Summary: UPS's (Uninterruptable Power Supplies)
Peter S. Shenkin
shenkin at cunixf.cc.columbia.edu
Fri Jul 27 02:11:57 AEST 1990
Now that I've gotten answers to my queries, they are more coherent if the
questions are phrased a bit differently; therefore this summary is
organized around questions that are a little bit different than the ones
I orginally asked.
This summary is longer than I'd like it to be, but the excerpts I've
included do add information not included in my four- or five-line summary
for each topic, plus they give my original sources of information.
In my excerpts I don't quote everyone who replied; but I hereby thank
all who replied.
==========================================================================
OVERALL SUMMARY:
A ballasted standby unit -- inverter normally off, but kicks in on failure,
with delay buffered by a ferroresonant transformer -- appears adequate for
all computational equipment. A pure standby unit (no transformer) will work
for some computers. True on-line -- inverter always on -- appears unnecessary.
==========================================================================
Question:
Of the three commonly available types of UPS -- standby (with ca 4ms
delay to battery startup following power failure), ballasted standby
(with ferroresonant transformer buffering this delay time), and true
on-line (inverter always on) -- which is the minimum level of protection
necessary for computational equipment?
Summary:
Standby works for most computers, but not always for other things like
modems; ballasted standby should always be adequate. It was also noted
that various machines (SparcStations and SGI PI's) seem immune to hardware
damage due to the power going out suddenly (as in yanking the plug). If
this is the case, then a UPS is a frill, rather than an important
insurance item.
Excerpts: [[ My interpolations are in brackets like this. -P. ]]
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From: seibel at zeno.mmwb.ucsf.edu
Organization: Computer Graphics Lab, UCSF
We have quite a variety of hardware here,
including about a dozen irises. We've had perhaps four or five power
outages in the last few years. The interesting thing is that the irises
have always come through with no damage or data loss. It looks to me
like SGI builds a pretty solid package, so if you're pressed for funds,
running on line power may not be that risky.
George Seibel
Dept of Pharmaceutical Chemistry
UCSF
seibel at cgl.ucsf.edu
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>From poage at sunny.ucdavis.edu Wed Jul 25 13:46:09 1990
We have a Sun 3/150 as our departmental computer. I frequently see the
lights flicker due to power transients; PCs reboot or hang, dumb
terminals go whacky, yet the Sun never seems to be affected. Overall I'm
very happy about this. In contrast, our old departmental computer (a
Z80 based S-100 bus system) was very sensitive and had to have a UPS.
--
Tom Poage, Clinical Engineering
Universiy of California, Davis, Medical Center, Sacramento, CA
poage at sunny.ucdavis.edu {...,ucbvax,uunet}!ucdavis!sunny!poage
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From: "David W. Abraham" <DWABRA at IBM.COM>
I have a PI 4D/25 for which I bought a UPS from a company named BTG in
Vienna, VA. (703)-556-6518. It is their model 1200VX quality 1 , which
is I believe 1200 VA, for $906.00, plus $210 for the RS-232 shutdown
software. It works fine, and is of the on-line sort as you have
described. More grist for the mill...
[[ At this price can it really be true on-line? -P. ]]
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From: guy at phy.duke.edu (Guy Metcalfe)
Sorry, no personal experiance to give, but in the oct and nov 1988
issues of Byte magizine there are 2 articles by Mark Waller on this
subject. I found them informative enough at the time to keep them
around in a box as reference.
Guy Metcalfe
Duke University Dept. of Physics guy at phy.duke.edu
& Center for Nonlinear Studies guy at physics.phy.duke.edu
Durham, N.C. 27706 guy%phy.duke.edu at cs.duke.edu
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From: karn at thumper.bellcore.com (Phil R. Karn)
Organization: Bell Communications Research, Inc
[[ He comments that standby + Ferroresonant has disadvantage that it is
large, massive and acoustically noisy, and that just standby is enough
for most things in his actual experience; true on-line clearly provides
the most consistent power signal. ]]
So, in general, SPS's [[ (standby inverter plus relay) ]]are just fine for
most computer equipment. And if you have more sensitive loads (modems, etc),
add a small ferroresonant transformer to protect them.
[[ I asked him by email if the above didn't mean that he simply built something
like the BEST unit himself; and if so, why not start with a BEST: ]]
No, it's not the same as buying a BEST UPS because you only need put
the ferroresonant transformer on the more sensitive loads; the switching
power supplies can be powered directly from the inverter. This minimizes
the size of the ferro that you have to buy, thus minimizing noise and
wasted power. Ferros are not like ordinary isolation transformers that
you already use; they operate in the saturated mode, so they are quite
accoustically noisy. They are also less efficient.
----------------------------------------------------------------------------
From: eho at clarity.princeton.edu (Eric Ho)
[[ Eric Ho recently also asked questions about UPSs, and summarized
to comp.sys.sun; look there for the excerpts he quoted. -P. ]]
Well, here we go for the summary. Basically, my impression is that Best
Power Technology is probably the way to go.
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From: poage at sunny.ucdavis.edu (Tom Poage)
I've been talking to some of our Senior Electronics Techs--they seem to
think that half of a cycle (about 8 ms) should be the high end cut off
for critical (in our case, medical) computer equipment. Your mileage
may vary, depending on the size of the capacitors in your power supply
versus the load being drawn. I'd say that the 8 ms is a worst-case
estimate--how paranoid do you want to be? However, they also say that
on-line UPS's are the most recent recommedation.
----------------------------------------------------------------------------
From: Florence GAUDRON <flg at litp.ibp.fr>
On the sparcstation, if you are quick enough, you can manually unplug
the machine and replug it without disturbing the machine, needless to
say, the sparcstation can support to be plugged behind any UPS
even the slower ones.
On the other hand, 4ms can be long for some supplies, some manufacturers
rate the switching time of the relay only. Befrore the relay trips, the
UPS has to decide that the power has gone out, it takes very sensitive
and carefully designed circuitry to sense that in 4ms, then you
have to add the relay tripping delay, that makes 8 ms.
After the relay has tripped, the power section of the UPS has to go to
full power, I have no figures, but it seems to me that this is the
main cause of the delays (ie you can add 10 2 40 ms).
The bottom line (for me)is: either you have a sensitive power supply
in your computer, in this case you need an UPS that never goes off line
(ie no static bypass switch, the UPS must support the full load all time)
or you have an insensitive power supply, in this case anything will do,
you have to select another criteria for choosing you UPS.
Hope this helps
Flo
==========================================================================
Question:
Why do you want the UPS to turn itself off after it brings the computer
down?
Summary:
After a power failure and bringing the computer down, the UPS batteries
could drain if the power didn't come back up soon. This would damage
the UPS (or at least the batteries), and in addition would leave the
computer with no protection should the power fail again shortly after it
came on again. Also, startup loads can be too high for the UPS to handle.
Note: I did ask Best whether their unit does this, and the answer is yes,
it is programmed to switch itself off two minutes after it signals the
computer to go down.
Excerpts: [[ My interpolations are in brackets like this. -P. ]]
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From: mk59200 at tut.fi (Kolkka Markku Olavi)
When your machine is "down" it's still ON and consuming power. Discharging
the accumulators in the UPS all the way may damage them, or at least
decrease their lifetime. Switching the power output off saves the
accumulators.
Markku Kolkka
mk59200 at tut.fi
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From: don at zl2tnm.gp.govt.nz (Don Stokes)
Probably not much of an issue on a small system, but on a big one, once
you have pulled power out from under a system, you want to bring it back
up again in a controlled fashion. The reason for this is that a startup
loads can be *much* higher than normal loads; some devices, eg large
disk drives, can pull something like ten times the normal running load.
Just loading power supply capacitors can pull surprisingly large
currents for a short time. An uncontrolled startup load on the
VAXcluster at GPO stands a pretty good chance of tripping the power
conditioner out (we don't have a UPS; 100kVA UPSs cost *real* money).
Don Stokes, ZL2TNM / / Home: don at zl2tnm.gp.govt.nz
Systems Programmer /GP/ Government Printing Office Work: don at gp.govt.nz
__________________/ /__Wellington, New Zealand_____or:_PSI%(5301)47000028::DON
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From: moe!paul%ppgbms at uunet.UU.NET (Paul Evan Matz)
In our application, we have to support power outages for long periods,
and many such outages in a single day (rural hospitals). Any
unattended machine's UPS should be able to shut itself off after the
machine has halted. Otherwise the battteries could conceivably be
totally discharged; In this condition, if the power comes up and the
machine reboots, the machine is totally vulnerable to power outages.
[[ Following is reply from email from me to him: -P. ]]
>I just called Elgar, and I think your prices are a bit out of date....
The IPS 400+600 is available for $525 from one of Elgar's distributors.
(Call Bob Roth, Manchester Equipment, 516-434-8700). Unisafe for
SunOS WAS $99, and is worth a bit less in my opinion.
> Also, these and their
>smaller units are standby units, not online units. This means that there is
>a gap -- they quoted 4ms -- between the time the power goes off and the UPS
>kicks on...
This gap (2-4msec, typical) doesn't bother a SPARCstation or 386i. A 3/60
seems to be sensative to a sneeze. I don't know about 3/50s.
>It also means that you get little or no power filtering or surge
>protection from the UPS.
I don't think this is true. Their spec sheet shows noise,
surge/transient protection for the UPS's output. I'm not sure what you
mean by "little", but assume it's not bad compared to nothing.
___________________________________________________________________
|Regards, PPG Biomedical Systems |
|Paul Matz One Campus Drive |
|914-741-4685 Pleasantville, NY. 10570 |
|{..}!uunet!philabs!ppgbms!paul ppgbms!paul at philabs.philips.com |
|___________________________________________________________________|
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==========================================================================
Question:
What about this UL-544 spec (low leakage current) that Clary says they meet?
Summary:
This has nothing to do with the UPS as a power supply; it has to do wit
the UPS as a device. It is a measure of how much current will leak to
the chassis from the hot wire under controlled conditions if the ground
wire is disconnected. This spec is used for equipment used for medical
and dental purposes.
Excerpts: [[ My interpolations are in brackets like this. -P. ]]
----------------------------------------------------------------------------
>From poage at sunny.ucdavis.edu Wed Jul 25 13:46:09 1990
The Standard for Medical and Dental Equipment, UL-544, recommends a
number of different limits for leakage current, depending on the
application. In general, medical and dental equipment are categorized
into patient care and non-patient equipment.
Leakage current, as far as your application is concerned, is defined as
that AC or DC current that "leaks" (via electromagnetic coupling
through insulation and air) from active ("hot") circuitry inside of the
device through the chassis and to ground through the third grounding
prong (since the ground prong is attached to the chassis). There are
also other definitions that don't apply to you.
The limit for non-patient equipment is 0.5 milliamperes for frequencies
up to one kilohertz; this happens to be about the perception threshold
in humans for 60 Hz sinusoidal current. The UL chassis-ground current
leakage limit for patient care equipment is 0.1 milliamperes.
...
I wouldn't worry about it.
[[ Following in response to email request from me for clarification. -P. ]]
To perform this kind of test, the tester is plugged into an electrical
outlet, the device power cord is plugged into the tester and a
measurement wire is connected from the tester to the device chassis.
The plug ground is then temporarily lifted (disconnected) and the
leakage measured as that current which flows from the device active
circuit (the hot wire) to the device chassis and into the tester wire
and its measurement circuitry. Some measurement levels are down to a
few microamperes. As you might imagine this requires some sensitive
circuits.
--
Tom Poage, Clinical Engineering
Universiy of California, Davis, Medical Center, Sacramento, CA
poage at sunny.ucdavis.edu {...,ucbvax,uunet}!ucdavis!sunny!poage
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****************************************************************************
THE END!
-P.
************************f*u*cn*rd*ths*u*cn*gt*a*gd*jb**************************
Peter S. Shenkin, Department of Chemistry, Barnard College, New York, NY 10027
(212)854-1418 shenkin at cunixc.cc.columbia.edu(Internet) shenkin at cunixc(Bitnet)
***"In scenic New York... where the third world is only a subway ride away."***
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