If you're at all typical about buying a surge suppressor, you either know or have been told that you need one for the safety of your computer, so you've gone to the store to buy one or more. What greets you is a marvelous array of products, all aspiring to be your next purchase.
The difficulty you face is that you can find commercial surge suppressors priced from $5 to $75 with little to distinguish the cheapest from the best. To see for ourselves whether the extra bucks buy anything more than a brand name and hype, we opted for autopsy. We bought an almost-generic Woods No. 0418 surge suppressor and outlet strip from the local Lowe's home warehouse store for $7.95 and a brand-name product, the American Power Conversion SurgeArrest Surge Protector, Model Pro8T2C with a suggested retail price of $39.95. Then we got out our screwdriver and desoldering tool, and disassembled each.
The results surprised us. We had expected the cheap model to be worthless. Instead, we found adequate surge suppression circuitry inside. Although the design, materials, and construction rate are not the best, the overall product proved better than we had expected for the price. And the high-priced model? It justified its higher price on every count: better design, better materials, and more protection. Although we hope to reveal a shocking conclusion or two, our dissection revealed only that paying more does indeed get you more, at least as far as these surge suppressors are concerned. As to the protection the budget and premium surge suppressors afford your equipment, our autopsy would rate the two about equal despite the differences in rating and construction.
The critical aspect of a surge suppressor is its let-through voltage, the amplitude of the surge a suppressor allows to reach your equipment, discussed below. In that the two products use the same circuit design and comparable components (with the same voltage rating), we cannot dispute their equal let-through voltage rating (330 volts). The premium unit is, however, built for the longer run. The larger MOVs (metal oxide varistor), heftier capacitors, and use of better materials, indicate that it should last and protect longer.
A surge suppressor is aptly names. It protects against surges, which are a special case of a general problem called overvoltage-- more voltage than a circuit is designed to handle. Voltage is the force behind the punch of electricity, and high voltages can punch through many physical barriers. The high-voltage punch can change the semiconductor silicon into a true conductor, creating a short circuit that renders the device unusable.
Overvoltages can last long periods or brief intervals, as long as forever or as brief as a fraction of one of your computer's clock cycles. You don't usually have to worry about long-term overvoltages. 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. The internal voltage regulation circuitry of your PC can take these fluctuations in stride.
Sometimes power companies do make errors and send too much voltage down the line, causing your lights to glow brighter and your PC to teeter closer to disaster. The occurrences are simply termed overvoltages. Unless something goes dramatically wrong at the power company, however, such overvoltages are rare because your utility carefully regulates the voltage it supplies to you.
Brief overvoltages are more worrisome--not because of their duration, but because they are unpredictable. They are not compensated for by the utility voltage regulators, and they tend to be larger (in voltage), and thus more dangerous to your computer equipment than long-term fluctuations. Most spikes and surges sneak into the power line as electricity travels from the utility to you, long after the power company can regulate them or do anything about them. Some spikes are, in fact, generated inside your home or office.
The most dangerous of all spikes and surges are those caused by lightning striking power lines. Brief peaks as high as 25,000 volts have been measured on power lines. Lightning doesn't have to hit a power line to induce a voltage spike that can damage your PC; it can induce damaging voltages in power lines without actually touching them. Or it could hit or induce voltages in telephone wires connected to your modem. When it does hit a wire, however, everything connected to that circuit is likely to take on the characteristics of a flash bulb.
During normal operation, surge suppressors do essentially nothing. Their design is such that they act as electrical insulators until the voltage across their leads reaches a potential called the clamping voltage at which they start conducting electricity to create the surge-killing short circuit. In effect, they almost instantly switch from being an electrical insulator to a good conductor. They give the electrical surge a low-resistance direct path that bypasses your computer. The clamping voltage of the components inside a commercial surge suppressor produces the most important rating of the product, the let-through voltage. This measure indicates the maximum voltage the entire surge suppressor allows to reach your equipment after surge suppression, noise reduction, and power-line conditioning.
The nominal voltage of line current in the United States, 120 volts, is what engineers call the average or RMS voltage. But every cycle of alternating current reaches a peak value about 1.414 times higher than this voltage. That is, the voltage at your wall outlet varies between 0 volts and 170 volts 60 times per second.
Obviously a 130-volt surge suppressor will see a 170-volt surge 60 times a second. To guard against these 170-volt surges, the 130-volt surge protector would short-circuit the power line 60 times a second and would soon wear out, overheat, or (more likely) explode.
Underwriters Laboratories rates surge suppressors based on let-through voltage. Under UL specifications, the minimum let-through voltage that a product can claim--and thus the best let-through voltage rating of a UL product--is 330 volts. Lower claims are not possible; let-through voltage ratings higher than 330 volts indicate a product that affords less protection.
Protecting Phone Lines
The telephone surge protection built into power-line surge protectors use the electrical ground wire of your utility power. Note that if you install such a telephone-line surge protector in a home with older wiring that lacks a ground--one that has two-prong rather than three-prong outlets--it will not function properly. Similarly, if you install such a combined power-and-telephone surge suppressor at the end of a two-wire extension cord, it will not protect against surges in the telephone line--and will only protect against normal-mode power-line surges.