The Siemens 5SY4440-8CV is a 4-pole miniature circuit breaker (MCB) with a D40 characteristic, rated for a 15 kA breaking capacity (Icu) at 400 V. For a mill environment, that 15 kA Icu at 400 V gives you the fault-clearing headroom to maintain selectivity downstream without cascading upstream breakers on a bolted fault.
The 4-pole configuration (3 phases + neutral) is standard for three-phase four-wire systems where the neutral needs overcurrent protection. The D40 thermal-magnetic curve: thermal trip at 40 A continuous, magnetic trip between 10x and 20x In (400–800 A), so it won't nuisance-trip on motor starting but clears fast on a hard short.
Panel integration
Snap-on DIN rail mounting per EN 60715. The 4-pole width is 72 mm (4 x 18 mm modules). Wire termination accepts up to 25 mm² for the line and load terminals. Busbar comb can bridge the incoming side across all four poles for a common feed.