shenkin@cunixf.cc.columbia.edu (Peter S. Shenkin) (07/26/90)
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. Q: 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? A: 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. ]] X-From: seibel@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. X-From: poage@sunny.ucavis.edu (Tom Poage) 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. X-From: "David W. Abraham" <DWABRA@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. ]] X-From: guy@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. X-From: karn@thumper.bellcore.com (Phil R. Karn) [[ 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. X-From: eho@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. ]] Q; Why do you want the UPS to turn itself off after it brings the computer down? A: 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. ]] X-From: mk59200@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. X-From: don@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). X-From: moe!paul%ppgbms@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. Q: What about this UL-544 spec (low leakage current) that Clary says they meet? A: This has nothing to do with the UPS as a power supply; it has to do with 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. ]] X-From: poage@sunny.ucdavis.edu (Tom Poage) 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. Peter S. Shenkin, Department of Chemistry, Barnard College, New York, NY 10027 (212)854-1418 shenkin@cunixc.cc.columbia.edu(Internet) shenkin@cunixc(Bitnet)