In short. Closed transition makes the second source connect before the first disconnects, so loads see no interruption. The brief overlap puts two sources in parallel, so it needs synchronism, a strict time limit, protection that remains active and, for utility sources, the utility’s permission.
Sequence
Both sources healthy → synchronise (voltage, slip, angle) → close second breaker → parallel timer starts → open first breaker → confirm transfer → timer stops. If the first breaker fails to open before the time limit, the control must trip one source to end the parallel.
Why it is used
- Tests and maintenance transfers without load outage.
- Retransfer from generator to utility after an outage, avoiding a second interruption.
- Sensitive loads that cannot ride through a break.
Requirements
| Item | Requirement |
|---|---|
| Synchronism | Voltage, frequency and angle matched. See synchronising control |
| Overlap time | Short and limited, often under a second; set by the utility if applicable |
| Fault level | Equipment must withstand the sum of both sources during overlap. See fault studies |
| Protection | Reverse power, over-current and sync-check active during overlap |
| Permission | Utility agreement for parallel operation. See interconnection |
| Earthing | Neutral arrangement for both sources. See earthing |
Failure responses
- Sync not achieved in time: abort and fall back to open transition if allowed.
- First breaker fails to open: trip the second source to end the parallel.
- Loss of sync-check or sensing: block.
- Source disturbance during overlap: protection trips as defined.
Soft load transfer
For generators, ramp load from one source to the other during overlap rather than stepping. This reduces the transient on the generator.
Illustrative example
A pharmaceutical site retransfers from generator to utility with closed transition. The controller matches the generator to the utility, closes the utility breaker, ramps load over several seconds and opens the generator breaker, with a 60 second maximum overlap alarm. A test with a stuck generator breaker triggers the failure response and the utility breaker opens to end the parallel.
What closed transition buys
In a closed-transition transfer, both sources are briefly paralleled so the load is never de-energised. It is useful for sensitive loads, but it introduces risks that open transition avoids.
| Item | Open transition | Closed transition |
|---|---|---|
| Interruption | Short outage | None |
| Parallel time | None | Under 100 ms for many switches, or seconds for soft transfer |
| Fault level during overlap | One source only | Sum of both sources |
| Utility approval | Not required for parallel | Typically required |
| Control complexity | Lower | Higher: sync-check, protection |
Control requirements
- Sync-check: voltage, frequency and angle inside a narrow window before closing.
- Time limit on overlap, with a trip of one source if exceeded.
- Protection active during overlap: reverse power, over/under voltage and frequency.
- Ramp transfer for momentary paralleling: shift load gradually between sources.
- Fallback to open transition if conditions are not met in time.
Fault level check
During overlap, switchgear sees both sources. Confirm the ratings cover the sum. See utility plus DG synchronisation.
When not to use it
Where utility rules forbid paralleling, where generator is weak relative to the load or where maintenance capability is limited. See interconnection study.
The master engineering question
If anything goes wrong during the overlap, which protection ends the parallel and what is the worst fault current the equipment must survive?
What to send us
Send utility rules and equipment ratings. Send your application details.
Questions engineers ask
Is closed transition always better?
No. It adds complexity and requirements. Use only where the benefit is real.
Does the utility need to approve?
Usually yes, for any parallel operation.
How long can the overlap last?
As short as practical and within the utility’s limit.
Can generators do it alone?
Between generators and generator buses, yes, with sync control.
This guide gives general principles. Follow the utility’s rules and local regulation.