.