alexis@dasys1.UUCP (Alexis Rosen) (08/28/88)
I need to put a Mac II file server on a UPS. There seem to be three major
classes of UPSs: <4ms, <1ms, and instantaneous. The question is, is there
any advantage to the <1ms or instantaneous models? The power supply in the
Mac II has so far proven itself to be amazingly stable. Power fluctuations
which blow out air conditioners and dim all the lights don't bother it in
the slightest (whereas various PC-AT clones tend to go up in smoke). On the
other hand, I don't want to push the power supply too far, no matter how good
it is.
So: given the Mac II's rock-solid power supply, what should I get?
And while I'm at it, how many watts for a Mac II w/8MB ram, 300 MB internal
CDC drive, monochrome monitor, and ethernet card? What if I use a 21-inch
greyscale monitor as well?
Thanks in advance
----
Alexis Rosen {allegra,philabs,cmcl2}!phri\
Writing from {harpo,cmcl2}!cucard!dasys1!alexis
The Big Electric Cat {portal,well,sun}!hoptoad/
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Best path: uunet!dasys1!alexisric@arizona.edu (Ric Anderson) (09/05/88)
Power Protection - Some thoughts from an end-user. I would like to start by saying I am NOT (repeat, NOT) an electrical engineer or a power systems person. I have a fair amount of experience from a victim's perspective with dirty power, and some attempts at cleaning it up. My experience is that there are three levels of power protection: A. Placebo (cutsie little things that turn off their neon lights when they can't handle the line jolts, but still leave your equipment open to damage). B. Single point solutions (Isolated attempts to protect an individual piece of equipment without regard to the total power environement). C. Serious protection for the total power environment. Type A protection (if you want to call it that) may still be a very cost effective solution, when coupled with a full replacement value insurance policy (making sure the insurance company accepts your placebo as "reasonable effort"), and faithful backups. Basically, you accept that the machine is going to get fried, and you just want to get another one when it does. Type B involves placing interruptors (devices that monitor voltage and/or current, and disconnect your equipment from the power line if things get out of hand), power filters (essentially transformers that deliver "clean power" over a wide range of voltages), and battery backup systems. Some caution has to be exercised in the selection of equipment, as some items interact adversly with each other, and with the power supplies in the protected equipment. If wandering voltages (not surges, just drifts and mild lows) are involved, a constant voltage transformer may meet the need. If the ability to "finish up and save a day's work" is important even if the lights go out, then a battery backup is in order. Short duration units (20 minutes or less) usually meet this need. Some of these Battery backup units (also called Uniterruptable Power Supplies or UPS (some are, some aren't)), also provide surge supression of one kind or another. Reaction time for the surge supression equipment, the amount of energy they can dissapate, and the amount of time for the batteries to "engage" are all important. In most cases (not all, but most) a battery unit that engages in 1/4 of a cycle (1/240 of a second with 60 cycle power) is totally adequate. Also, consider whether the equipment you are protecting requires a pure sine wave for input. Some battery units output a Square Wave or a Modified Square Wave (sometimes called a Modified Sine Wave) instead of the Pure Sine Wave delivered by the electric company. Square wave units are cheaper (generally) than their pure sine wave counterparts for a given size (wattage). The size of the unit depends on how much stuff you plan to hang on it. My MAC-II with a monitor and internal hard disk runs nicely on a 300 watt Battery Backup. If I were to add another disk, I would either have to plug it in the wall (bad trouble if the wall plug I pick isn't connected to the same ground as the rest of the system) or buy a bigger backup system. Another factor to consider is "in rush"; this is, loosely, the amount of instaneous current used by the equipment during startup or major component activation (like some laser printers do just before printing the first page in a batch). Last month I saw two backup units blown to bits when a laser printer cranked up while the units were running on batteries. The amp load of the laser printer was within the specs for the units, but the instantaneous load was not... Be generous in sizing your unit for future growth (mine was way too big for the dumb PC it originally guarded, but that was five years ago, and now it is barely adequate for my current system). Compare oranges to oranges; some vendors throw Kva (Kilo (=1000) volt-amps) or va (Volt-amps) around in their ads; others use watts. Va is easy: it is just voltage x amps (120 volts at 2 amps = 240va). Watts are a little more involved; for very loose rule-of-thumb approximations, figure 0.8 watts per Va (so my 240 Va above would be 300 watts). Type C protection begins by analyzing the entire operating environment, which includes the building you are in, its electrical attributes, and the people in it (static electricity is one thing a battery unit offers NO protection from). For those interested, the National Technical Information Service U. S. Department of Commerece Springfield, Va 22161 has a 100 page document called Federal Information Processing Standard (FIPS) 94 Guideline on Electrical Power for ADP Installations which discusses power conditioners, grounding (more is not better), life safety, static electricity control,... It makes very good reading. I don't have current pricing, but I recall it being under $15.00. Lastly, no matter what anyone tells you, no surge suppressor, battery backup unit, constant voltage transformer,... is worth anything if your location takes a lightening strike. If you want REAL lightening protection, forget type A and B solutions; "C" might get you there, if your luck is good that day (my opinion, and, as stated in the intro, a power specialist I am not). Hope some of this is useful out there in net land, Ric Disclaimer: The above are my opinions, and do not represent the University of Arizona in any way. Consult a power system professional before buying anything!!!! Ric Anderson Bitnet: Ric@Arizrvax Computer Center Internet: Ric@rvax.ccit.arizona.edu University of Arizona AT&T: (602) 621-6701 Tucson, Arizona 85715