Transformer-protection circuit breaker, Trip Class 10
That means it trips fast enough to protect a transformer winding from sustained overcurrent while riding through inrush — the standard choice for distribution transformers feeding motor control centers or lighting panels. Breaking capacity is 30 kA at 400 V AC (65 kA at 400 V AC for the higher-rated version), so it handles fault currents on a typical 400 V industrial service without upstream fuses. No ground-fault detection on this variant, so for grounded-wye services you'd add an external GF relay.
Width is 55 mm — three 18 mm pole spaces on the rail. Depth 149 mm, height 140 mm. That's a tight fit for a 55 mm-wide breaker; no extra breathing room needed behind the panel door.