STS (Static Transfer Switch)

An STS is a solid-state switching device that transfers a load between two independent power sources — typically the A and B feeds of a dual-corded facility — in a few milliseconds when one source fails or drifts out of tolerance, fast enough that server power supplies never see an interruption. It sits downstream of the UPS and upstream of the PDU or busway, and is standard equipment in Tier III and Tier IV designs that guarantee concurrent maintainability, since it lets technicians take one entire power path offline for testing or repair without dropping single-corded IT load. Typical switching time is under 4 milliseconds — a quarter of an AC cycle at 60 Hz — well inside the ride-through window of a standard server power supply, and units are commonly rated from 30 kVA up to several hundred kVA to match rack- or row-level busway capacity.