Stamp out power surges before they stomp on your PC

The surge suppressor is one of the unsung heroes of the computer world. Often valued more for its ability to multiply one electrical receptacle into many than for its role as protector of all things electronic, the surge suppressor is your first line of defense against transient power surges that can damage or destroy sensitive components inside your PC. Let’s take a look at how they work.

Before we tackle the concept of surge suppression, we should first understand what exactly a surge is. In the United States, electrical energy flows through standard household wiring at an average rate of 120 volts. Because the system used is alternating current, the voltage level of every AC cycle reaches a peak value that’s roughly 1.414 times higher than 120 volts. A surge occurs when the voltage level suddenly rises significantly higher than that. A lightning strike on a power line, for instance, will cause a transient spike in the electrical power entering your house. Problems with your utility company’s equipment (anything from a downed power line to a defective transformer) can also cause power surges.

Appliances and other electrically powered devices inside your home, however, are much more common causes of power surges. Any device that requires a large amount of energy to switch on or off—examples include refrigerators, vacuum cleaners, and air conditioners—can disrupt the flow of voltage through your home’s electrical wiring. Surges such as these don’t pack as much destructive power as a lightning strike, but they can cause as much damage, instantly or over time.

The First Line of Defense

When you plug a surge suppressor into an electrical outlet in your wall, it will pass the electrical current from your home’s wiring to each of the devices plugged into its several receptacles. But the current will first pass through at least one component called an MOV, which stands for metal oxide varistor. If the voltage entering the MOV is higher than a specified value, the MOV will conduct the extra electricity either to the surge suppressor’s neutral wire or to its grounding wire.

An MOV consists of a mass of metal oxide grains (typically zinc oxide, with small amounts of bismuth, cobalt, and manganese) sandwiched between two metal plates that function as electrodes. One electrode is connected to the surge suppressor’s hot wire (the one carrying the electrical current) and the other is connected to its neutral or ground wire. These electrodes exhibit variable resistance (hence the term “varistor”) dependent on the voltage passing through them.

When the voltage is below a defined level, the electrodes present very high resistance, so the current bypasses the MOV and runs through to the outlet strip’s receptacles. When the voltage exceeds that defined level, the electrodes change character and present very low resistance. At this point, the MOV absorbs some of the excess current (dissipating the energy in the form of heat) and conducts more of it to the ground wire to bleed off the excess voltage. As the surge passes and the voltage in the hot wire returns to normal, the MOV reverts to its previous state of high resistance.

Surge Suppressor Ratings

A surge suppressor’s clamping voltage rating specifies the amount of voltage that will cause its MOVs to conduct electricity into its ground wire. A reputable manufacturer will submit its surge suppressor to Underwriters Laboratory for testing according to its UL 1449 standard. A lower value indicates better protection, but 330 volts is the minimum clamping value a manufacturer can claim according to the UL standard. A surge suppressor’s joule rating, which defines now much energy it can absorb and/or dissipate before it fails, provides a second means of measuring its effectiveness at smoothing energy spikes. In this case, a higher number indicates better protection.

There will always be a slight delay before a surge suppressor can respond to a power surge; the longer the delay, the longer the devices connected to it will be exposed to the surge. Fortunately, surges also take a few microseconds to reach their peak voltage, which gives the surge suppressor time to react. A suppressor with a one-nanosecond response time will be fast enough to shield connected devices from damaging energy spikes.

MOVs degrade after responding to a few major surges or many smaller ones. A fully degraded MOV offers no protection, but it also won’t prevent power from reaching the outlets. A high-quality surge suppressor will include a thermal fuse or a circuit breaker that can cut off power if a particularly strong surge exceeds the MOVs’ capacity to absorb it or redirect it to ground. A good surge suppressor will also feature an LED to indicate that its MOVs remain viable. When the LED fails to light, the surge suppressor should be replaced.