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I am trying to figure out what the maximal interruption for an offline/standby uninterruptible power supply (UPS) would be. According to wikipedia: "The switchover time can be as long as 25 milliseconds depending on the amount of time it takes the standby UPS to detect the lost utility voltage."

Does this detection time depend on the grid frequency (50/60Hz), the quality of the UPS or the type of outage? Are there some general guide line for what is acceptable for servers? Are offline UPS-Systems even suitable for servers?

Lucas
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I wish I could find authoritative references for you, but I can't recall where I read this info in the recent past (this year sometime). I believe it was on APC's site itself, but possibly Liebert's or another well known mfg.

Here's what I recall:

  1. Most offline UPS' will "failover" to battery between 2-10ms. It can be as long as 25, but most shoot for sub 10ms. Why? Because most PC power supplies (which is the bulk of the offline model market) will continue to flow power for about 15-20ms after the AC direct power is off. So if an offline model can hit 10ms or less, they are usually fine. So why "as long as 25ms"? Beats me...I guess certain power supplies have longer tolerances and can accept a longer "outage".
  2. Online UPS' are often misunderstood. Most people assume they do what an offline UPS really does...which is to switch off of building power and onto battery when the power goes out. In reality, they are NORMALLY on battery at all times, with building power there to keep the batteries charged during normal use. I say normally because you can cut it over to "maintenance" typically so that you can run straight off building power and bypass the batteries entirely if need be.

So, that said...general guidelines for servers?

There's nothing I can think of as far as proof NOT to run a server on an offline UPS. However, all of the ones I've ever setup had online UPS'. The ability to create a much stricter output signal coupled with immediate "failover" (again because it isn't really failing over but simply the battery is no longer being kept charged) is worth the small extra cost associated with choosing an online UPS over an offline one.

TheCleaner
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