What is a static transfer switch?

A static transfer switch, or STS, is an automatic switch with no moving parts in the power path. It connects a critical load to one of two independent AC sources and moves it to the other within a few milliseconds when the first one fails.

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¼ cycleas fast as, per phase, to sense a bad source and complete the transfer

0moving parts in the power path

2independent sources, never connected to each other

How it works

Each source reaches the output through silicon controlled rectifiers, or SCRs. Every switched conductor has a pair of them wired back to back, one for each half of the AC cycle. Control logic gates the SCRs of the preferred source on and holds the alternate set off.

Sensing circuits watch both sources all the time. When the preferred source leaves the window you have set, the logic stops gating it, confirms that it has stopped conducting, and gates the alternate source on. The sequence is break-before-make, so the two sources are never tied together, and in phase the whole thing takes as little as a quarter of a cycle; out of phase, an automatic Vt-proportional delay of up to 12 ms is added. When the preferred source has been healthy again for a set period, the switch returns the load to it.

Why "static"

Static means solid state. There are no contacts to arc, bounce, weld or wear, and no coils that must stay energized. SCRs also tolerate the overloads and inrush currents that shorten the life of relays. That is why static switches have been the standard choice in data centers, air traffic control, telecom and process control for decades, and why TwinSource units have logged more than 70 million hours online without dropping a load.

The preferred source is your choice. On TwinSource switches you select it with a push button on the front panel, remotely over RS-485, or in the settings.

What to look for

Transfer time, in and out of phaseA failing UPS cannot stay synchronized. Ask for the out-of-phase number as well as the in-phase one.

A true continuous ratingEvery TwinSource rating is 100% continuous: a 30 A switch carries 30 A, with no derating.

A bypassEvery other critical power device has one. Without it, servicing the switch means dropping the load.

Switched neutralTwo-pole and four-pole designs keep separately derived sources separate.

Accessible fault protectionKnow where the fuses are and whether they can be changed without sending the unit back.

MonitoringAlarm contacts and serial communications so a transfer never goes unnoticed.

Where it is used

Anywhere a load with one power input has to stay up and two sources are available: single-corded servers and network gear, coolant distribution units, chiller and plant controls, SCADA systems, fab tools and air traffic control positions. The application pages show where the switch sits in each.

Applications Static vs relay transfer switches Transfer time basics

Questions engineers ask

Is a static transfer switch the same as a UPS?

No. A UPS stores energy and carries the load when its input fails. An STS stores nothing. It chooses between two sources that are already live. Most critical sites use both: two UPS systems, and static switches that let single-corded loads reach either one.

Some rack products are labelled STS but use relays. How can I tell?

Ask what the switching element is. A few vendors use STS to mean "source transfer system" for a relay-based product. Transfer time alone will not tell you, because a good relay switch is also inside the 20 ms ride-through of IT power supplies.

Can the two sources be out of phase?

Yes. TwinSource switches transfer at any phase angle. If a transformer sits downstream of the switch, talk to our engineers about inrush during an out-of-phase transfer.

Put it to work

Narrow the range to your voltage, current and mounting, or send us your one-line diagram and talk to an engineer.

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