|[ Team LiB ]|
Normal line voltage is often far from the 115-volt alternating current you pay for. It can be a rather inhospitable mixture of aberrations such as spikes and surges mixed with noise, dips, and interruptions. None of these oddities is desirable, and some can be powerful enough to cause errors in your data or damage to your computer. Although you cannot avoid them, you can protect your computer against their ill effects.
Power Line Irregularities
The deadliest power-line pollution is over-voltage—lightning-like high potential spikes that sneak into your computer and actually melt down its silicon circuitry. Often the damage is invisible—except for the very visible lack of image on your monitor. Other times, you can actually see charred remains inside your computer as a result of the over-voltage.
As its name implies, an over-voltage gushes more voltage into your computer than the equipment can handle. In general—and in the long run—your utility supplies power that's very close to the ideal, usually within about ten percent of its rated value. If it always stayed within that range, the internal voltage regulation circuitry of your computer could take its fluctuations in stride.
Short duration over-voltages larger than that may occur too quickly for your utility's equipment to compensate, however. Moreover, many over-voltages are generated nearby, possibly within your home or office, and your utility has no control over them. Brief peaks as high as 25,000 volts have been measured on normal lines, usually due to nearby lightning strikes. Lightning doesn't have to hit a power line to induce a voltage spike that can damage your computer. When it does hit a wire, however, everything connected to that circuit is likely to take on the characteristics of a flash bulb.
Over-voltages are usually divided into two classes by duration. Short-lived over-voltages are called spikes or transients and last from a nanosecond (billionth of a second) to a microsecond (one millionth of a second). Longer-duration over-voltages are usually termed surges and can stretch into milliseconds.
Sometimes power companies do make errors and send too much voltage down the line, causing your lights to glow brighter and your computer to teeter closer to disaster. The occurrences are simply termed over-voltages.
Most AC-power computers are designed to withstand moderate over-voltages without damage. Most machines tolerate brief surges in the range of 800 to 2,000 volts. On the other hand, power cords and normal home and office electrical wiring break (by arcing between the wiring conductors) at potentials between about 4,000 and 6,000 volts. In other words, electrical wiring limits the maximum surge potential your computer is likely to face to no more than about 6,000 volts. Higher voltage surges simply can't reach your computer.
Besides intensity and energy, surges also differ in their mode. Modern electrical wiring involves three conductors: a hot, neutral, and ground. Hot is the wire that carries the power; neutral provides a return path; and ground provides protection. The ground lead is ostensibly connected directly to the earth.
A surge can occur between any pairing of conductors: hot and neutral, hot and ground, or neutral and ground. The first pairing is termed normal mode. It reflects a voltage difference between the power conductors used by your computer. When a surge arises from a voltage difference between hot or neutral and ground, it is called common mode.
Surges caused by utility switching and natural phenomena—for the most part, lightning—occur in the normal mode. They have to. The National Electrical Code requires that the neutral lead and the ground lead be bonded together at the service entrance (where utility power enters a building) as well as at the utility line transformer, typically hanging from a telephone pole near your home or office. At that point, neutral and ground must have the same potential. Any external common mode surge becomes normal mode.
Common mode surges can, however, originate within a building because long runs of wire stretch between most outlets and the service entrance, and the resistance of the wire allows the potential on the neutral wire to drift from that of ground. Although opinions differ, recent European studies suggest that common mode surges are the most dangerous to your equipment. (European wiring practice is more likely to result in common mode surges because the bonding of neutral and ground is made only at the transformer.)
An under-voltage occurs when your equipment gets less voltage than it expects. Under-voltages can range from sags, which are dips of but a few volts, to complete outages or blackouts. Durations vary from nearly instantaneous to hours—or even days, if you haven't paid your light bill recently.
Very short dips, sags, and even blackouts are not a problem. As long as they are less than a few dozen milliseconds—about the blink of an eye—your computer should purr along as if nothing happened. The only exceptions are a few old computers that have power supplies with very sensitive Power Good signals. A short blackout may switch off the Power Good signal, shutting down your computer even though enough electricity is available.
Most computers are designed to withstand prolonged voltage dips of about 20 percent without shutting down. Deeper dips or blackouts lasting for more than those few milliseconds result in a shutdown. Your computer is forced to cold start, booting up afresh. Any work you have not saved before the under-voltage is lost.
Noise is a nagging problem in the power supplies of most electronic devices. It comprises all the spurious signals that wires pick up as they run through electromagnetic fields. In many cases, these signals can sneak through the filtering circuitry of the power supply and interfere with the signals inside the electrical device.
For example, the power cord of a tape recorder might act like an antenna and pick up a strong radio signal. The broadcast could then sneak through the circuitry of the recorder and mix with the music it is supposed to be playing. As a result, you might hear a CB radio maven croaking over your Mozart.
In computers, these spurious signals could confuse the digital thought coursing through the circuitry of the machine. As a practical matter, they don't. All better computers are designed to minimize the leakage of their signals from inside their cases into the outside world to minimize your computer's interfering with your radio and television. The same protection that prevents signals leaking out works well in warding off other signals from getting in. Personal computers are almost automatically well-protected against line noise. You probably won't need a noise filter to protect your computer.
Then again, noise filtering doesn't hurt. Most power-protection devices have noise filtering built in to them because it's cheap, and it can be an extra selling point (particularly to people who believe they need it). Think of it as a bonus. You can take advantage of its added protection—but don't go out of your way to get it.
Surges are dangerous to your computer because the energy they contain can rush through semiconductor circuits faster than the circuits can dissipate them—the silicon junctions of your computer's integrated circuits fry in microseconds. Spike and surge protectors are designed to prevent most short-duration, high-intensity over-voltages from reaching your computer. They absorb excess voltages before they can travel down the power line and into your computer's power supply. Surge suppressors are typically connected between the various conductors of the wiring leading to your computer. Most short out surges that rise above a preset level.
The most important characteristics of over-voltage protection devices are how fast they work and how much energy they can dissipate. Generally, a faster response time or clamping speed is better. Response times can be as short as picoseconds—trillionths of a second. You get better protection from devices that have higher energy-handling capacities, which are measured in watt-seconds or joules. Devices claiming the capability to handle millions of watts are not unusual.
Four kinds of devices are most often used to protect against surges: metal-oxide varistors (MOVs), gas tubes, avalanche diodes, and reactive circuits. Of these, the MOVs dominate because the parts are inexpensive and they work effectively against most surges.
The MOV is a disc-shaped electronic component typically made from a layer of zinc-oxide particles held between two electrodes. The granular zinc oxide offers a high resistance to the flow of electricity until the voltage reaches a break-over point. The electrical current then forms a low-resistance path between the zinc-oxide particles that shorts out the electrical flow. The energy-handling capability can be increased simply by enlarging the device (typical MOVs are about an inch in diameter; high-power MOVs may be twice that). Figure 31.1 shows a typical MOV.
The downside to MOVs is that they degrade. Surges tend to form preferred paths between the zinc-oxide particles, reducing the resistance to electrical flow. Eventually, the MOV shorts out, blowing a fuse or (more likely) overheating until it destroys itself. The MOV can end its life in flames or with no external change at all—except that it no longer offers surge protection.
Thanks to the laws of thermodynamics, the excess energy in a surge cannot just disappear; it can only change form. With most surge suppression technologies (all except reactive devices), the over-voltage is converted into heat that's dissipated by the wiring between the device and the origin of the surge as well as inside the surge suppressor itself. The power in a large surge can destroy a surge suppressor so that it yields up its life to protect your computer.
Because they degrade cumulatively with every surge they absorb, MOVs are particularly prone to failure as they age. Eventually, an MOV will fail, sometimes in its own lightning-like burst. Although it's unlikely this failure will electrically damage the circuits of your computer, it can cause a fire—which can damage not just your computer, but your home, office, or self.
An MOV-based surge suppressor also can fail more subtly—it just stops sucking up surges. Unbeknownst to you, your computer can be left unprotected. Many commercial surge suppressors have indicators designed to reveal the failure of an internal MOV.
In any case, a good strategy is to replace MOV-based surge suppressors periodically to ensure that they do their job and to lessen the likelihood of their failure. How often to replace them depends on how dirty an electrical diet you feed them, but most MOV-based surge protectors should work effectively for a few years before needing replacement.
Protecting against total blackouts requires a local source of electricity, either a generator or storage batteries. A local generator is the choice when you want to continue to work as normal, running from local power for hours or days. Battery backup lasts only as long as the batteries, typically a few minutes to allow the orderly shutdown of your computer so you can start back up when normal utility power returns.
Battery backup systems are often called uninterruptible power systems (UPSs) because they supply power continuously, without interruption. UPSs are often used in conjunction with generators to bridge over the few seconds that power would otherwise not be available while the generator is starting. A battery backup system is built around powerful batteries that store substantial current. An inverter converts the direct current from the batteries into alternating current that can be used by your computer. A battery charger built in to the system keeps the reserve power supply fully charged at all times. Long runtime UPSs, which have extra batteries, sometimes substitute for generators, keeping computer servers (particularly those in remote locations where they are not readily accessible) running for hours.
Although the term UPS has become the industry standard for any kind of battery backup system, there are actually two kinds of UPSs, only one of which provides truly uninterruptible power. An offline or standby power system switches the input of your computer from utility power to backup power when the utility fails. An online power system keeps your computer constantly connected to the backup power source, so it never has to switch.
Offline Backup Systems
As the name implies, the standby power system constantly stands by, waiting for the power to fail so that it can leap into action. Under normal conditions—that is, when utility power is available—the battery is offline and its charger draws only a slight current to keep its source of emergency energy topped off. The AC power line from which the offline supply feeds is directly connected to its output, and thence to the computer. The batteries are out of the loop.
When the power fails, the offline supply switches into action—switch being the key word. The current-carrying wires inside the power supply that lead to your computer are physically switched (usually by a mechanical relay) from the utility line to the current coming from the battery-powered inverter.
Most offline power systems available today switch within one-half of one cycle of the AC current they are supplied—that's less than ten milliseconds, quick enough to keep nearly all computers running as if no interruption occurred. Although the standby power system design does not protect again spikes and surges, most offline power systems have other protection devices installed in their circuitry to ensure that your computer gets clean power.
Line-interactive power systems are offline designs with an added feature. They react to changes in line voltage and compensate when the voltage gets too high or too low. To change the voltage, they use multitap autotransformers. When the voltage falls low, the line-interactive UPS switches to a tap that boosts the voltage back to the appropriate level. When the voltage gets too high, it switches to a different tap that bucks the voltage down, reducing it. If the line voltage is too far from normal for the transformer taps to compensate, the UPS reacts as if it suffered a power failure, switching to battery power. The line-interactive design offers the big benefit of compensating for most brownouts without draining its battery reserves.
Online Backup Systems
Online backup systems use several designs to guarantee that there is never an interruption in their output power. The traditional design is the double-conversion UPS. These devices earn their name from converting the power twice. First, incoming utility power is transformed down to battery level and rectified to direct current. Then this direct current is inverted and transformed back up to utility voltage to be supplied to your computer. The process would seem to be wasteful and redundant except that a set of batteries connects in the middle. When utility power is available, the constant supply at low voltage keeps the batteries charged. When utility power fails, the batteries maintain the low voltage that feeds the inverter, so the power to your computer is never interrupted.
This traditional double-conversion design is wasteful and expensive. It requires two large transformers capable of carrying the entire load (your computer's and its peripherals), which are very costly and resistant to miniaturization technology. UPS-makers consequently have shifted to a slightly different design that eliminates the transformers. These newer double-conversion UPSs simply rectify the incoming utility voltage to DC and then send it to an inverter, which changes it back to AC. The batteries connect in the middle, but instead of a direct link, they couple through a DC-to-DC converter that matches their voltage to the power line. DC-to-DC converters are high-tech electronic devices that cost substantially less than conventional transformers. As a result, this kind of double-conversion UPS not only has replaced other online designs, but it can also be cost-competitive with offline designs.
In effect, a double-conversion UPS acts like your computer's own generating station, one that is only inches away from the machine it serves. It keeps your system safe from the polluting effects of lightning and load transients found on long-distance power lines. Moreover, dips and surges can never reach the computer. Instead, the computer gets a genuinely smooth, constant electrical supply, exactly like the one for which it was designed.
An alternate online design is more like an offline standby system but uses clever engineering to bridge over even the briefest switching lulls. These UPSs connect both the input power and the output of their inverters together through a special transformer, which is then connected to your computer or other equipment to be protected. Although utility power is available, this kind of UPS supplies it through the transformer to your computer. When the utility power fails, the inverter kicks in, typically within half a cycle. The inductance of the transformer, however, acts as a storage system and supplies the missing half-cycle of electricity during the switchover period.
The double-conversion UPS provides an extreme measure of surge and spike protection (as well as eliminating sags) because no direct connection bridges the power line and the protected equipment—spikes and their kin have no pathway to sneak in. Although the transformer in the new style of UPS absorbs many power-line irregularities, overall it does not afford the same degree of protection. Consequently, these newer devices usually have other protection devices (such as MOVs) built in.
The most important specification to investigate before purchasing any backup power device is its capacity as measured in volt-amperes (VA) or watts. This number should always be greater than the rating of the equipment to which the backup device is to be connected.
In alternating current (AC) systems, watts do not necessarily equal the product of volts and amperes (as they should by the definition that applies in DC systems) because the voltage and current can be out of phase with one another. That is, when the voltage is at a maximum, the current in the circuit can be at an intermediary value. So the peak values of voltage and amperage may occur at different times.
Power requires both voltage and current simultaneously. Consequently, the product of voltage and current (amperage) in an AC circuit is often higher than the actual power in the circuit. The ratio between these two values is called the power factor of the system.
What all this means to you is that volt-amperes and watts are not the same thing. Most backup power systems are rated in VA because it is a higher figure thanks to the power factor. You must make sure the total VA used by your computer equipment is less than the VA available from the backup power system. Alternatively, you must make sure that the wattage used by your equipment is less than the wattage available from the backup power system. Don't indiscriminately mix the VA and watts in making comparisons.
To convert a VA rating to a watt rating, multiply the VA by the power factor of the backup power supply. To go the other way—watts to VA—divide the wattage rating of the backup power system by its power factor. (You can do the same thing with the equipment you want to plug into the power supply, but you may have a difficult time discovering the power factor of each piece of equipment. For computers, a safe value to assume is 2/3.)
Both online and offline backup systems also are rated as to how long they can supply battery power. This equates to the total energy (the product of power and time) that they store. Such time ratings vary with the VA the backup device must supply—because of finite battery reserves, it can supply greater currents only for shorter periods. Most manufacturers rate their backup systems for a given minutes of operation with a load of a particular size instead of in more scientific fashion using units of energy. For example, a backup system may be rated to run a 250 volt-ampere load for 20 minutes.
If you want an idea of the maximum possible time a given backup supply can carry your system, check the ratings of the batteries it uses. Most batteries are rated in ampere-hours, which describes how much current they can deliver for how long. To convert that rating to a genuine energy rating, multiply it by the nominal battery voltage. For example, a 12-volt, 6 amp-hour battery could, in theory, produce 72 watt-hours of electricity. That figure is theoretical rather than realistic because the circuitry that converts the battery DC to AC wastes some of the power and because ratings are only nominal for new batteries. However, the numbers you derive give you a limit. If you have only 72 watt-hours of battery, you can't expect the system to run your 250 VA computer for an hour. At most, you could expect 17 minutes; realistically, you might expect 12 to 15.
You probably will not need much time from a backup power system, however. In most cases, five minutes or less of backup time is sufficient because the point of a backup supply is not to keep a system running forever. Instead, the backup power system is designed to give you a chance to shut down your computer without losing your work. Shutting down shouldn't take more than a minute or two.
UPS-makers warn that no matter the rating of your UPS, you should never plug a laser printer into it. The fusers in laser printers are about as power hungry as toasters—both are resistive heaters. The peak power demand when the fuser switches on can overload even larger UPSs, and the continuing need for current can quickly drain batteries. Moreover, there's no need to keep a print job running during a power failure. Even if you lose a page, you can reprint it when the power comes back at far less expense than the cost of additional UPS power capable of handling the laser's needs. Some printers, such as inkjets, are friendlier to UPSs and can safely be connected, but you'll still be wasting capacity. The best strategy is to connect only your computer, your monitor, and any external disk drives to the UPS. Plug the rest of your equipment into a surge suppresser.
To handle such situations, many UPSs have both battery-protected outlets and outlets with only surge protection. Be sure to check which outlets you use with your equipment, making sure your computer has battery-backed protection.
An ordinary UPS works effectively if you're sitting at your computer and the power fails. You can quickly assess whether it looks like the blackout will be short or long (if the world is blowing away outside your window, you can be pretty sure any outage will be prolonged). You can save your work, haul down your operating system, and shut off your computer at your leisure. When a computer is connected to a network or is running unattended, however, problems can arise.
During a prolonged outage, a simple UPS only prolongs a disaster with an unattended computer—it runs another dozen minutes or so while the power is off, then the UPS runs out of juice and the computer plummets with it. Of course, if a server crashes without warning, no one is happy, particularly if a number of files were in the queue to be saved.
To avoid these problems, better UPSs include interfaces that let them link to your computer, usually through a serial port. Install an appropriate driver, supplied by the UPS-maker, and your computer can monitor the condition of your power line. When the power goes off, the software can send messages down the network warning individual users to save their work. Then, the UPS software can initiate an orderly shut down of the network.
Some UPSs will continue to run even after your network or computer has shut itself down. Better units have an additional feature termed inverter shutdown that automatically switches off the UPS after your network shuts down. This preserves some charge in the batteries of the UPS so that it can still offer protection if you put your computer or network back online and another power failure follows shortly thereafter. A fully discharged UPS, on the other hand, might not be ready to take the load for several hours.
The gelled electrolyte batteries most commonly used in uninterruptible power systems have a finite life. The materials from which they are made gradually deteriorate, and the overall system loses its ability to store electricity. After several years, a gelled electrolyte battery will no longer be able to operate a UPS, even for a short period. The UPS then becomes nonfunctional. The only way to revive the UPS is to replace the batteries.
Battery failure in a UPS usually comes as a big surprise. The power goes off and your computer goes with it, notwithstanding your investment in the UPS. The characteristics of the batteries themselves almost guarantee this surprise. Gelled electrolyte batteries gradually lose their storage capacity over a period of years, typically between three and five. Then, suddenly, their capacity plummets. They can lose nearly all their total storage capability in a few weeks. Figure 31.2 illustrates this characteristic of typical gelled electrolyte batteries.
Note that the deterioration of gelled electrolyte batteries occurs regardless of whether they are repeatedly discharged. They deteriorate even when not used, although repeated heavy discharges will further shorten their lives.
To guard against the surprise of total battery failure, better UPSs incorporate an automatic testing mechanism that periodically checks battery capacity. A battery failure indication from such a UPS should not be taken lightly.
Phone and Network Line Protection
Spikes and surges affect more than just power lines. Any wire that ventures outside holds the potential for attracting lightning. Any long wiring run is susceptible to induced voltages, including noise and surges. These over-voltages can be transmitted directly into your computer or its peripherals and cause the same damage as a power line surge.
The good news is that several important wiring systems incorporate their own power protection. For example, Ethernet systems (both coaxial and twisted pair) have sufficient surge protection for their intended applications. Apple LocalTalk adapters are designed to withstand surges of 2000 volts with no damage. Because they are not electrical at all, fiber-optical connections are completely immune to power surges.
The bad new is that two common kinds of computer wiring are not innately protected by surges. Telephone wiring runs long distances through the same environments as the power-distribution system and is consequently susceptible to the same problems. In particular, powerful surges generated by direct lightning hits or induction can travel through telephone wiring, through your modem, and into the circuitry of your computer. In addition, ordinary serial port circuitry includes no innate surge suppression. A long unshielded serial cable can pick up surges from other cables by induction.
The best protection is avoidance. Keep unshielded serial cable runs short whenever possible. If you must use a long serial connection, use shielded cable. Better still, break up the run with a short-haul modem, which will also increase the potential speed of the connection.
Modem connections with the outside world are unavoidable in these days of online connectivity and the Internet. You can, however, protect against phone-line surges using special suppressors designed exactly for that purpose. Better power-protection devices also have modem connections that provide the necessary safeguards. Standalone telephone surge suppressors are also available. They use the same technologies as power-line surge suppressors. Indeed, the voltage that rings your telephone is nearly the same as the 110–120 volt utility power used in the United States. Most phone-line suppressors are based on MOV devices. Better units combine MOVs with capacitors, inductors, and fuses.
|[ Team LiB ]|