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Computer Power Protection

Even the best power supply is helpless without a source of reliable, steady AC power. Simply plugging your system into a wall receptacle and hoping for the best is a sure road to disaster, sooner or later. Before we smartened up, we lost many hours' work to power failures, and more than one system to lightning damage. All of that was preventable, if only we'd installed proper power protection. There are two types of power protection.

Passive power protection

Passive power protection defends your system against spikes and other power anomalies that might damage the system or cause it to hang, but does nothing to protect against power failures. The most common form of passive power protection is the familiar surge suppressor outlet strip.

Active power protection

Active power protection provides backup power to allow the system to continue running when utility power fails. The most common form of active power protection is a battery-backed backup power supply. Most active power protection devices also provide at least minimal passive protection.

In this section, we'll take a brief look at both types of protection.

Passive power protection

The best first step in protecting your computer from surges, spikes, and other garbage on the utility power line is to install some form of passive power protection. There is a bewildering array of passive power protection devices available, from the $5 outlet strips sold by hardware stores to $500 power conditioners sold by specialty vendors. As you might expect, the more expensive devices are superior in reliability, the level of protection they provide, and their ability to withstand damage.

You don't need to spend $500 on passive power protection, but we do recommend using high-quality surge protectors on all your systems. Stick with high-end models from APC (http://www.apc.com), Belkin (http://www.belkin.com), or Tripp Lite (http://www.tripplite.com), and you won't go far wrong. Plan to spend at least $40 to $50 for a high-quality surge protector with basic AC protection, and as much as $100 for one of similar quality with additional features such as video and broadband Internet ports. Figure 16-17 shows the $90 Tripp Lite HT10DBS surge protector, which is designed for home theater systems, but is equally at home protecting our computer room.

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Figure 16-17: The Tripp Lite HT10DBS surge protector (image courtesy of Tripp Lite)

Active power protection

For a corporation, active power protection can mean anything up to standby generators and alternative power grids. In a home or SOHO environment, though, active power protection means a backup power supply.


BPS types

All BPSs have three common elements: a battery, which stores electrical energy against power failures; an inverter, which converts DC voltage supplied by the battery to the AC voltage required by the load; and charging circuitry, which converts AC mains power to the DC voltage required to charge the battery. IEEE recognizes three categories of BPS:


An online UPS (often called a true UPS or a dual-conversion UPS, to differentiate it from an SPS) connects the load directly to the inverter, which converts DC voltage supplied by the battery to standard AC voltage. The charging circuitry charges the battery constantly while the UPS is operating, and the equipment always runs from battery power supplied by the inverter. Online UPSs cost more than SPSs, described shortly, but have two advantages. Because the PC runs on battery power all the time, there is no switch-over time, and no switch to fail. Also, because the PC does not connect to mains power, it is effectively isolated from AC line problems.


A line-interactive UPS, also called a single-conversion online UPS, differs from an online UPS in that the load normally runs primarily from utility power as long as that power is available. Rather than convert utility power to DC, use it to charge the battery, and then reconvert it to AC for the load (the "dual-conversion" part), a line-interactive UPS feeds utility power directly to the load under normal conditions. Minor variations in utility power are smoothed out by the inverter using battery power. The defining characteristics of a line-interactive UPS are that the inverter runs at all times, and that the load is always dynamically shared between inverter and utility power. During routine operation, utility power may support 99% of the load and the inverter only 1%. During a brownout, the inverter may support 10% or more of the load. Only during a blackout does the inverter assume 100% of the load. A true line-interactive UPS has no switch-over time, because the inverter and utility power dynamically share the load at all times, so a power failure simply means that the inverter instantaneously assumes 100% of the load. Although line-interactive units do not isolate the load from the AC line to the extent that an online UPS does, they are quite good at maintaining clean, steady AC to the load. Line-interactive UPSs are common in data centers, but uncommon in the PC environment.


The most common form of BPS used with PCs is an offline power supply, sometimes called a standby power supply (SPS). BPS marketers dislike "standby" and downright hate "offline," so offline power supplies are always described as "uninterruptable" power supplies, which they are not. The defining characteristics of an SPS are that it has a switch and that the inverter is not always running. During normal operation, the switch routes utility power directly to the load. When utility power fails, that switch quickly disconnects the load from the utility power and reconnects it to the inverter, which continues to power the equipment from battery. SPSs are less expensive than online and line-interactive units, because they can use a relatively inexpensive inverter, one rated for low duty cycle and short run time.

Unlike online and line-interactive units, SPSs do not condition or regenerate incoming AC before supplying it to the load. Instead, they pass utility AC power through a passive filter similar to an ordinary surge suppressor, which means that SPSs do not provide power as clean as that provided by online and line-interactive units. In theory, SPSs have another drawback relative to online and line-interactive units. Actual switching time may be considerably longer than nominal under extended low-voltage conditions and with partially depleted batteries. Because the hold-up time of a PC power supply decreases under marginal low-voltage conditions, in theory an SPS may require longer to switch than the hold-up time of the PC power supply, resulting in a system crash. In practice, good SPSs have typical switching times of 2 to 4 ms and maximum switching times of 10 ms or less, and good PC power supplies have hold-up times of 20 ms or longer at nominal voltage and 15 ms or longer during sustained marginal under-voltage conditions, which means this is seldom a problem. Two types of SPS are common:

Standard SPS

A standard SPS has only two modes full utility power or full battery power. As long as utility power is within threshold voltage limits (which can be set on many units), the SPS simply passes utility power to the equipment. When utility power dips beneath threshold, the SPS transfers the load from using 100% utility power to using 100% battery power. Some standard SPSs also transfer to battery when utility voltage exceeds an upper threshold. That means that the SPS switches to battery every time a surge, sag, or brownout occurs, which may be quite frequently. This all-or-nothing approach cycles the battery frequently, which reduces battery life. More important, frequent alarms for minor power problems cause many people to turn off the alarm, which may delay recognition of an actual outage so long that the battery runs down and work is lost. Most entry-level SPS models are standard SPSs. The American Power Conversion (APC) Back-UPS series, for example, are standard SPSs.

Line-boost SPS

A line-boost SPS adds line-boost mode to the two modes of the standard SPS. A line-boost SPS is sometimes advertised as a line-interactive UPS, which it is not. Unlike line-interactive units, which use battery power to raise AC output voltage to nominal, line-boost units simply have an extra transformer tap, which they use to increase output voltage by a fixed percentage (typically, 12% to 15%) when input voltage falls below threshold. For example, when AC input falls to 100VAC, a line-interactive unit uses battery power to raise it 15V to 115VAC nominal. For 95VAC input, the line-interactive unit raises it 20V to 115VAC nominal. For 100VAC input, a line-boost unit uses the extra tap to raise output voltage by the fixed percentage (we'll assume 12%), yielding 112VAC output. For 95VAC input, the line-boost unit raises it by the same fixed percentage, in this case to 106.4VAC. That means that output voltage follows input voltage for line-boost units, with the resulting transients and current surges on the load side as the inverter kicks in and out. Most midrange and high-end PC SPS models are lineboost SPSs. The American Power Conversion (APC) Back-UPS Pro and Smart-UPS series, for example, are line-boost SPSs.

BPS characteristics

Here are the most important characteristics of a BPS:

Volt-Ampere (VA) rating

The VA rating of a BPS specifies the maximum power the unit can supply, and is determined by the capacity of the inverter. VA rating is the product of nominal AC output voltage and the maximum amperage rating of the inverter. For example, a 120V 650VA unit can supply about 5.4A (650VA/120V). Connecting a load greater than the amperage rating of the inverter overloads the inverter, and soon destroys it, unless the BPS has current-limiting circuitry. Watts equal VA only for 100% resistive loads (for example, a light bulb). If the load includes capacitive or inductive components, as do PC power supplies, the draw in VA is equal to the wattage divided by the Power Factor (PF) of the load. Non-PFC PC power supplies typically have Power Factors of 0.65 to 0.7. For example, one of our SPSs is rated at 1000VA but only 670W, which means that the manufacturer assumes a PF of 0.67 when rating wattage for this unit.

Run time

The run time of a BPS is determined by many factors, including battery type and condition, amp-hour capacity; state of charge; ambient temperature; inverter efficiency; and percentage load. Of those, percentage load is most variable. The number of amp-hours a battery can supply depends on how many amps you draw from it, which means that the relationship between load and run time is not linear. For example, a 600VA SPS may be able to supply 600VA for 5 minutes, but 300VA (half the load) for 20 minutes (four times longer). Doubling load cuts run time by much more than half; halving load extends run time by much more than twice.

Output waveform

Utility AC voltage is nominally a pure sine waveform, which is what power supplies and other equipment are designed to use. The output waveform generated by BPSs varies. In order of increasing desirability (and price), output waveforms include: square wave, sawtooth wave, modified square wave (often somewhat deceptively called near sine wave, stepped approximation to sine wave, modified sine wave, or stepped sine wave marketers are desperate to get the words "sine wave" in there, especially for units that don't deserve it). The cheapest units generate square wave output, which is essentially bipolar DC voltage with near-zero rise time and fall time, which allows it to masquerade as AC. Midrange units normally provide pseudo sine wave output, which may be anything from a very close approximation to a sine wave to something not much better than an unmodified square wave. The output waveform is determined by the inverter. The inverter is he most expensive component of a BPS. Better inverters those that generate a sine wave or a close approximation are more expensive, so the quality of the output waveform generally correlates closely to unit price. Astonishingly, we once saw specifications for a no-name BPS that listed output waveform as "pure square wave," presumably intending to confound buyers with "pure" (a Good Thing) and "square wave" (a Bad Thing).

Choosing a BPS

Use the following guidelines when choosing a BPS:

Select BPS type according to your needs and budget.

Nowadays, you can buy $40 BPSs at big-box stores. They're not very good BPSs, true, but they're better than nothing. When new, a cheap BPS may give you only a minute or two of run time; as the battery ages, the run time may drop to only a few seconds. Still, the vast majority of power outages last for one second or less, so even a five-second run time provides some protection.

The next step up is a consumer-grade SPS, such as one of the APC Back-UPS or Back-UPS Pro units. These units provide much better protection and much longer run times than the low-end units. We consider these units to be the minimum for "serious" power protection, and use them on some of our secondary systems. Better still are the line-boost units, such as the APC Smart-UPS and the Falcon Electric (http://www.falconups.com) SMP and SUP series, which we consider the minimum acceptable for important systems. Finally, there are the true UPSs, such as the Falcon Electric SG and SSG series units, which we use on servers and primary desktop systems.

Pick a unit with adequate VA and run time.

You can calculate VA requirements by checking the maximum amperage listed on the PC power supply and on each other component the UPS will power. Total these maximum amperages and multiply by the nominal AC voltage to determine VA requirements. The problems with this method are that it is time-consuming and results in a much higher VA than you actually need. A better method is to use one of the sizing tools that most BPS makers provide on their web sites. For example, the APC UPS Selector (http://www.apc.com/sizing/selectors.cfm) allows you to specify your system configuration, the run time you need, and an allowance for growth. From that information, it returns a list of suitable APC models, with the estimated run times for each.

Consider buying one BPS for multiple PCs.

If you need to protect multiple PCs in close proximity, consider buying one larger unit rather than several inexpensive smaller units. The larger unit will probably cost less for the same cumulative VA and run time, and will likely provide superior features (such as line-boost and a better waveform).

Get the best waveform you can afford.

The very cheapest units provide square wave output, which PC power supplies can use for short periods without damage. However, running a computer on square wave power for extended periods stresses the power supply and may eventually damage it. Also, square wave units are entirely unsuitable for other electronic devices, which they can quickly damage. Buy a square wave unit only if the alternative is not being able to afford a BPS at all. For general use, buy a unit that provides simulated sine wave if you expect to run the PC for 10 minutes or less on backup power before shutting it down. Buy a true sine wave unit if you expect to run the PC for extended periods on backup power, or if you also plan to power equipment that is intolerant of pseudo sine wave power (such as some displays).

So, what do we actually use? For years, we used and recommended APC units exclusively. Then one of our APC Smart-UPS units failed prematurely. We wrote that off to bad luck. Then, a couple months later, a Back-UPS Pro failed. Then a Back-UPS. Then another Smart-UPS. These weren't battery failures, either, which we expect with any UPS. These were failures of the inverters or control circuitry, and all but one of the failed units was two years old or less.

Obviously, four failures isn't a statistical universe, even among the limited number of units we run, but it did give us pause. Then we began hearing from readers whose experiences were similar to our own. Like us, they'd used APC units for many years with no problems, and, like us, they'd recently begun experiencing a higher rate of premature failures with their newer APC units. Obviously, even that didn't prove anything, but we became very concerned.

Then one day we were talking to our friend and colleague, Jerry Pournelle, who for more than 20 years has written the BYTE "Chaos Manor" column. We told him of our concern with failure rates on the APC units. "Talk to Falcon Electric," said Jerry, "I've been using their UPSs for years. They're bulletproof. One of mine even got knocked over by an earthquake and never missed a beat."

We took Jerry at his word and ordered some Falcon Electric (http://www.falconups.com) units. After researching and testing them, we decided Jerry was right. Falcon Electric makes the best UPSs available, so we've standardized on them. Falcon's customer list is heavily skewed towards military, industrial, telecommunications, and medical organizations, which was no small factor in our decision. Those folks need rock-solid reliable power protection, and what's good enough for NATO, Lucent, and General Atomics is good enough for us.

Robert uses the Falcon Electric 2 kVA SG-series UPS Plus shown in Figure 16-18 to protect his entire office servers and desktop systems. These units resemble a standard mini-tower PC (including the cooling fans). The unit on the bottom you might have already guessed is an external battery bank that extends run time long enough to outlast about 95% of the power outages we're likely to suffer. Barbara uses a similar Falcon Electric online UPS in her office, and we run most of our secondary systems on Falcon Electric SMP and SUP series line-boost units. We've been through dozens of thunderstorms and several power outages since we converted to Falcon Electric units, and have never had the slightest glitch.

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Figure 16-18: A Falcon Electric SG Series 2 kVA online UPS with external battery bank

Computer Power Supplies and Protection

Hello! Nice article! But I have a question. Recently I have installed this povervoltage system http://hardware.eu/overvoltage-protectio... to protect my house devices from power surges and spikes. But I am worry it is not enough for my desktop computer. Can you tell me please do I need to buy UPS in addition to overvoltage system?

Scott Waschlerner - Responder

what are the recommended maintenance period for power protection devices and Cleaning / Tool Kit Items Required and Checking / Cleaning Methods to Use .System Diagnostic / Maintenance Tool(s) to Use .

wafaa amoun - Responder

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