In short. Automatic transfer switches differ mainly in how they move the load between sources. Open transition breaks before it makes, giving a short interruption. Closed transition briefly parallels both sources so the load sees no break. Delayed transition holds the load disconnected long enough for motors and transformers to decay safely. Bypass-isolation allows the switch to be maintained without dropping the load. The right type follows from the load's tolerance to interruption and the site's maintenance needs.
Where to start
| If your question is… | Start with |
|---|---|
| A short break on transfer is acceptable | Open-Transition ATS |
| The load must not see any interruption | Closed-Transition ATS |
| Large motors or transformers are being switched | Delayed-Transition ATS |
| The ATS must be serviceable without a shutdown | Bypass-Isolation ATS |
All 4 guides in this section
- Open-Transition ATS — What happens to your load during an open-transition (break-before-make) transfer, which loads accept the gap, and how to set delays and manage motor restart.
- Closed-Transition ATS — How a closed-transition ATS briefly parallels generator and mains to avoid the retransfer break, and the synchronising, protection and DISCOM consent it needs.
- Delayed-Transition ATS — When a delayed-transition ATS should hold the load off both sources, how motor residual voltage and drive resets set the pause, and how to choose the timer.
- Bypass-Isolation ATS — How a bypass-isolation ATS keeps the load supplied while the automatic switch is isolated for test or repair, where it is justified, and how to operate it.
Questions engineers ask
Which ATS type is most common?
Open transition, because it is the simplest and suits most commercial and industrial loads that can tolerate a few seconds' break.
Does closed transition need utility approval?
Usually yes. Because it briefly parallels the generator with the grid, the utility's interconnection rules and protection requirements apply.
Engineering content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. For advice on your own site, send us your requirement or call 098852 01608.