A1: What is this document
01: What is this document?
02: How is this document made
03: Who maintains this?
04: Where did this information come
05: How can I contribute?
06: How may this document be
07: Got anything else you'd like to add?
02: What is a UPS and how does is work?
01: What is a UPS?
02: How do
you pronounce " UPS" ?
03: Vendor X says that (description) is a UPS, is
04: Describe the types of UPS's?
05: How can a UPS help me?
What sort of stuff does a UPS do?
07: How long can equipment on a UPS keep
08: What is a " good" UPS?
09: Support contracts on UPS's.
10: Self maintenance tips.
11: Is a UPS a glorified power strip?
How important is the UPS output waveform?
03: UPS monitoring/shutdown
01: Can a UPS shut the computer down when power is low?
Can I write my own shutdown routines?
03: What freely distributable
solutions are there?
04: No UPS software works on my machines, what to
05: What other software is out there.
04: How big a UPS do I
01: How are UPS sizes determined?
02: What VA rating do I
03: How do I determine this?
04: What else should I consider?
05: Can I use an UPS with a laser printer?
06: What UPS sizes do you use on
05: Specific manufacturer's info.
01: What vendors are
02: UPS Hardware.
03: UPS Software only.
TOPIC: What is this document all
Q: What is this document?
A: This is a FAQ
document on Uninterruptable 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.
Note that most of this document is very
US-centric. The power
numbers, companies and services all emphasize US
needs. Sorry, but that's what I have to work with. All the
principles discussed here should be applicable just about
Q: How is this document made available?
A: Currently, its "
home" is comp.misc. It is also cross posted to
comp.sys.ibm.hardware, comp.sys.dec, comp.answers
news.answers. This posting is automated and will occur on or
the 10th of each month. If there are other groups to
which this document
should be posted, please let me know, but
if I post it to every group where
UPS questions get asked, that
would be a lot of groups. I'm open to
This document is also available via anonymous FTP. The
sits on navigator.jpl.nasa.gov (188.8.131.52) in pub/doc/faq
the file UPS.faq. It is also available via anonymous FTP
Q: Who maintains this?
A: Right now, this document is
writen by Nick Christenson
Q: Where did this information come
A: Thankfully, many people have rallied to my cry to fill in
many gaps in my original draft. This is now a group work,
although I claim
full responsibility for misstatements and
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
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: This document is copyright by the author.
You are encouraged
to distribute this document for any non-commercial
as long as the contents remain unchanged and a pointer to
up-to-date version is included.
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
This may reflect a bias toward or against certain brands,
functions, etc.. Please keep in mind that the suggestions,
names and functions here are by no means exhaustive, or even
necessarily applicable to your situation. Also, if you have
that is not in this document, please submit it to
the maintainer listed
above. If you submit information, please
say whether you'd like it to be
attributed to you or not. I am
more than glad to give credit to the fine
people who helped with
this document, but I want to respect the anonymity
people who would prefer it.
One more caveat: While the
principles of UPS design and
maintenance are likely to be fairly
universal, the power
figures in this FAQ are *very* US-centric. Sorry, but
they're the only numbers I have.
This was contributed almost entirely by some kind soul. I just
up a bit.
Blackout: Complete loss of power. Some literature considers
drop below about 80V to be a blackout as well since most
equipment will not operate below these levels.
Sag or Brownout:
Decrease in voltage levels which can last for
periods ranging from
fractions of a second to hours. Can be
caused by heavy equipment coming on
line such as shop tools,
elevators, compressors etc. Also occurs when
deliberately do this to cope with peak load times.
Spike: An instantaneous and tremendous increase in voltage often
caused by a direct lightning strike on a power line or when
after a blackout.
Surge: An substantial increase in voltage lasting a
of a second, often caused when high powered appliances such
air conditioners are switched off.
Electromagnetic Interference and Radio Frequency
Interference. Caused by,
inter alia, lightning, generators,
radio transmitters, industrial
MOV: Metal Oxide Varistors used to control spikes. These
common in Power Strips. If you see more than two, you likely
fairly decent Power Strip. They look like largish disk
Inverter: Circuitry that converts DC battery power
to AC power required by
most computer equipment.
Surge Protector: Circuitry consisting of
MOVs, capacitors, rod-core
inductors etc. for suppressing surges and spikes
in a power strip.
Line Conditioner: A transformer
that attempts to smooth out
fluctuations in input voltage to provide near
voltage or voltage waveform.
What is a UPS and how does is work?
Q: What is a UPS?
An Uninterruptable Power Source is a device that sits between
supply (e.g. a wall outlet) and a device (e.g. a computer)
undesired features of the power source (outages, sags,
harmonics, etc.) from the supply from adversely
affecting the performance
of the device.
Q: How do you pronounce " UPS" ?
pronounce it " ups" , but most of the literature seems to favor
" you pee
ess" , since they use " a UPS" instead of " an UPS" . This
try to follow the literature.
Q: Vendor X says that (fill in
description) is a UPS, but it's
different that what you describe above.
A: There really is no standard definition of what a UPS is.
Anything ranging from a 9 volt battery backup in a clock radio
building/compound wide backup generator has been called
a 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 temporarily operational when
changes to the
power they receive would otherwise interrupt their
Maintaining power to a minicomputer (like a VAX 11) is
the scope of this document. This FAQ deals with UPS equipment
that can be installed by a computer owner/administrator. If you
requirements that large, you need to talk to a qualified
Q: Can you give me some more information on this?
provided by Don Deal, my additions
are in [square brackets] )
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
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 directly from the power line, until power
After power failure, a battery powered inverter turns on to
continue supplying power. Batteries are charged, as necessary,
power is available. This type of supply is sometimes
called an " off-line"
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,
called the switchover time, varies by unit. While some
computers may be
able to tolerate long switchover times, your
mileage may vary. [ Some
articles in the trade press have claimed
that their testing shows that
modern PCs can withstand transfer
times of 100ms or more. Most UPS units
claim a transfer time to
battery of about 4ms. Note that even if a
computer can stay up
for 100ms, it doesn't mean that 100ms switchover is
can still be done to a computer or data on it even if it
Other features to look for in this class of supplies
filtering and/or other line conditioners. Since appliances
connected to the supply are basically connected directly from
line, SPSs provide relatively poor protection from
line noise, frequency
variations, line spikes, and brownouts.
[Some SPS's claim to have
surge/spike suppression circuitry as
well as transformers to " boost"
voltage without switching to
the battery if a modest voltage drop occurs.
An example is the
" APC Smart UPS" which claims it will switch to this
if voltage drops below 103V (from the normal expected 120V)
switches to battery only at 90V and below. This, it is claimed,
allows operation of the equipment indefinitely under brownout
as long as voltage does not drop below 90V. I have
not tested this, and
would be interested in independent data.
There are other vendors products
that make similar claims.]
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
power using a ferroresonant transformer. This transformer
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
Best claims that their inverter then goes online so quickly that
operating without any interruption in power. Other UPS
that the transition is less than seamless, but
then again it's not in
their best interest to promote Best's
Best has a sizable
part of the UPS market.
[ Note: According to some sources,
ferroresonant transformers in an
UPS system can interact with ferroresonant
transformers in your
equipment and produce unexpected results. The Moral:
before you buy. -npc ]
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
devices are increased cost, increased power consumption, and
heat generation. Despite the fact that the inverter in a " true"
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.
Note, though, that given the same quality inverter, you'd expect the
that runs least to last longest. ]
Q: How can it help me?
A: A UPS has internal batteries to guarantee that continuous power
provided to the equipment even if the power supply stops
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
1) Computer jobs don't stop because the power
2) Users not inconvenienced by computer shutting down.
Equipment does not incur the stress of another (hard)
Data isn't lost because a machine shut down without
doing a " sync" or
equivalent to flush cached or
real time data.
sort of stuff does a UPS do?
A: A UPS traditionally can perform the
1) Absorb relatively small power surges.
out noisy power sources.
3) Continue to provide power to equipment during
4) Provide power for some time after a blackout has
In addition, some UPS or UPS/software combinations provide
1) Automatic shutdown of equipment during long
2) Monitoring and logging of the status of the power
3) Display the Voltage/Current draw of the equipment.
Restart equipment after a long power outage.
5) Display the voltage
currently on the line.
6) Provide alarms on certain error conditions.
7) Provide short circuit protection.
Q: How long can
equipment on a UPS keep running after the power
A: How big a UPS
do you have and what kind of equipment does it
protect? For most typical
computer workstations, one might
have a UPS that was rated to keep the
machine alive through
a 15 minute power loss. If you need a machine to
hours without power should probably look at a more robust
power backup solution. Even if a UPS has a very small load,
it must still
operate it's DC (battery) to AC converter,
which costs power. A rough
extrapolation from APC's
documentation, leads me to guess that a 2000 VA
operate it's own converter (with no extra load) for just
hours. A 1250 VA UPS could run its converter for
about 5. These are *very*
rough guesses based on information
provided by one vendor for one
Q: Given the same vendor claims, how can I tell a "
UPS from a " poor" quality UPS?
A: Testing, testing,
testing. I can't emphasize this enough. There
are many good and bad units
out there that call themselves UPS's.
There are many good units that are
wrong for your situation.
Some properties you might
look for are:
1) Sinusoidal power output. In general, the closer the AC
of the UPS is to a sine wave, the better it is for your
Many UPS units, especially the cheaper ones, deviate a
deal from a sinusoidal output. Some of them generate
waves. Waveform effects are dealt with in section 2.12.
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 a 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
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 a UPS! Also, beware, I have heard of
advertising " Network UPS" monitoring where the network is
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
If you do have a UPS that
does not output a sinusoidal waveform,
some manufacturers *strongly* urge
you to not put a surge protector
between the UPS and the computer. The
surge protector might mistake
the non-sine waveform as a power surge and
try to send it to ground.
This could be bad for your UPS. I don't know if
this has happened
or not, but I wouldn't chance it.
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
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 a 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.
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
in and you've got full backups of the machines. Throw the
circuit breaker with the UPS on it to simulate and outage and
see how the
transition goes. Note that some UPS vendors suggest
that testing an UPS by
pulling the plug from the wall is *not*
a good idea (Tripp Lite is one of
them). These UPS units like
to have a good idea of what ground looks like.
It is likely that
unplugging just about any UPS for a short amount of time
not be too dangerous (don't take my word for it, though!), but
all cases, throwing a circuit breaker would be a better thing
It might be useful to install a GFI (Ground Fault Interrupter)
to facilitate this testing without having to pull the plug,
you don't have your UPS protected machines on
an isolated circuit (which
you probably should). These are the
sockets found in most modern kitchens
and bathrooms with a red
and a black button. You push the latter to cut
power and the
former to restore power.
Those UPS units that use
lead-acid batteries (that's most of
them, I'm told) do not have a battery
memory and should be run
dry as few times as possible. It's probably not a
to do this once on one UPS out of a largish batch to learn
much UPS time you can expect in a real power outage. Note:
depending on the manufacturer, UPS batters can be expected to
about 1 and 5 years before they ought to be replaced.
As a UPS gets
older, its battery life will become shorter. Of
course there's no way to
reliably test how long it is without
running the battery down and you don't
want to do that because
they have lead acid batteries. All of these are
good reasons to get a support contract for them that includes
periodic battery replacement. At the very least, you can figure
batteries will still be good at the end of the UPS
warranty figure, so
that's a good place to start guesswork.
Q: Isn't a 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
and some other circuitry.
It may also have a timer, thermometer or other
Q: How important is the UPS output waveform?
That's a good question, and one is worthy of some debate. One
thought holds that one should always run equipment on
approximation of sinusoidal input that one can, and that
harmonics which may either be interpreted as
signal if they get through a
power supply, or may actually damage
the equipment. Another school holds
that since almost all
computers use switching-type power supplies, which
power at or near the peaks of the waveforms, the shape of the
input power waveform is not important. Who's right? I don't
*opinion* is that sinusoidal output is worth the
extra money, especially
for on-line UPS systems that continually
provide their waveform to the
computer. Also, if you don't *know*
that your equipment has a
switching-type power supply, you might
want to think twice before buying a
low quality UPS.
TOPIC: UPS monitoring/shutdown
Q: If the power is out for a long time, I would
like to have my
computer automatically shut itself down gracefully before
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: Okay, how about
restarting the system for me once power returns?
A: Fewer software products
do this, but many do. Again, ask your
vendor. I do not know of any freely
distributable products that
will do this. It doesn't mean that they can't
be built, but
vendor software is cheap enough (usually) that it's probably
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 a
your computer. The UPS sends information along the serial line
as it goes. If you can decode which pins contain which information,
information is formatted and figure out what it wants to
hear from the
computer side, you're all set. Make sure you have
the right serial cable
and know how the pins map between DB9 and
DB25 as both your computer and
your UPS may take either.
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, firstname.lastname@example.org.
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
: > $PORT
# If we reach this point, we've lost
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
the pub/src/UPS directory as upsd.tar.Z. 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
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.
Another good example, that probably works
straight away for
SunOS 4.1.X machines using APC Back-UPS devices, is also
available on navigator for anonymous FTP in the pub/src/UPS
is pf.c. It was written by Ronald Florence
(email@example.com). It looks like
a nice framework for expansion
to other OS platforms and UPS
implementations. Give it a try.
Q: I can't find monitoring
software that will work on my configuration.
What should I do?
it seems you have a few choices:
1) Build your own. See item 03.02.
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
See item 05.01.
Q: What other software is
A: Software packages for UPS machines are getting more
Most provide some level of power and status monitoring, but
lately there are more GUI's, more interactive packages, SNMP
and even call-out paging. See the software section
05.03 for more info.
TOPIC: How big a UPS do I need?
Q: How are
the " sizes" of UPS's determined?
A: Typically, a UPS has a VA rating. The
VA rating is the maximum
number of Volts * Amps it can deliver. The VA
rating is not the
same as the power drain (in Watts) of the equipment.
are notoriously non-resistive. A typical PF (power factor:
Watts/VA) for workstations may be as low as 0.6, which means that
record a drain of 100 Watts, you need a UPS with a VA
rating of 167. Some
literature suggests that 0.7 may be a
good conversion factor, but this
will depend heavily on the
machine. WARNING: Don't take my word for it!
Note: Some UPS's
can continue to deliver power if the VA rating is
merely can't provide above their VA rating if the power
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*
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
1) Direct measurement. You can get equipment to
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
type people, they're likely to be able to do this.
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
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 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
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. There are some
examples in section 4.6, but the
information is probably worth
what you paid for it :-)
Q: Hmmm... seems like a tough thing to determine.
A: Yeah, it can be. It's
also very important. If you get a UPS
that's too big, then you've
overpaid, but your equipment can
survive a longer outage. If you get a UPS
that's too small,
then you could be in deep trouble. Therefore, I
you be conservative in buying these things, unfortunately,
Q: What else should I consider?
A: It would be nice to know how long your site's typical power
are. In some places, with nice weather and a flaky
power grid, the power is
almost never out for more than 5
minutes, but this could happen quite
frequently. In this case, you
may as well use a UPS with a VA rating close
to your equipment
rating with no extra batteries. If your area has longer
in the half hour or hour range, as is often the case in
thunderstorm country, you can either buy UPS's with multiples
of the VA
rating of the equipment, since oversizing a VA rating
for a UPS has the
effect of lengthening the amount of time your
equipment can stay up in
case of a power outage, or you can buy
additional battery units for a
smaller UPS. You can probably
get away with doing simple math to determine
how much longer a
larger UPS will keep your equipment running, but I
running a few tests before committing to a large purchase
order. Also, your UPS vendor will almost certainly be glad
to help you
size the equipment you need. If all else fails
and you guess wrong, or
move equipment to a location with
different power status, you may be
really, really glad if you
bought a UPS that can have additional battery
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
of a laser printer during startup (and during printing)
be stunned at what it pulls. All UPS manufacturers I know of
you not to do this.
Okay, I have to back down from this. I know APC,
just as an
example, now does rate some of their UPS units for use with
certain laser printers. Not that I think this is a good
you. In general, they are difficult to size and
rarely do they require the
same level of uptime as servers.
In any case, don't do this without
specific approval of
your UPS vendor.
Q: So, what sorts
of UPS sizes do you use on your equipment?A:
BIG DISCLAIMER. I disclaim everything about these
At best, they are very, very rough. Heck, I may be lying.
Don't trust them. Here they are anyway.
Note also, this is what the
equipment apparently PULLS, not
the UPS sizes that are on them. Generally,
I've been using
UPS's that are about 2X the VA ratings shown. At the
least, I would using UPS sized 1.5X the VA ratings here.
Sparc 2 with 3 600 MB disks, 1 200 MB disk, 1 exabyte 8200
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.
SPARC 2GX clone. 1 1.2 GB disk, 4
2.0 GB disks, 2 tape drives,
1 CD-ROM drive, " big" monitor.
Sparc 2 clone with 100W power supply, internal 424 disk, 16"
monitor, external 1 GB disk drive.
These are U.K. numbers, based on
240 V wall current. Most of
these VA numbers are very close to " American"
VA numbers, but
if my caveats weren't strong enough earlier... .
SGI Indigo R4400, 48 MB RAM, 1 GB int. disk, no graphics, no
SGI Indigo R4400 configured as above with 17"
Sony monitor plus
3 19" monochrome X terminals.
Another word of
warning, don't assume that power requirements
scale with compute power and
number of peripherals, ESPECIALLY
if they are different architectures.
Older equipment, CPU's,
disks, monitors, whatever almost universally
power than new equipment. For example, it seems that an
9000/425e with 1 internal 420 MB disk and 19" color monitor
lot more power than a much more modern and much faster
HP 9000/715 with an
internal 1.3 GB disk, CD-ROM drive and
more modern 19" color monitor.
Again, the moral is don't assume.