How to Protect Your Computer from Power Surges
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How to Protect Your Computer from Power Surges

A dicussion of surge protection devices and components to protect your computer from voltage spikes and other power related problems.

A surge protector protects your computer from the surges in supply voltage, transients, and other fluctuations in electrical current. Without a surge protector, an electrical surge or spike could destroy your computer and all of its components instantly.

Most computers today have some sort of built-in surge protection as part of the power supply, but for full protection, you should purchase an effective surge protector. Here are a few tips on choosing, installing, and using a surge protector.

Surge Protection Components

MOV - Metal-Oxide Varistors

This type of surge diverter component has a high clamping voltage (300V to 500V) but a slow response time. This means that, in best-case scenarios, voltage impulses of less than 500V usually enter the computer system unimpeded. In addition, higher voltage events with very fast rise times may pass by the MOV before it is able to respond. Although MOVs can handle a significant amount of energy, they are physically degraded each time they clamp. This characteristic alters their future performance and ultimately leads to physical failure.

SAD - Silicon Avalanche Diodes

The disadvantages of the MOV have led to the use of the SAD, either in conjunction with the MOV or in standalone applications. Compared to MOVs, SADs have a faster response time and are not subject to the physical degradation that characterizes an MOV design. The overall energy handling ability of the SAD, however, is not as high. An impulse that merely degrades an MOV may also cause outright destruction of the SAD. To overcome this disadvantage, many surge strip manufacturers whose designs use standalone SADs will parallel multiple SADs to increase the overall energy handling capability of the protector. Manufacturers and standards agencies often debate the effectiveness of this design method.

Gas tubes

These devices have a high clamping voltage but are comparatively slow. However, they handle almost limitless amounts of energy. Some surge strip designs use gas tubes as the final line of “brute force” protection to spare the lives of the other surge diverter components in the presence of a catastrophic power line disturbance. In fact, many designs incorporate paralleled MOVs, SADs, and/or gas tubes in an effort to improve performance by combining the relative strengths of each particular component.

The event commonly referred to as a surge is more accurately defined as a high-voltage transient or impulse. Surge strips are designed to divert the impulse away from the sensitive electronic system. The term diverter is more appropriate for this type of protective device. (Figure from Electrical Construction and Maintenance)


Consider a unit's features, performance, longevity, manufacturer's reputation, and price. Consider these tips when making a purchase.

Know the difference among surge protectors. An electrical outlet has three slots, one for each of the prongs on a power cord: a neutral slot, a hot slot, and a grounding slot. Surge protectors differ in how they handle passing current among the various slots, and they come in two main types: MOVs (metal-oxide varistors) and series mode.

MOVs have a series of tiny MOV disks that use semiconductors on each side to connect the hot wire to the grounding wire. When voltage increases, the MOVs lessen their resistance, and excess voltage passes through to the grounding wire. Series-mode surge protectors, on the other hand, absorb excess voltage and then gradually let the current pass through the hot wire after the surge ends. In general, series-mode surge protectors have less inherent risk because and an MOV protector may send excess electricity through a grounding wire. Many people choose MOVs because they are typically less expensive.

Standard metal-oxide varistors surge protectors typically cost much less than series-mode models; generally, you can expect to spend $50 to $75 more for the series-mode device. You should expect to pay $15 or more for a typical surge protector.

Know the UL testing information. The Underwriters Laboratories ( ) is an independent product safety testing and certification organization. Look for a UL label on the device package that states "UL Listed Transient Voltage Surge Suppressor," which will tell you that it's a surge protector and not just a multiple-outlet power strip. Then, check for "UL Code 1449," which indicates that the product is safe for home use. Don't purchase the item for protection if you don’t see these two things.

When a surge hits a surge protector, it may destroy the device, which is why good surge protectors typically include an indicator light or series of lights that shows the surge protector is still working. Unfortunately, these lights aren't standardized, and the mere presence of a light may not indicate whether a device is no longer operational. Be sure to read the product box to make sure that the product has a light to indicate it's working properly, and consult the instruction manual to find out exactly what the lights mean.

Most equipment and outlets in the United States are designed for a typical supply voltage of 120 volts, and voltage rising above 120V constitutes a power surge or power spike. Voltage regularly fluctuates so it is when the voltage increases “significantly” that a surge protector will work. The definition of significant depends on the manufacturer. Normal residential line voltage can vary from 110 to 125 volts depending on such factors as age of the system and weather conditions.

It is important to look at the clamping level, the maximum voltage an MOV surge protector will let your PC reach before it goes to work and sends electricity to the grounding line. The lower the clamping level (also known as a suppressed-voltage rating), the better it is. Look for a clamping level of less than or equal to 330V.

When the surge protector detects a power surge, it will take some time to respond and halt the surge; this is called the clamping response time. The faster the surge protector can react, the more likely it is to protect your PC. Look at surge protectors that fit your budget and choose the one with the shortest clamping response time. Series-mode surge protectors respond immediately, so this tip only applies to choosing an MOV surge protector with the fastest response time.

Some surge protector manufacturers state the device's Joule rating, or the amount of energy that the MOV can safely handle. But even though many manufacturers advertise high Joule ratings, implying that this is equivalent to a more powerful surge protector, the joule rating doesn’t count for much. To raise the Joule rating, all a manufacturer needs to do is put more MOVs inside the surge protector. But when one varistor goes out, the surge protector is useless.

Keep in mind that UL doesn't measure the Joule rating on surge protectors so you should focus on other UL codes and clamping data. In addition, if you use a modem to connect to the Internet or another network, your surge protector should have a telephone-line or cable input jack.

You shouldn't buy a surge protector without a warranty. In general, the more expensive surge protectors include more comprehensive warranties. Purchase one that covers damage to equipment and not just offer a replacement surge protector.


Proper installation is important to protecting your computer from surges.

Know the basics of setup. Setting up a surge protector is typically a fairly simple process. You plug your PC and its components into the surge protector, plug the surge protector into a properly grounded outlet, and then turn on the surge protector. But if you use a dial-up modem to connect to the Internet, you'll want to make sure you protect your phone line, as well. To do so, plug a standard phone cord from the telephone wall connector into the input jack of the surge protector and plug one end of a second phone cord into the modem and the other end into the output jack.

Surge protectors protect many other types of devices such as Internet-enabled televisions and cable modems, and standard televisions. If your surge protector has coaxial cable surge protection, take advantage of it. Connect the incoming cable or broadband line to the input coaxial connector on the surge protector. Then, connect one end of a coaxial cable to the output coaxial connector on the surge protector and connect the other end to the input coaxial connector of the cable modem or other device you want to protect.

You can place the surge protector on the floor or mount it on the side of a desk or on the wall if you don’t have enough space on your desk. It should be in a spot where you can see the indicator lights and readily access the plugs. Don’t place it close to curtains or other highly flammable materials. In the case of a catastrophic event, a great deal of heat can be generated. You should also clean up around the area where you are placing the surge protector; remove any dirt and loose papers on the floor and move any boxes away from the surge protector.

Comply with the warranty on the device; using any extension cord, adapters, other grounding wires, or electrical connections in conjunction with a surge protector can void all warranties.

You should connect your laptop computer to a surge protector if you're using the AC adapter when charging or using it at home. You can buy a surge protector specifically made for notebooks, or you can use a standard plug-in surge protector and make sure that the head of the adapter doesn’t hit the surge protector's power switch. While some AC adapters include built-in surge protection, it usually isn't as strong as the protection a separate surge protector provides.

While surge protectors are designed to protect your computer from voltage spikes like those caused by lightning strikes, they will not protect against a direct hit on your home or the electrical lines leading to your home. It is best to shut down your computer and unplug the surge protector from the wall.

Some surge protectors have reset buttons, and you may have accidentally hit the reset button or the surge protector blew a breaker that you need to reset. Push the reset button on the surge protector and see if your computer has power again.


Here are some guidelines on determining when it is time to replace a surge protector.

If your surge protector has warning lights you need to pay attention to them. Sometimes a light will still display when strikes or surges have damaged the surge protector. You may want to replace the surge protector every 3 to 4 years, when the warranty has expired, the manufacturer no longer supports the product, or you can’t remember the last time you replaced your current surge protector.

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Comments (3)
leverton Lisa

this is good advise, not sure if mine has one or not?

I use them with all my electronic equipment. Very wise.

This was a little technical for me. I think I got the gist of it though.