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:
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.
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
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 (220.127.116.11) 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)
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
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
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
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.
# 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)
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
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.
Sparc 2 with 3 600 MB disks, 1 200 MB disk, 1 exabyte 8200
tape drive, 19" color monitor.
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) 18.104.22.168.00
World Wide Phone: (401) 789-5735
Email: none known
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.
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
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
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)
ITT Power System Corp
I'd appreciate any information I can get on these.
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.