Which UPS for two server?

Ben Scott dragonhawk at gmail.com
Thu Dec 21 09:48:48 CST 2006

On 12/21/06, Alessandro FAGLIA <a.faglia at farmol.it> wrote:
> My concern is about redundant power suppliers. I think that during
> normal operations, only one of them is actually working, and in case the
> first breaks, the second takes over the faulty one and stars working.

  From my limited testing with our PE 2900, that is not correct.  It
appears that, during normal operations, the current draw is balanced
between the two power supplies.  If I yank the cord from one PS, then
the other PS of course picks up all the load.  Now, it may be that
there is a momentary increase in current draw over nominal when the
second PS has to pick up the full load.  My monitoring equipment is
not good enough to tell me that.

> If the total power is 1420W, I could go for the APC Smart-UPS 3000i,
> 2700 watt (probably the APC Smart-UPS 2200i, 1600 watt is enough,

  As Gordon Henderson points out, the specified figures for the
current draw of equipment are maximums.  (Dell has to quote the
maximum for safety reason.)  That means fully loaded (all disk drive
bays populated, all PCI slots populated, etc.).  It's rare to see that
load in practice.

  In particular, the biggest loads occur when the machine is first
powered on.  All the capacitors have to charge, resistance overcome,
and disk drive motors spun up.  Once the system is running, load drops

  You may also want to consider getting two UPSes, and plugging one PS
from each server into each UPS.  That way, your redundancy extends to
the UPSes as well.  I have had UPSes fail before (bringing the loads
down with them), so that's not just a theoretical case.

  If budget is tight, you can get two smaller capacity UPSes.  Unless
you need to cold start your servers during a power failure (see above
about start up draw), that will not be a problem.

-- Ben

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