Why Do Lines Equipped with STS Still Need a UPS?

Simply put: STS and UPS address completely different issues related to “power outages” and “power supply.” They are not substitutes for one another, but rather the perfect pair. Here’s an analogy that makes it easy to understand: A UPS (Uninterruptible Power Supply) is like a “high-capacity power bank” with its own built-in battery. When the external power source is completely cut off, it can generate its own power to keep things running. ​STS (Static Transfer Switch) is an “ultra-high-speed automatic transfer switch.” It doesn’t generate power itself, but if the main line (Line A) goes down, it can instantly switch you to the backup line (Line B) at lightning speed (typically < 5 milliseconds). ​

Why install a UPS if you already have an STS? If both lines lose power, the STS becomes inoperable. The STS operates on the premise that “there are two mains power sources (Path A and Path B).” The risk of using an STS alone: If a widespread regional power outage, grid fluctuation, or lightning strike causes both Path A and Path B to lose power simultaneously—or if Path B is already undergoing maintenance—then no matter how fast the STS switches, it will merely switch from “powerless Path A” to “powerless Path B.” The equipment will still shut down immediately due to the power loss. ​

The necessity of adding a UPS: A UPS contains a battery. When both Power Paths A and B are completely down, the UPS immediately takes over, using its own battery to continue supplying power. This provides servers or production equipment with tens of minutes or even hours of “breathing room” to shut down properly or wait for the generator to start. ​

Conversely, why install an STS if you already have a UPS? The UPS itself is also a “single point of failure.” If you set up only a UPS system without an STS, the risk remains significant. Risks of using a UPS alone: A UPS is a machine; it can fail on its own (e.g., inverter failure, battery aging or short circuits, internal fires). If your critical equipment is connected only to this single UPS, once the UPS fails, the equipment will still lose power. The necessity of an STS: For absolute safety, large data centers implement a “dual-bus system”—UPS System A and UPS System B. The STS is placed at the very front end. When UPS A suddenly fails or needs to be shut down for maintenance, the STS will switch the load to the stable UPS B within 3–5 milliseconds (so quickly that the equipment won’t even notice).