If one phase drops out, the breaker trips — that prevents single-phasing a motor, which would otherwise overheat the remaining windings quickly.
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. That matters when you're packing breakers into a panel — you need that 50 mm top/bottom gap for arc exhaust and cooling. 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.
It's a standard SIRIUS catalog item, so it's broadly stocked across distribution.
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.