I think the question is about when you use a UPS in line interactive mode (power is normally sourced via the STS). When the supply goes out of tolerance, the STS will kick in to run the load from the UPS. So when do you decide to switch it back, is it auto or manual.
This is how I read the question.
If line interactive is fine for you, make it automatic after the supply stays within tolerance for 15 minutes.
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