What it is and what it fits
The Siemens 3RV2021-4NA20 is a SIRIUS motor protection circuit breaker designed for motor protection, with phase failure detection built in. It snaps onto 35 mm DIN rail per DIN EN 60715 and accepts spring-loaded terminals for the main current circuit — solid or stranded wire up to 2x 10 mm². The 45 mm width and 97 mm depth fit standard panel layouts, with 50 mm clearance needed above and below, and zero clearance required forwards or backwards.
Ratings that decide the fit
Trip Class 10 means it opens within 10 seconds at 7.2x the thermal setting — standard for general-purpose induction motor starting where the inrush doesn't linger. The interrupting capacity at 400 V is 25 kA, which covers most industrial distribution panels; at 240 V it's 100 kA, and at 690 V it drops to 2 kA. The rated operating voltage spans 20 to 690 V, so it works on 480 V or 600 V systems as well. Maximum switching frequency under AC-3 duty is 15 operations per hour — fine for a conveyor or pump that cycles a few times per shift, not for a rapid-reversing application.