In short. ATS, STS and UPS devices each handle a different time scale. The UPS covers milliseconds to minutes, the STS switches in milliseconds between clean sources, and the ATS moves between utility and generator in seconds. Their thresholds, delays and sequences must be coordinated so they do not fight, chatter or cause a second interruption.
The time scales
| Device | Time scale | Job |
|---|---|---|
| UPS | Immediate to minutes | Carry the load, hide the gap |
| STS | Milliseconds | Switch between two clean sources |
| ATS | Seconds | Switch between utility and generator |
| Generator controls | Seconds to tens of seconds | Start, stabilise, accept load |
A typical sequence
- Utility fails. The UPS switches to battery with no break to the load.
- The ATS controller waits a short delay to ignore momentary dips, then signals the generator to start.
- The generator reaches voltage and frequency; the ATS transfers the generator onto the bus.
- The UPS rectifier restarts, usually with a walk-in delay so the generator is not hit by a step.
- When utility returns and is stable for a set time, the ATS retransfers, ideally in an in-phase or controlled manner.
Settings to coordinate
- ATS undervoltage and delay: long enough to ignore dips the UPS can ride through, short enough that the generator is ready before the battery runs out.
- UPS input window: the generator’s voltage and frequency must stay within the UPS acceptance range, or the UPS will stay on battery.
- Rectifier walk-in: staggered start prevents a block step on the generator.
- Retransfer delay: prevents repeated switching on unstable utility.
- Bypass operation: avoid an unwanted static bypass transfer during generator transfer.
Common interaction problems
- Generator frequency hunting causes the UPS to reject the supply and drain the battery.
- Transformer inrush on ATS transfer trips breakers. See inrush screening.
- Motor residual voltage at fast transfer can cause damaging torque. A time delay or in-phase transfer avoids it.
- Retransfer break: a second interruption at return of utility that the UPS then has to ride through.
- Different transfer logic on parallel devices causes sections to transfer at different times.
Open, delayed and closed transition
An open-transition ATS breaks before it makes, giving a brief interruption. Delayed transition adds a deliberate pause in the dead position to let residual voltages decay. Closed transition briefly parallels the sources, with no interruption but needing synchronism checks and the utility’s permission. See closed-transition control.
Worked illustration
A UPS has 8 minutes of battery. The ATS delay to start is 5 seconds, the generator takes 12 seconds to start and accept load, and the UPS rectifier walks in over 20 seconds. The bridging needed is under a minute, well inside the battery. The risk is not time but acceptance: if the generator voltage drifts outside the UPS input window during the load pick-up, the UPS stays on battery, and the 8 minutes become the limit. The test is to confirm the UPS accepts the generator on its first presentation.
Test plan
- Utility loss with generator starting normally, then with the first start failing.
- Utility return and retransfer, recording any break.
- Generator voltage and frequency recording at the UPS terminals.
- Repeat with the load at low, medium and full levels.
Three switches, three speeds
| Device | Typical speed | Between |
|---|---|---|
| STS | 4 to 8 ms | Two UPS outputs or two clean sources |
| UPS static bypass | Under 4 ms | Inverter and bypass source |
| ATS | 50 ms to several seconds, plus generator start | Utility and generator |
Coordination goals
- No two devices fight: only one device decides each transfer.
- No transfer onto a fault.
- Sources in phase before a fast transfer.
- No overload of the next source during recharge or walk-in.
Example sequence (illustrative)
| Time | Event |
|---|---|
| 0 ms | Mains fails; UPS inverter carries the load from battery |
| 1 to 3 s | ATS confirms failure, issues generator start |
| 10 to 15 s | Generator at rated voltage; ATS transfers |
| 15 to 45 s | UPS rectifier walks in on generator supply |
| Minutes | Battery recharge at limited current |
Typical pitfalls
- UPS rejecting generator frequency or voltage, so it stays on battery and drains it.
- ATS transfer while the UPS bypass is active, giving repeated disturbances.
- STS transfer into a bad source because the sync was not monitored.
Test the whole chain, not each device alone. See UPS, STS and ATS control coordination.
Common mistakes
- Setting the ATS time delays without reference to UPS autonomy.
- Ignoring the retransfer sequence.
- Not recording a baseline of voltage and frequency during the transfer.
What to send us
Send the UPS, ATS and generator data and the present sequence. Send your application details. See also static transfer switches.
Questions engineers ask
Why does my UPS stay on battery when the generator runs?
The generator voltage or frequency is likely outside the UPS input window, or frequency is hunting. Check generator regulation and the UPS settings.
Is closed transition better?
It avoids an interruption but needs synchronism and permission, and it raises fault levels momentarily. Open or delayed transition is simpler where a short break is acceptable.
What is a retransfer delay for?
It waits until utility is stable, to avoid repeated transfers and unnecessary interruptions.
Do ATS and STS need to be coordinated?
Yes, so one does not transfer while the other is changing state and cause an unintended break.
This guide gives general principles. Settings should be set and tested on the actual installation.