What this breaker does on the line
The Siemens SIRIUS 3RV6321-4BC10 is a three-pole circuit breaker built for starter combinations — meaning it lives right ahead of a contactor in a motor branch circuit. It gives you both overload and short-circuit protection in one package, and the magnetic instantaneous trip is set at 260 A, so it clears a fault fast before the contactor takes damage. Rated operational current is 20 A in AC-3 duty at 400 V, which covers a fair range of small-to-mid motor loads on a 50/60 Hz line. This is a size S0 frame, which is the compact workhorse form factor in the SIRIUS line — 45 mm wide, 97 mm tall, 96 mm deep. Power loss is 8 W typical, so it won't cook the enclosure if you've got a row of them.
Mounting and wiring in the panel
Clearance requirements: 50 mm above and below, 30 mm to the sides, zero forward or backward — so you can butt it against a backplane or gland plate without extra breathing room. Terminals are screw-type on top and bottom, accepting 2x (1 to 2.5 mm²) solid/stranded or 2x (2.5 to 10 mm²). Torque to 2 to 2.5 N·m on the main contacts. AWG equivalents are 2x (16 to 12) and 2x (14 to 8).
RoHS compliance date is noted as May 1, 2012, which covers the EU RoHS directive. Relative humidity tolerance is 10 to 95% during operation. Shock resistance per IEC 60068-2-27 is 25 g for 11 ms. Installation altitude max is 2000 m above sea level.