Uninteruptable Power Source FAQ

Uninteruptable Power Source FAQ

Post by Nick Christenson, Jet Propulsion Laborato » Sat, 12 Feb 1994 04:07:00

This is the 1.0 version of the Uninteruptable Power Souce FAQ (UPS.faq).
It is available via anonymous FTP from navigator.jpl.nasa.gov in
the pub/doc/faq directory.  

Many people have a been a great help in improving this document from
my first release.  I want to thank them most sincerely.

The one thing that I didn't get help on was which groups to post this to.
Right now I've settled on the following:


I plan to try to get it posted to comp.answers as well.

Are there any groups that should be added to the list above?  Are there
any groups that this is posted to where it is inappropriate?  

Here it is:
------------------cut here-------------
Uninteruptable Power Source (UPS) FAQ.
VERSION 1.0, February 10, 1994

        01: What is this document all about?
        02: What is an UPS and how does is work?
        03: UPS monitoring/shutdown software.
        04: How big an UPS do I need?
        05: Specific manufacturer's info.
        06: Acknowledgments

TOPIC:  What is this document all about?

Q:      What is this document?
A:      This is a prototype for a FAQ document on Uninteruptable Power
        Sources.  It is intended to provide a starting point for those
        people that want to find out what they are, what they do, and
        what's available.

Q:      How is this document made available?
A:      Well, we kinda need to decide this.  I suggest it be posted to
        the newsgroup comp.unix.admin, news.answers and comp.answers at
        regular intervals, say once a month.  It probably ought to be
        posted to other groups as well, but I don't know which ones.
        If I post it to every group where UPS questions get asked, that
        would be a lot of groups.

        This document is also available via anonymous FTP.  The master
        sits on navigator.jpl.nasa.gov ( in pub/doc/faq
        as the file UPS.FAQ.  It will probably be mirrored on other
        machines and contributed to rtfm.mit.edu.

Q:      Who maintains this?
A:      Right now, this document is maintained by Nick Christenson.  My
        preferred email address is n...@minotaur.jpl.nasa.gov, and I
        would like it very much if questions regarding this document
        could have the word "UPS or UPS FAQ" or some such in the Subject
        line.  Note:  I am maintaining this on my own time, so please
        don't be upset if it takes a while for me to respond to your

Q:      Where did this information come from?
A:      Thankfully, several people have rallied to my cry to fill in
        the many gaps in my original draft.  This is now the work of
        many people, although I claim full responsibility for
        misstatements and inaccuracies.  

Q:      How can I contribute?
A:      You should mail new information, corrections, suggestions, etc.
        to the current maintainer of this FAQ.  If you provide a suggestion,
        make sure you reference where the information is located in the
        document.  I guarantee that suggestions of the form "Change the
        word 'always' to 'almost always' in the part about surge suppression."
        will be ignored.

Q:      Are there any restrictions on distribution of this document?
A:      You are encouraged to freely distribute this document for any
        non-commercial purpose as long as the contents remain unchanged.

Q:      Got anything else you'd like to add?
A:      Yes, now that you mention it.  The people who contribute to this
        document can speak only about equipment they have experience with.
        This may reflect a bias toward or against certain brands, features,
        functions, etc..  Please keep in mind that the suggestions, brand
        names and functions here are by no means exhaustive, or even
        necessarily applicable to your situation.  Also, if you have
        information that is not in this document, please submit it to
        the maintainer listed above.

TOPIC:  What is an UPS and how does it work?

Q:      What is an UPS?
A:      An Uninteruptable Power Source is a device that sits between
        a power supply (e.g. a wall outlet) and a device (e.g. a computer)
        to prevent power outages from the supply from affecting the

Q:      Vendor X says that (fill in description) is an UPS, but it's
        different that what you describe above.  Who's right?
A:      There really is no standard definition of what an UPS is.  
        Anything ranging from a 9 volt battery backup in a clock radio
        to a building/compound wide backup generator has been called
        an UPS by someone.  The majority of this document refers to
        objects larger than a beer can and smaller than a desk that
        help devices remain functional when their power supply would
        otherwise interrupt their function.

        Maintaining power to a minicomputer (like a VAX 11) is beyond
        the scope of this document.  This FAQ deals with UPS equipment
        that can be installed by a computer owner/administrator  If you
        have requirements that large, you need to talk to a qualified

Q:      Can you give me some more information on this?
A:      (Kindly provided by Don Deal, Don.D...@oit.gatech.edu)

        The UPS industry is made up of many manufacturers, and there is
        a lack of standard terms within the industry.  I think this
        sometimes borders on deliberate misdirection. (It's a jungle out

        There are basically three different types of devices, all of
        which are occasionally passed off as UPSs.

        1. Standby power supply (SPS). In this type of supply, power is
        usually derived from directly from the power line, until power
        fails. After power failure, a battery powered inverter turns on
        to continue supplying power.  Batteries are charged, as necessary,
        when line power is available.  This type of supply is sometimes
        called an "offline" UPS.

        The quality and effectiveness of this class of devices varies
        considerably; however, they are generally quite a bit cheaper than
        "true" UPSs.  The time required for the inverter to come online,
        typically called the switchover time, varies by unit.  While some
        computers may be able to tolerate long switchover times, your
        mileage may vary.

        Other features to look for in this class of supplies is line
        filtering and/or other line conditioners. Since appliances
        connected to the supply are basically connected directly from
        the power line, SPSs provide relatively poor protection from
        line noise, frequency variations, line spikes, and brownouts.

        2. Hybrid UPS systems. I only know one vendor who sells them -
        Best Power, Inc. The theory behind these devices is fairly simple.
        When normal operating line power is present, the supply conditions
        power using a ferroresonant transformer. This transformer maintains
        a constant output voltage even with a varying input voltage and
        provides good protection against line noise.  The transformer also
        maintains a constant output voltage even with a varying input
        voltage and provides good protection against line noise.  The
        transformer also maintains output on its secondary briefly when
        a total outage occurs.  Best claims that their inverter then
        goes online so quickly that it is operating without any
        interruption in power.  Other UPS vendors maintain that the
        transition is less than seamless, but then again it's not in
        their best interest to promote Best's products.  Best has a
        sizable part of the UPS market.

        3. What I call "true" UPS systems, those supplies that continuously
        operate from an inverter. Obviously, there is no switchover time,
        and these supplies generally provide the best isolation from power
        line problems.  The disadvantages to these devices are increased
        cost, increased power consumption, and increased heat generation.  
        Despite the fact that the inverter in a "true" UPS is always on,
        the reliability of such units does not seem to be affected.  In
        fact, we have seen more failures in cheaper SPS units.

Q:      How can it help me?
A:      An UPS has internal batteries to guarantee that continuous power
        is provided to the equipment even if the power supply stops
        providing power.  Of course the UPS can provide power for a while,
        typically a few minutes, but that is often enough to ride out
        power company glitches or short outages.  
                1) Computer jobs don't stop because the power fails.
                2) Users not inconvenienced by computer shutting down.
                3) Equipment does not incur the stress of another (hard)
                   power cycle.

Q:      What sort of stuff does an UPS do?
A:      An UPS traditionally can perform the following functions:
                1) Absorb relatively small power surges.
                2) Smooth out noisy power sources.
                3) Continue to provide power to equipment during line sags.
                4) Provide power for some time after a blackout has occurred.
        In addition, some UPS or UPS/software combinations provide the
        following functions:
                1) Automatic shutdown of equipment during long power
                2) Monitoring and logging of the status of the power supply.
                3) Display the Voltage/Current draw of the equipment.
                4) Restart equipment after a long power outage.

Q:      How long can equipment on an UPS keep running after the power
A:      How big an UPS do you have and what kind of equipment does it
        protect?  If you put a clock radio or laptop computer on an UPS
        rated for 2000 Volt-Amps, you could probably run it for days.
        For most typical computer workstations, one might have an UPS
        that was rated to keep the machine alive through a 15 minute
        power loss.  If you need a machine to survive hours without
        power should probably look at a more powerful power backup

Q:      Given the same vendor claims, how can I tell a "good" quality
        UPS from a "poor" quality UPS?
A:      Some properties you might look for are:
        1) Sinusoidal power output.  In general, the closer the AC output
           of the UPS is to a sine wave, the better it is for your equipment.  
           There are rumors of UPS units that generate square waves.  This
           is not good.  
        2) Does the UPS have a manual bypass switch?  If the UPS is
           broken or is being serviced, can you pass power through it to
           your equipment?  The last thing you want is for a broken UPS to
           be the cause of extra downtime.
        3) The more information about an UPS's operation you can get from
           watching the unit itself, the better.  How much power (or
           percentage load) the equipment is drawing, how much battery
           life is left and indications of the input power quality are
           all very useful.
        4) Some newer UPS's can communicate with their monitoring software
           via network connection and SNMP!  This is wonderful *if* your
           network is on an UPS!  Also, beware, I have heard of dealers
           advertising "Network UPS" monitoring where the network is
           the normal serial connection (no SLIP or PPP).  
        5) Does the UPS vendor offer support/maintenance contracts.  If
           they don't even offer them, I would suspect the quality of the

Q:      Should I make sure I have a support/maintenance contract for my
        UPS systems?
A:      Some people strongly recommend this, but to be honest, I don't
        know how important it is.  I haven't had any UPS's long enough
        to have enough of them fail to know what the failure modes are
        likely to be.  Some people, with more experience than I in these
        matters, insist that an UPS support/maintenance contract is as
        important as your computer support/maintenance contract.  I can't
        argue with them.  In any case, it's almost certainly worth
        pricing at any rate.

Q:      What sort of maintenance can I perform myself?
A:      One good thing you might want to do is periodically test the
        UPS's and their failure modes.  A good time to do this might be
        right after after a periodic level 0 backup.  Nobody is logged
        in and you've got full backups of the machines.  Pull the plug
        on the UPS to simulate and outage and see how the transition
        goes.  Also, as far as I know, UPS batteries are susceptible to
        "battery memory" problems.  It probably wouldn't be a bad idea
        to run the battery down periodically to "erase the memory."  Maybe
        once every year or half year.  Note that, depending on the
        manufacturer, UPS batters can be expected to last between about
        1 and 5 years.  

Q:      Isn't an UPS just a glorified power strip/surge protector with
        some batteries and a little power conditioning thrown in?
A:      Basically.  It's also got a power inverter, some other circuitry.
        It may also have a timer and other gadgets.

TOPIC:  UPS monitoring/shutdown software.

Q:      If the power is out for a long time, I would like to have my
        computer automatically shut itself down gracefully before the
        UPS batteries die.  Can I do this?
A:      Yes.  Most UPS manufacturers support software that will do this
        for some UPS's on at least some platforms.  Ask your UPS vendor
        for details.

Q:      How does it work, I'm a starving (fill in the blank) and I really
        don't want to pay for software unless I absolutely have to.
A:      Usually, there is a serial connection running from an UPS into
        your computer.  The UPS sends information along the serial line
        as it goes.  If you can decode which pins contain which information,
        how the information is formatted and figure out what it wants to
        hear from the computer side, you're all set.

        Since UPS units with network based monitoring capabilities are
        appearing on the market, we can hopefully get something that will
        communicate with those units.

        Here is a skeleton script provided by Joe Moss, j...@morton.rain.com.
        Definitely check this out as a starting point, but don't expect
        it to do anything meaningful without some work.

        ---------start upsd.sh-------------
        #! /bin/sh

        # Shut down system in case of extended power failure

        # This should be the serial port to which the UPS is connected
        # This port must be set to block on open until the DCD line
        # is asserted - many UNIX systems have this determined by
        # the minor device number, if not, see if there is some way
        # to enable this behavior on your system

        # Ok, this should block until there is a power failure

        : > $PORT

        # If we reach this point, we've lost power
        wall << EOF
        The sky is falling!! The sky is falling!!

        # call shutdown (or init or whatever)
        exec shutdown

Q:      Hmmm... that sounds kinda complicated.  Has someone already done
A:      Any solution would almost certainly be vendor specific.  However,
        some brave souls have provided partial functionality for certain
        vendors' UPS's.  I don't know the original source, but I have a
        copy available for anonymous FTP at navigator.jpl.nasa.gov in
        the pub/src/upsd directory.  I haven't tried it and I don't
        honestly know if it even works.  

        Note:  Different UPS's produce different sorts of signals.  Just
        installing this already built package may require a great deal of
        work.  The cabling can be complicated, etc..  I would be
        interested in hearing where this software does/doesn't work.

Q:      I can't find monitoring software that will work on my configuration.
        What should I do?
A:      Well, it seems you have a few choices:
                1) Build your own.  See item 03.02.
                2) Use something freely distributable.  See item 03.03.
                3) Lean on your UPS vendor to port to your platform.
                4) Try a different vendor that supports your platform.
                   See item 05.01.

TOPIC:  How big an UPS do I need?

Q:      How are the "sizes" of UPS's determined?
A:      Typically, an UPS has a VA rating.  The VA rating is the maximum
        number of Volts X Amps it can deliver.  The VA rating is not the
        same as the power drain (in Watts) of the equipment.  Computers
        are notoriously non-resistive.  A typical PF (power factor:
        Watts/VA) for workstations may be as low as 0.6, which means that
        if you record a drain of 100 Watts, you need an UPS with a VA
        rating of 167.  WARNING:  Don't take my word for it.  Note:  
        Some UPS's can continue to deliver power if the VA rating is
        exceeded, they merely can't provide above their VA rating if the
        power goes.  Some can't provide power above their VA rating at all.  
        Some may do something really nasty if you try.  In any case, I
        *strongly* recommend not doing this under *any* circumstances.

Q:      How can I tell what VA rating I need for my equipment?
A:      First, when possible, get VA rather than wattage ratings.  See
        Q04.01 above.
        There are a couple of ways:
                1) Direct measurement.  You can get equipment to measure
                   the current draw of your equipment directly.  You may
                   or may not have access to this.  If you are part of an
                   organization that has it's own facilities/electrical
                   type people, they're likely to be able to do this.  They
                   might help you out if you ask nice.
                2) Compare notes.  If you know someone with the same setup
                   you're using, ask them what they use and how close they
                   are to the maximum VA rating.
                3) Use a chart.  Most vendors can help you out for common
                   equipment.  If you have an unusual setup, or a mix
                   vendors a lot, you're probably out of luck here.
                4) Use the equipment rating.  Most pieces of computer
                   equipment have a power rating on some back panel.  This
                   number is usually high, as it is necessary for the
                   manufacturer to play it safe or they'll get sued.

        Note:  Method 1 is by far the best, method 2 and 3 are secondary,
        method 4 is usually overkill, but pretty safe.

Q:      Hmmm... seems like a tough thing to determine.
A:      Yeah, it can be.  It's also very important.  If you get an UPS
        that's too big, then your equipment can survive a longer outage.
        If you get an UPS that's too small, then you could be in deep
        trouble.  Therefore, I recommend that you be conservative in
        buying these things, unfortunately, this costs money.

Q:      How about I use one of these UPS thingies for a laser printer?
A:      Don't *ever* do this.  If you ever measured the current draw
        of a laser printer during startup (and during printing) you'd
        be stunned at what it pulls.  All UPS manufacturers I know of
        tell you not to do this.

Q:      So, what sorts of UPS sizes do you use on your equipment?
A:      BIG DISCLAIMER.  I disclaim everything about these figures.  
        I may be lying.  Don't trust them.  Here they are anyway.

        400 VA:
        Sparc 2 with 3 600 MB disks, 1 200 MB disk, 1 exabyte 8200
        tape drive, 19" color monitor.

        600 VA:
        HP 750 with 4 1.3 GB disks, internal 4mm tape drive and internal
        CD-ROM drive, external disk cabinet and 19" color monitor.

TOPIC:  Specific manufacturer's information.

Q:      What vendors are there and what do they produce?
A:      Here is a very incomplete list, based only on what I know.
        Please give me information to expand it.

        Company:                APC, American Power Conversion
        US Address:             132 Fairgrounds Road
                                P.O. Box 278
                                West Kingston, RI 02892
        FR Address:             4, rue Ste Claire Deville
                                Zac du Mandinet-Batiment Espace
                                77447 MARNE LA VALLEE Cedex 2
        US & CAN Phone:             1-800-800-4272
        Europe Phone:           (+33)
        World Wide Phone:       (401) 789-5735
        Email:                  none known

        UPS Products:
                Smart UPS in sizes up to 2000 VA.  You can add battery
                packs to cover up to 20000 VA.  The Smart UPS's do
                monitoring and can shutdown multiple machines using the
                PowerChute software.  I recommend putting these on
                Back UPS same as Smart UPS but without the monitoring
                and multiple machine stuff.  I recommend putting these
                on dumb equipment like network equipment, X Terminals
                and Macintoshes (sorry, I couldn't resist.)    
                Matrix UPS a modular "fault-tolerant" system.  Any
                module can be "hot-swapped" at any time.  Also
                additional battery modules can be added, again, while
                the system is running.  Includes SNMP support.
                PowerChute, PowerChute PLUS.  They produce it themselves.  
                Supported on:  SunOS, HP-UX, SCO, AIX, AT&T UNIX,
                Interactive UNIX, XENIX, and probably others by now.
        Contributed by:
                APC information contributed by Nick Christenson,
                n...@minotaur.jpl.nasa.gov without consultation with
                APC.  Additional information provided by Joe Moss,
                j...@morton.rain.com.  I have no affiliation with APC
                except as a satisfied customer.

        Company:                Best Power Technology, Inc.
                                P.O. Box 280
                                Necedah, WI 54646-9899
        US Phone:               1-800-356-5794
        Email:                  None known

        UPS Products:
        FERRUPS:  Ferroresonant-Based, Line-Interactive UPS, sizes
                from 500 VA - 18 KVA.  
                Features:  Standard power features, serial line
                communications, runtime monitoring, logging,
                automatic shutdown with optional software, user
        FORTRESS:  Advanced, line-Interactive UPS, sizes from 360
                VA - 2 KVA.
        PATRIOT:  Low-Cost Standby Power Systems, 250 VA - 850 VA.

        Contributed by:  Scott Pinkerton, spink...@t4rta-gw.den.mmc.com

        Company:                Data General

        UPS Products:
                Data General repackages another vendor's UPS's (from
                Exide?) with some sort of special cable.  They deserve
                some mention since they provide UPS monitoring software
                built in to the AViiON (their UN*X boxen) line.  It can
                be managed through sysadm(1M).

        Contributed by:  Morris Galloway Jr., mmg...@presby.edu

        Other companies:  Emerson (also manufactures Liebert equipment)
                          Sola Electric
                          ITT Power System Corp

                I'd appreciate any information I can get on these.

TOPIC:  Acknowledgements

        I would like to thank Charles Rhoades (c...@zeus.jpl.nasa.gov) for
        his sage remarks on my draft of this document.
        I would like to thank Kevin R. Ray (ke...@kray.com) for sending me
        the freely distributable upsd software.
        Thanks also to Don Deal (Don.D...@oit.gatech.edu) for a great
        many valuable suggestions and that great section on the types
        of UPS units.
        The following people made valuable suggestions to this document:
                Scott Pinkerton, spink...@t4rta-gw.den.mmc.com
                Morris Galloway Jr., mmg...@presby.edu
                David E A Wilson, da...@cs.uow.edu.au
                Edward Hartnett, e...@larry.gsfc.nasa.gov
                Joe Moss, j...@morton.rain.com

        Please note that I take full blame for any errors or omissions.
Nick Christenson