In short. An automatic transfer switch moves load between utility and generator. The study chooses the transfer type, sets sensing and delay times, checks fault and withstand ratings, handles motor residual voltage and neutral switching, and ensures the failure of the ATS cannot take the site down.
Transfer types
| Type | Behaviour | Use |
|---|---|---|
| Open transition | Break before make, brief outage | Most standby systems |
| Delayed transition | Pause in the neutral position to let residual voltage decay | Motor and transformer loads |
| Closed transition | Make before break, momentary parallel | Sensitive loads; needs sync checks and utility approval |
| Soft load | Ramp load between sources | Reduce step on generator |
Study elements
- Sensing and thresholds. Under- and over-voltage, frequency, phase loss, with time delays to avoid nuisance transfer.
- Timing chain. Detect, start delay, engine start, warm-up, transfer, retransfer delay, cooldown.
- Ratings. Withstand and closing ratings against the fault level of both sources. See fault studies.
- Residual voltage. Motors and transformers keep voltage briefly after disconnect; reconnecting out of phase can cause large torque and current. Use delayed transition or in-phase monitoring.
- Neutral. Use a switched neutral where the generator neutral is earthed to avoid parallel neutral paths. See earthing architecture.
- Bypass. Maintenance bypass that keeps supply during ATS service.
- Test and failure modes. Test on load, fail-to-start, fail-to-transfer, stuck contactor.
Protection coordination
The ATS sits between sources, and its short-time rating must match the protective devices on both sides. The downstream breakers should clear faults before the ATS contacts are stressed. See ATS service, and ATS and AMF control logic.
Illustrative example
A pump station restarts after transfer and trips repeatedly. The study finds that the open-transition ATS reconnects a large motor within 100 ms, while residual voltage is still high and out of phase with the generator. Adding a delayed-transition interval of a few seconds, with the ATS pausing in neutral, solves the issue.
What an ATS study examines
| Item | Question |
|---|---|
| Transition type | Open, closed, delayed or in-phase; what outage can the load accept? |
| Switch rating | Continuous current, withstand and closing rating vs fault level |
| Load behaviour at transfer | Motor residual voltage, transformer inrush |
| Timing | Detection, start, stabilisation, transfer, retransfer |
| Neutral switching | Four-pole or three-pole, earth path |
Motor residual voltage
When power is lost, spinning motors keep a decaying residual voltage. If the transfer reconnects out of phase, the transient current and torque can be severe. In-phase monitoring or delayed transition, waiting for residual voltage to decay below about 25 percent, avoids this.
Typical safe closing condition: residual voltage below about 25 percent of rated, or phase within the allowed window
Timing budget (illustrative)
- Detection delay: 1 to 3 s.
- Generator start and reach rated voltage: 8 to 15 s.
- Stabilisation: 2 to 5 s.
- Transfer: under 1 s, or 100 to 500 ms for delayed transition.
Total outage is the sum, and loads must tolerate it or be backed by UPS.
Study outputs
Transfer sequence, outage duration, current transients and settings for delays. See ATS and AMF control logic and ATS switchgear.
Common mistakes
- Using ratings for the utility case only.
- No neutral switching strategy.
- Skipping on-load tests.
What to send us
Send single-line diagram, ATS data and load types. Send your application details.
Questions engineers ask
Do I need a bypass?
For critical loads, yes, so the ATS can be serviced without an outage.
Why is transfer delayed?
To allow stable source voltage and to let motors decay.
What is a four-pole ATS?
One that also switches the neutral.
How often should an ATS be tested?
Regularly on load, per site policy and standards.
This guide gives general principles. Confirm with manufacturer data.