What the 15 kA Icu means for your panel
The Siemens 5SY4614-8CV is a 3P+N miniature circuit breaker rated for a 15 kA ultimate breaking capacity (Icu) at 400 V AC. For a panel fed by a transformer sized under 630 kVA, 15 kA Icu typically gives enough headroom; above that, you'd want to check the prospective fault level at the breaker's busbar. That's the curve you spec for motor circuits, welding equipment, or transformer inrush — loads that draw a high starting surge without being a fault. The 0.3 A rating makes this a specialty breaker for very small motor or control transformer protection, not general lighting or socket circuits.