The ABB S803U-K40 is a 3-pole, 40 A miniature circuit breaker from the S800 High Performance series, with a trip curve K and a 50 kA ultimate short-circuit breaking capacity at 240 VAC. It carries approvals to IEC/EN 60947-2, UL 489, and CSA 22.2 No. 5-02, making it accepted across North American and IEC panels.
Trip curve K is the key spec for this part. It combines a magnetic trip threshold of 10–14× In (400–560 A instantaneously) with a thermal element that handles inrush without nuisance tripping. The 40 A rating covers a 15–20 kW motor on 400 V three-phase or a 50 A transformer primary. DIN-rail mounting means it clips directly onto a standard 35 mm rail in the panel — no bracket, no drilling. Three poles let it switch a three-phase load with a single handle; the handle ties all three poles mechanically for common trip on a phase-to-phase fault.