In short. A static transfer switch (STS) uses power electronics, not mechanical contacts, to move a load from one source to another in milliseconds. It gives single-corded loads access to two sources and protects against loss of one path. Because every load passes through it, the STS itself must be carefully designed and maintained.
What it does
The STS monitors a preferred and an alternate source. If the preferred source goes out of tolerance, thyristors switch the load to the alternate source, usually in a fraction of a cycle to a few milliseconds. To avoid large current surges, the two sources should be synchronised, normally by being fed from the same upstream system or by UPS inverters locked together.
STS versus ATS
| Aspect | Static transfer switch | Automatic transfer switch |
|---|---|---|
| Technology | Thyristors | Mechanical contactor or breaker |
| Typical transfer time | Milliseconds | Tens to hundreds of milliseconds or more |
| Typical use | Between two UPS or clean sources | Between utility and generator |
| Heat losses | Higher, needs cooling | Low |
They are complementary. See ATS, STS and UPS coordination.
Where it sits in the system
- At the output of two UPS systems, feeding single-corded loads.
- At a PDU level to give a zone of loads two sources.
- At rack level for individual devices.
Key specifications to check
- Continuous current and voltage rating, and overload capability.
- Fault withstand and downstream fault clearing, since an STS may transfer onto a fault.
- Transfer time and the equipment’s hold-up time.
- Sync window, and behaviour when sources are out of sync.
- Bypass arrangement for maintenance.
- Back-feed protection, so the failed source is not energised from the other.
Failure modes to design against
- STS failure: the load loses both paths. Use a maintenance bypass and dual STS or redundancy where critical.
- Transfer onto a fault: downstream short circuit can move to the other source and disturb it. Coordinate protection.
- Out-of-sync sources: a transfer can cause large inrush into transformers or motors.
- Transformer inrush: downstream transformers may draw inrush on transfer. See transformer inrush.
- Overheating: failed fans or blocked ventilation.
Worked illustration
Two 300 kVA UPS systems feed a 200 kVA group of single-corded loads through an STS. The STS is rated 400 A at the system voltage. On failure of UPS A, the STS moves to UPS B. If the downstream circuit includes a transformer, the inrush on transfer might reach several times rated current for a few cycles. The designer checks that UPS B can supply the inrush without tripping, or adds a transformer arrangement that limits it.
Maintenance
The STS needs a maintenance bypass so it can be serviced without dropping load. Test transfers periodically, check cooling, and review alarm history. Keep spares of fans and control boards.
What a static transfer switch does
An STS uses thyristors to move a load between two AC sources in about 4 to 8 milliseconds, fast enough that most IT power supplies see no interruption. It does so only if the two sources are in sync and within tolerance.
| Feature | STS | ATS |
|---|---|---|
| Transfer time | 4 to 8 ms | Tens of ms to seconds |
| Switching element | Thyristors | Contactor or breaker |
| Use | Between two UPS or sources in sync | Between utility and generator |
| Losses | Continuous small loss in thyristors | Negligible |
Transfer logic
- Preferred source selected.
- If preferred source goes out of tolerance, check alternate source is in tolerance and in sync.
- Transfer, with back-feed protection that stops the load feeding the failed source.
- Retransfer when the preferred source is restored, normally after a delay.
Design points
- Sources must be synchronised, normally from a common UPS bypass or from two UPS systems locked in phase.
- Mechanical bypass for maintenance.
- Fault clearing: the STS must not transfer into a downstream fault, or it can fail both sources in turn. Use fault detection on the load side to inhibit transfer.
- Rating for short-circuit current and inrush.
Single point of failure
An STS itself is a single device between two sources. Use redundant STS or place STS near loads to limit what a failure affects. See transfer coordination.
Common mistakes
- Treating the STS as having no failure risk.
- Feeding both STS inputs from the same upstream path.
- Ignoring fault current withstand.
- No maintenance bypass.
What to send us
Send the load, the sources and the single-corded devices involved. Send your application details. See also dual-cord and single-cord loads.
Questions engineers ask
Is an STS a single point of failure?
It can be. Design with a maintenance bypass and consider redundant STS units for the most critical loads.
Why must the two sources be synchronised?
If they are out of phase, a transfer can cause current surges and a voltage disturbance.
Can an STS replace an ATS?
No. An STS suits clean, synchronised sources, while an ATS typically moves between utility and generator.
What happens if the load has a fault when the STS transfers?
It may transfer the fault to the alternate source. Downstream protection must clear it quickly and selectively.
This guide gives general principles. Ratings and settings should be taken from manufacturer data and a protection study.