The site has 2 x 16A @ 230V, each server/SAN has dual PSU set up.


1) [2 x 16A] ---> [1 x STS or ATS]----> [1 xUPS]---->[2 x PDU]----->[DUAL PSU]

As can be seen the server is actually connected to the same UPS in the end even though via different PDU, however, the STS ensures that the UPS has dual connections to input supply

2) [1 x 16A] ---> [1 xUPS]---->[1x PDU]----->[PSU 01] AND [1 x 16A] ---> [1x PDU]----->[PSU 02]

In this set up the first PSU is connected to a UPS but the second is connected directly to the secondary supply.

Please advise on which set up is preferred; I am concerned about Surge Protection for the direct connection but all products seem to be in the home consumer or audio video segment.

There is insufficient space to have another UPS set up for the secondary supply; that would have been my first choice.



Option 1: If the UPS fails, your power goes out altogether, unless it's a UPS with a builtin bypass.

Option 2: If the power goes out, the UPS will suddenly be providing (nearly) twice the power.

If you're talking about a UPS that does have a bypass (this is going to be a system directly wired to mains power, not connected via plugs and outlets), Option 1 is undeniably a better choice.

If you're not talking about a bypass-type UPS, you have to guarantee that the UPS will never be at more than 50% capacity. I don't think that anyone can really provide such a guarantee. I still think Option 1 is the better choice, but it's less obvious.

  • Thanks for your response. But why would the UPS be supplying nearly twice the power for Option 2 if the power goes out as only 1 set of PSU would connected to the PDU connected to the UPS? The UPS datasheet states "Automatic Bypass in event of internal failure". Any thoughts on surge protection for servers connected to secondary mains? – BOBA_FETT_81 Apr 10 '11 at 2:51
  • @Boba, If power fails, the second PSU will shut down, and the first PSU will supply all power to the equipment, pulling all power from the UPS suddenly. Option 1 is the better choice of those two; though getting another UPS would be an even better option. – Chris S Apr 10 '11 at 3:34

Option 1 is how the datacenter at my old job worked, only it was a 460v 3-phase system with a single large UPS (and generator). There are several single points of failure in that system, and we managed to find most of them during the 10-odd lifetime of that datacenter:

  • Generator failure. If the mains outage is longer than the UPS can supply load, the room drops hard. Happened (actually, the circuitry that informs the ATS that the generator was online failed).
  • Automatic Transfer Switch failure. As above. Happened.
  • UPS failure. When power goes out, the room drops hard rather than come back up. Happened two months ago. Nearly stopped my heart. We survived with no equipment failures, though.
  • PDU failure. Can manifest as the entire PDU failing or just individual circuits/outlets. We hadn't had this one yet.

Option 2 is where we'd like to get, but as Wfaulk points out, you need to be very careful of loading. The reason for this is because power-supplies can function in two modes:

  • Hot-failover. One PS will be drawing 100% of power for the server while the other is idle.
  • Load-balanced. The power-supplies will be drawing 50% (or 33%) of power for the server.

What this means is that each power run will have an unpredictable amount of load on it ranging from 0 to 100% of the room's load (or in your case, rack). Each power run needs to be able to supply 100% of load. In the event of a mains outage, unbalanced load will mean that one of your two power paths will fail first (the one with more load, obviously) which will force all the devices to draw 100% from the remaining UPS. This needs to be engineered in from the beginning.

In a rack situation, Option 2 takes more rack-space, but does provide better protection. In my experience, Mains failures tend to take out all of your power circuits not just a single one (barring some unobservant person flipping the wrong breaker back at the panel), so the added protection is small. But running two UPSes able to handle 100% load means that your stuff will stay up longer in the event of a mains failure.

  • You have certainly raised an important point about the ATS being a single point of failure. Thank you for your detailed and insightful reply! – BOBA_FETT_81 Apr 10 '11 at 13:23

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