What happens when the normal source returns in an open transition transfer (OTT) automatic transfer switch?

In an open transition transfer (OTT) automatic transfer switch (ATS), when the normal power source is restored, the ATS typically remains in the alternate mode for a preset period before transferring back to the normal source. This delay allows time to ensure that the normal source is stable and reliable before making the switch.

This approach is crucial because if the normal source experiences fluctuations or is not fully restored, switching back too quickly could lead to power disturbances or equipment damage. The preset period serves as a safeguard, allowing the system to monitor the normal source for stability and reliability after an outage. Once it is determined that the normal source is stable, the ATS will then transfer back, ensuring that the transition occurs safely.

The other choices do not reflect the typical behavior of an OTT ATS. For instance, immediately transferring back to the normal source without any checking can pose significant risks if the source is unstable. Sending a startup signal to the generator is not relevant in this context, as the generator would already be running to supply the alternate source. Lastly, powering down the generator immediately would not allow for the necessary checks to ensure safety and stability before transitioning back to normal power.