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Plugging a UPS into a UPS.

Posted on 2008-10-16
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We have a Liebert UPS that provides power to most of our building, including the Cold Room.  I would like to make it so that the Liebert unit is not a single point of failure, and the first option is to install some APC SmartUPS units in the Cold Room.  It would be setup so that the servers are plugged into the APC UPSs, and the APC UPSs would be running off the Liebert UPS.  The design would be that each server power supply would be plugged into a separate UPS; so that one UPS going out would not kill the server.  Right now, if the Liebert UPS goes out, we lose the servers.  Or, if the Liebert goes on Stand-By, we lose power protection for the servers.  Either way, that single point of failure concerns me.

I asked a Liebert Field Tech about this configuration and he said they do not recommend installing one UPS in series with another UPS.  He said a lot of people do this, but it can cause problems on the power lines.

I have considered bypassing the Liebert UPS altogether (for the servers), but that would mean we would not be on the Emergency Generator; which would mean we'd have to shut down the servers during extended power outages.  I'm not sure if we could insert the APC UPSs between the Auto Transfer Switch and the Liebert UPS; so that the Emer Gen would supply power during power outages.

Do any of you have any recommendations/thoughts?

Respectfully,

Dave Tedder
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Question by:DaveTedder
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Mark earned 2000 total points
ID: 22735041
You would need to bring a new electrical circuit from the Genset transfer switch distribution panel to plug the APC UPS's into, and bypass the liebert.
The tech was correct when telling you that anomalies can occur when a UPS is plugged into a UPS, and if you are concerned with your servers the prudent thing to do is to supply the best power supply to them with the redundancy of multiple UPS's backed up by a Generator.

http://nam-en.apc.com/cgi-bin/nam_en.cfg/php/enduser/std_adp.php?p_sid=uz6B5zsh&p_lva=&p_faqid=397
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by:aleghart
ID: 22735316
First thing to analyze is if you are making the situation worse.

What is the failure rate of the Liebert system?
What is the failure rate of the APC standalone UPS?

If the failure rate of the APC unit is the same or worse, you are not helping yourself, statistically.  You are introducing a greater likelihood of unplanned downtime.  Two is not always better than one.

If the APC failure rate is less:
1. why?  Address this with the Liebert rep.
2. The equipment is not tested in a daisy-chain configuration, so there are no meaningful measures of failure.  Simple math does not apply.

A failure of equipment would likely not be honored under warranty.  Damage to the Liebert or APC system would be at your cost.  Your insurance may not cover costs, because installation of equipment was contrary to manufacturers safety instructions and/or applicable local codes.

Keep this in mind:  Your UL or CSA tag has the caveats of specific application(s).  I.e., lamp cords rated at xAmps, in polarized outlets, no multiple cords.  Holiday lights chained together with no more than 3 strings or 150 feet.

Neither UPS system was certified to be used as a supplier of power to another UPS, nor to be installed chained to another UPS.  The certification becomes invalid.  Your commercial property lease may prohibit the installation.  Local code will probably prohibit this as well.  Your facility manager can call or AHJ (authority having jurisdiction) for clarification.

Another issue is with power quality.  If the first power controller does not put out pure sine wave AC ("modified" or "step-approximated"), then the second UPS will detect dirty power and run off batteries.  After discharge, the second UPS will shut down, even though the Liebert may have many hours or days of power remaining.

Even if the Liebert pumps out pure sine wave, we're back to the invalidation of the UL certification.

Property owners, building engineers, city/county inspectors, and fire marshals look for UL certification tags.  There's a reason for trusting that certification.
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