In short. Utility–DG transfer is a timed sequence: detect failure, confirm it is real, start the generator, confirm the generator is acceptable, transfer, then later retransfer without a second needless outage. Each timer and permissive exists to prevent a specific failure.
The sequence
Utility fault detected → fail-confirm timer → start command → generator reaches voltage and frequency → stability timer → transfer → running on generator → utility returns → return-confirm timer → retransfer → cool-down → stop → AUTO standby.
Timers and their reasons
| Timer | Prevents |
|---|---|
| Mains-fail delay | Starting on brief sags and blips |
| Start attempts and rest | Starter damage; allows retries |
| Generator stable delay | Loading before voltage and frequency settle |
| Transfer neutral delay | Reconnecting motors while residual voltage is high |
| Mains-return delay | Retransfer onto an unstable utility |
| Cool-down | Shutting a hot engine off load |
Transfer types
Open transition, delayed transition and closed transition each fit different loads. Closed transition needs sync and utility approval. See closed transition and ATS studies.
Source acceptance windows
Each source must pass voltage, frequency, phase sequence and, for some loads, rate-of-change checks. Windows should match what the loads tolerate, for instance UPS input limits. Measure the actual generator at the UPS input during commissioning.
Failure cases
- Generator fails to start: alarm, retry, call backup set or shed load.
- Generator starts but is unacceptable: do not transfer.
- Transfer switch fails to move: alarm and manual fall-back.
- Utility flickers during retransfer: hold on generator.
See ATS and AMF logic.
Measure the sequence
Record T0 (utility failure), T1 (start command), T2 (generator running), T3 (source acceptable), T4 (transferred) and T5 (load accepted, such as UPS). These times show where delay occurs and how long batteries must bridge.
Illustrative example
On a test, T0 to T4 is 14 seconds. Start takes 4 seconds, stabilisation 6 and transfer 4. The UPS battery autonomy of 10 minutes is far above this, but when the first start fails and the second succeeds, the time grows to about 30 seconds, still within reserve. The test also reveals the stable timer was too long and is shortened.
Transfer control: four design choices
| Choice | Options |
|---|---|
| Transition | Open (break before make), delayed, in-phase, closed (parallel) |
| Detection | Voltage, frequency, phase loss, sequence, rate-of-change |
| Return policy | Automatic after stable mains, or manual and scheduled |
| Test mode | On-load test, off-load test, simulated failure |
Timers that shape the experience
- Mains failure delay (1 to 5 s): ignores momentary dips.
- Start and warm-up: until voltage and frequency are stable.
- Transfer pause: lets motor residual voltage decay on delayed transition.
- Mains return delay (typically 1 to 10 minutes): confirms mains is stable before retransfer.
- Cool-down (typically 3 to 5 minutes unloaded): protects the turbocharger.
Why the return delay matters
Utility supply often fails again within minutes of a first restoration. A short return delay causes repeated transfers, each of which disturbs the load. A longer delay keeps the generator carrying load until mains is proven.
Test regime
Run an on-load test regularly. Many standby failures are found only when the transfer is attempted. See ATS service.
See ATS and AMF logic and closed transition.
The master engineering question
If the first start attempt fails or the utility returns unstable at the worst moment, does the sequence still protect the load without causing a second outage?
What to send us
Send your current transfer scheme and load types. Send your application details.
Questions engineers ask
What mains-fail delay is right?
Long enough to avoid nuisance starts, short enough for your load. Typical values are seconds.
Why a return delay?
To confirm the utility is stable before retransfer.
Does cool-down matter?
Yes, for turbocharged engines. Follow the manufacturer.
Can we skip retransfer delay?
Not safely; repeated transfers stress equipment.
Values are illustrative. Set timers to site requirements.