What the ratings mean for fit
The S201-K4 is a 1-pole miniature circuit breaker rated 4 A continuous at 277 VAC, with a Trip Curve K characteristic. Curve K is a specialty magnetic trip curve designed for highly inductive loads — think motor starters, solenoid valves, or transformer primaries where the inrush current is high but short. The K curve's magnetic trip threshold (typically 8–12× rated current) lets it ride through those inrush spikes without nuisance tripping, unlike a Curve C (5–10×) or Curve D (10–14×) which might trip on a motor start or hold too long on a fault. The 6 kA interrupting rating at 277 VAC means it can safely clear a bolted fault up to 6,000 A without welding contacts or rupturing the case — adequate for most branch circuits in commercial and light industrial panels where the available fault current stays under that level.
Where it's used — the deployment context
This is a DIN-rail mounted supplemental protector per UL 1077, not a branch-circuit rated breaker per UL 489. That distinction matters: UL 1077 devices are intended for overcurrent protection within equipment — control panels, HVAC units, machine tool enclosures — not as the main service disconnect. It snaps onto standard 35 mm DIN rail, occupying one module width (17.5 mm). In a panel, it protects a single 4 A load circuit, typically a contactor coil, a small power supply, or a PLC output group. The K curve makes it the right choice for circuits feeding inductive loads where a standard C curve would nuisance-trip on startup.
Compliance and approvals
Listed under UL 1077 as a supplemental protector and CSA 22.2, the S201-K4 carries the safety certifications needed for North American panel builds. The S200 series also holds IEC/EN 60898 and IEC/EN 60947-2 ratings for global installations, though the specific listing here is UL/CSA. RoHS and REACH compliance documentation is available from the manufacturer upon request — standard for ABB's current-production S200 line.