The ABB 1SDA053999R1 is a T4N 250 molded case circuit breaker (MCCB) fitted with a PR221DS-LS/I electronic trip unit, rated at 250 A. This is the core overcurrent protection device for a distribution panel or motor control center branch — the trip unit gives you LS/I curves (long-time, short-time, instantaneous) for selective coordination downstream.
If your panel already carries a T4N frame with a different trip unit, the PR221DS-LS/I is a drop-in swap — same mechanical dimensions, same mounting hardware. No re-drilling, no adapter plate.
The 250 A continuous rating means the lug size accepts cable up to the breaker's terminal rating — typically 185 mm² Cu/Al for this frame class. Watch the heat dissipation at full load: the PR221DS draws its own operating power from the line-side sensing, so no auxiliary supply needed, but the breaker body needs free air circulation per the mounting clearance diagram.
The LS/I trip unit lets you set the long-time pickup (Ir) down to 0.4× In (100 A) and the short-time and instantaneous thresholds independently — so this one breaker covers a wide range of downstream loads from a 100 A feeder to a 250 A main. For a motor branch circuit, set the instantaneous above the locked-rotor inrush; for a distribution feeder, coordinate the short-time delay with downstream breakers.