What is the difference between an ATS and an STS in a data center?

An ATS (automatic transfer switch) is a mechanical switch that transfers a facility's power source between utility and generator, switching in tens of milliseconds. An STS (static transfer switch) is an electronic switch with no moving parts that transfers a load between two live, redundant UPS outputs in a fraction of an AC cycle, fast enough that sensitive equipment sees no interruption.

Why does a data center need both an ATS and an STS?

They protect against different failure scenarios. The ATS handles the relatively slow transition between utility and generator power, which the UPS's battery runtime already bridges without interruption. The STS protects single-corded equipment that lacks dual power supplies, by switching fast enough between two redundant UPS outputs that the equipment never sees a gap.

How fast does an STS switch compared to an ATS?

An STS switches in a fraction of a single AC cycle because it uses purely electronic, solid-state switching with no moving parts. An ATS takes tens of milliseconds to a few cycles because it involves physically moving mechanical contacts — much slower, but fast enough for its specific job of transferring between utility and generator.

Where does an STS typically get used in a data center power system?

An STS is typically placed close to the IT load, positioned between the outputs of two redundant UPS systems in a 2N architecture, specifically to protect single-corded equipment that would otherwise have no way to benefit from having two independent power paths available.

Engineering·7 min read·September 19, 2026

🔀 Data Center ATS vs. STS Explained

The difference between an automatic transfer switch (ATS) and a static transfer switch (STS) in data center power design — how fast each one switches, what kind of failure each protects against, and why a facility typically needs both, not one or the other.

🎓Data Center Engineering Basics — Start to Finish (Full Access)

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Two Different Switches, Two Different Jobs

An automatic transfer switch (ATS) and a static transfer switch (STS) both move electrical load from one power source to another, but they solve different problems, operate on very different timescales, and are typically both present in the same data center rather than being alternatives to each other. Confusing the two — or assuming one can do the other's job — is a common source of misunderstanding about how data center power switching actually works.

Topics covered

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