When we run production serves and when the power goes down UPS will kicked in, after that when would you normalize? Is there a rule? Or Just we can switch it back to the PDU?

I encourage someone to correct me if I'm wrong here as I'm not sure that I'm in 100% understanding of the question - but generally power is run through the UPS systems, such that it should not be necessary to do anything to revert back to standard power after a disruption has occurred (although I would imagine there are exceptions in some cases). Switching back and forth between generator power would likely be another story.

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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.