What it is and what it does
The Siemens SIRIUS 3RV2021-1FA20-ZX95 is a motor protection circuit breaker — the device that sits between the contactor and the motor in a standard motor starter lineup. Its job is to protect the motor and the wiring against overload and short-circuit, and it does that with a Trip Class 10 characteristic, meaning it will trip within 10 seconds at 7.2 times the thermal setting — fast enough for most standard induction motors on pumps, fans, and conveyors. It carries a 100 kA breaking capacity at 400 V AC, so it can safely interrupt a fault current up to that level without welding its contacts or cascading the fault upstream to the feeder breaker. That rating holds at 240 V, 400 V, and 500 V AC — a solid performance across common industrial voltages. Phase failure detection is built in. If one phase drops out, the breaker trips — that prevents single-phasing a motor, which would otherwise overheat the remaining windings quickly.
Mounting and wiring
Mounts via screw or snap-on onto 35 mm DIN rail per DIN EN 60715. The 45 mm width means it takes up one standard 45 mm module slot on the rail — a common footprint for motor protection breakers in this class. Depth is 97 mm, height 119 mm. Clearance requirements: 50 mm above and below, 30 mm to the side, zero forwards or backwards (–). That matters when you're packing breakers into a panel — you need that 50 mm top/bottom gap for arc exhaust and cooling. Main circuit terminals are spring-loaded, accepting 2x 1 to 10 mm² solid or stranded wire. Spring terminals are faster to wire than screw clamps and hold better under vibration — good for panels that get shipped and bounced around. Mounting position is any, so you can lay it sideways or upside-down if the panel layout demands it — no derating needed for orientation.
Lifecycle and sourcing
It's a standard SIRIUS catalog item, so it's broadly stocked across distribution. Availability and current pricing are confirmed at quote time against an RFQ — we source through independent channels and can quote to order.
How it compares to the 3RV2021-1FA15
The 3RV2021-1FA15 is the closest sibling in the same SIRIUS family — same form factor, same Trip Class 10, same 100 kA breaking capacity. The difference is the thermal setting range: the -1FA15 covers a lower current band than the -1FA20. If your motor's full-load amps fall inside the -1FA15's range, it's a drop-in swap — same 45 mm width, same DIN rail mount, same terminal layout. No rewiring needed.