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The technical info in the remainder of this section is edited from material supplied by David E. Wexelblat .
There are several levels of power protection available to the home computer user. I break this down into 4 levels; others may have different ways of classifying things. The levels are:
This is a bare minimum level of protection that any piece of expensive electronics should have. Note that this applies to more than just AC power; surge suppressors are available for (and should be used on) phone lines, and RS-232 and parallel connections (for use on long lines; generally not needed if the devices is colocated with the computer and all devices are protected from outside sources). Note also that all devices connected to your computer need to be protected; if you put a surge suppressor on your computer but not your printer, then a zap on the printer may take out the computer, too.
An important fact about surge suppressors is that they need to be replaced if they absorb a large surge. Besides fuses, most suppressors rely on on components called Metal-Oxide Varistors (or MOVs) for spike suppression, which degrade when they take a voltage hit. The problem with cheap suppressors is that they don't tell you when the MOV is cooked, so you can end up with no spike protection and a false sense of security --- better ones have an indicator.
You can buy surge suppressors at any Radio Shack; for better prices, go mail-order through Computer Shopper or some similar magazine. All of these are low-cost devices ($10-50).
My Tripp-Lite 1200 was typical of the better class of line conditioners --- a box with a good big soft-iron transformer and a couple of moby capacitors in it and no conductive path between the in and out sides. With one of these, you can laugh at brownouts and electrical storms. You can get for $139 or so by mail order. A fringe benefit of this little beauty is that if you accidentally pull your plug out of the wall you may find you actually have time to re-connect it before the machine notices (I did this once). But a true SPS or UPS is better.
Netter Trey McLendon has good things to say about Zero Surge conditioners. He says: "Our systems at work [...] have been protected for 2.5 years now through many a violent storm...one strike knocked [out] the MOV-type suppressors on a Mac dealer's training setup across the street from us. The Zero Surge just sort of buzzed when the surge came in, with no interruption whatsoever. The basic principle is this: ZS units slow down the surge with a network of passive elements and then sends it back out the neutral line, which is tied to ground _outside at the box_ by code. MOV units shunt the surge to ground _at the computer_, where it leaps across serial ports, network connections, etc. doing its deadly work."
Price vary widely, from $40-400, depending on the power rating and capabilities of the device. Mail-order from a reputable supply house is your best bet. Line conditioners typically don't need to be replaced after a surge; check to see if yours includes MOVs.
Note: these devices do not come on line until after the power fails, and have a certain amount of delay (typically some milliseconds) before they come on line. If the capacitors in your power supply are not large enough, the SPS may not cut in soon enough to prevent your computer from seeing the power failure.
Note also that many SPSs are marketed as Uninterruptable Power Supplies (see below). This is incorrect. Any device with a non-zero cutover time cannot be a true UPS. If the ad mentions a cutover time, it's an SPS, and not a UPS.
The price range for these devices (increasing with increasing peak load capacity and with decreasing cutover time) is $200-2000. An SPS will not need to be replaced after absorbing a large surge.
I have an SPS called an APC 400 that replaced the Tripp-Lite (which now protects my girlfriend's machine). The 400 is meant for light-duty use, and seems to be able to supply current for about 20 minutes. I do not recommend APCs for Unix users, however, for reasons I'll go into below.
Now, given this information, how does one decide what to get? For a system that runs unattended, like most Unix systems, it is best to have a device that provides both power holdover and a power failure signal. Hence, for a Unix system, a UPS or SPS with Unix monitoring software is the best choice.
If the vendor isn't secretive about interface specs, it's fairly simple to write your own daemon to monitor a serial port, and send init a SIGPWR signal when it sees a powerdown notification on the port. Freeware power-monitor demons are available for Linux.
Many UPS/SPS signal ports work by asserting a pin, so that one could use a modem-control serial port on the PC and wire this pin to "Carrier Detect" in order to monitor it. Some, like the APC "SmartUPS" series, actually conduct an ASCII dialog with the host through a serial line in order to accomplish the monitor functions.
Our recommendation for a production Unix environment is a configuration like the following:
An important question is "How do I know how big a UPS/SPS to get?" The watt rating of the UPS/SPS should be at least the sum of the peak ratings off all equipment connected to it (don't forget the console monitor). Power-supply marketroids tend to quote you capacities and formulas like "sum of VA ratings + 20%" which (surprise!) push you towards costlier hardware. Ignore them. If a watt rating is not given, watts = 0.75*VAmax.
One other consideration is that you typically shouldn't put a laser printer on a UPS --- toner heaters draw enough current to overload a UPS and cause a shutdown within seconds. The other thing is that you can't even put the laser printer on the same circuit with a UPS --- the heater kicks on every 20-30 seconds, and most UPSs will see the current draw as a brownout. So buy a separate line conditioner for the laser printer.
Finally, read the UPS's installation manual carefully if you're going to use it with other power-protection devices. Some UPSs don't like having surge suppressors between them and the equipment.
David personally recommends surge suppressors and line conditioners from Tripp-Lite (available both mail-order and retail), and UPSs from Best Power Technologies (Necedah, WI - 1-(800)-356-5737). I can enthusiastically second the TrippLite recommendation, but haven't dealt with Best Power at all.
Tripp-Lite has a whole range of products, from a $10 phone-line surge-suppressor, to line conditioners and SPSs with prces in the hundreds of dollars. They have a line of $50-80 line conditioners that are good for most peripherals (including your home stereo :->).
Best Power Technologies sells two lines of UPSs in the range for home systems. The older and more expensive FERRUPS line (which is what David has) has a smart interface, and very good filtering and surge-suppression capabilities. He says "I have a 1.15kVA FERRUPS for my home system, which is overkill with my current hardware (although it rode out a 45 minute power failure with nary a whisper - no reboot). In 1990, I paid ~$1600 for this device, and that has since gone up. They also sell a newer line of Fortress UPSs. These are better suited in price for home systems. I don't know much about them, as they were not available when I bought my UPS. I expect that this is what most people will want to consider, though. In addition, Best sells Check-UPS, a software package (in source form) for monitoring the UPS and shutting it down. I have found Best to be a good company to deal with, with competent, knowledgeable sales people (who will be able to help you pick the right device), and helpful, courteous, and responsive technical support."
I used to recommend American Power Conversion SPS/UPSs here, but I can't anymore. I didn't try monitoring my first one, which I'd inherited without docs. When it died and I bought a second one, I discovered that (1) the APC people are appalling assholes about releasing their cable-interface specs, and (2) apparently none of their stock cables maps the power-event signals to RS232 lines that Unix can readily read (they rely on the fact that Windows programs can hack a serial port's UART directly!).
To use an APC box on Linux you actually have to hand-build a custom cable. If you want to try this, see the UPS HOWTO at the Linux Documentation Project site.
On top of all this, my second APC SPS (the 420 PNP) turned out to be a lemon with a bad battery. At this point I'd have to say don't buy APC.
The UPS-HOWTO Suggests avoiding TrippLite SPSs for the same reasons you should avoid APC's; apparently they're twits about their interface info too. It recommends Best Power and Upsonic.
Other things to know:
A UPS should be wired directly to (or plugged directly into) the AC supply (i.e. a surge suppressor is neither required nor suggested between the wall and the UPS). In addition, a surge suppressor between the UPS and the equipment connected to it is redundant.