What it is and what the K-curve means
The S801U-K90 is a single-pole miniature circuit breaker (MCB) from the ABB S800 series, rated 90 A with a K-trip characteristic. The K-curve is engineered for motor and inductive-load protection — it tolerates the inrush of a starting motor without nuisance tripping, then clears a hard fault fast. In a panel, that means you can size it for the running current, not the start surge.