How Surge Protectors Work

By: Tom Harris & Talon Homer  | 
surge protectors
Surge protectors protect your electronics from power surges in your electrical system. Erik Von Weber/Getty Images

When you put together a computer system, one piece of standard equipment you'll probably buy is a surge protector. They serve one obvious function; they let you plug multiple components into one power outlet. With all the different components that make up a computer system, surge protectors are useful devices.

But the other function of a surge protector power strip — protecting the electronics in your computer from power surges — is far more important. So how do surge protectors work and when you need them? And how do you know if the one you have is good quality?


The main job of a surge protector system is to protect electronic devices from "surges." So if you're wondering what a surge protector does, the first question is, "What are surges?" And then, "Why do electronics need to be protected from them?"

A power surge, or transient voltage, is an increase in voltage significantly above the designated level in a flow of electricity. In normal household and office wiring in the United States, the standard voltage is 120 volts. If the voltage rises above 120 volts, there is a problem, and a surge protector helps prevent surges from damaging your electronics.

To understand the problem, it is helpful to understand something about voltage. Voltage is a measure of a difference in electric potential energy. Electric current travels from point to point because there is a greater electric potential energy on one end of the wire than there is on the other. This is the same sort of principle that makes water under pressure flow out of a hose — higher pressure on one end of the hose pushes water toward an area of lower pressure. You can think of voltage as a measure of electrical pressure.

Various factors can cause a brief increase in voltage:

  • When the increase lasts three nanoseconds (billionths of a second) or more, it's called a surge.
  • When it only lasts for one or two nanoseconds, it's called a spike.

If the surge or spike is high enough, it can cause major damage on a machine. The effect is very similar to applying too much water pressure to a hose. If there is too much water pressure, a hose will burst. A similar thing happens when too much electrical pressure runs through a wire — the wire "bursts." Actually, the wire heats up like the filament in a light bulb and it burns, but it's the same idea.

Even if increased voltage doesn't immediately break your electronics, it may put extra strain on the components, wearing them down over time. In the next section, we'll look at what surge protectors do to prevent this from happening.

Surge Protection

A standard surge protector passes the electrical current along from the outlet to several electrical and electronic devices plugged into the power strip. If the voltage from the outlet surges or spikes — rises above the accepted level — the surge protector diverts the extra electricity into the outlet's grounding wire.

metal oxide varistor (MOV) surge protector
This graphic shows a simple metal oxide varistor (MOV) surge protector with line conditioning and a fuse.

In the most common type of surge protector, a component called a metal oxide varistor (MOV) diverts the extra voltage. As you can see in the diagram to the left, an MOV forms a connection between the hot power line and the grounding line.


An MOV has three parts: a piece of metal oxide material in the middle, joined to the power and grounding line by two semiconductors.

These semiconductors have a variable resistance that is dependent on voltage. When voltage is below a certain level, the electrons in the semiconductors flow in such a way as to create a very high resistance. When the voltage exceeds that level, the electrons behave differently, creating a much lower resistance. When the voltage is correct, an MOV does nothing. When voltage is too high, an MOV can conduct a lot of current to eliminate the extra voltage.

As soon as the extra current is diverted into the MOV and to ground, the voltage in the hot line returns to a normal level, so the MOV's resistance shoots up again. In this way, the MOV only diverts the surge current, while allowing the standard current to continue powering whatever machines are connected to the surge protector. Metaphorically speaking, the MOV acts as a pressure-sensitive valve that only opens when there is too much pressure.

Gas Discharge Arresters

Another type of surge protection device is a gas discharge arrester, or gas tube. These tubes do the same job as an MOV — they divert the extra current from the hot line to the ground line. Gas discharge arresters are typically used in places like power substations, industrial plants or inside office buildings, and they can be installed in the gas tube of a power transformer or mounted on an outside wall.

When the voltage is at a certain level, the makeup of the gas is such that it is a poor conductor. When the voltage surges above that level, the electrical power is strong enough to ionize the gas, making it a very effective conductor. It passes on current to the ground line until the voltage reaches normal levels, and then becomes a poor conductor again.


Both methods have a parallel circuit design — the extra voltage is fed away from the standard path to another circuit. A few surge protector products suppress surges with a series circuit design — the extra electricity isn't shunted to another line, but instead is slowed on its way through the hot line. Basically, these suppressors detect when there is high voltage and then store the electricity, releasing it gradually. The companies that make this type of protector argue that the method offers better protection because it reacts more quickly and doesn't dump electricity in the ground line, possibly disrupting the building's electrical system.

As a backup, some surge protectors also have a built-in fuse. A fuse is a resistor that can easily conduct current as long as the current is below a certain level. If the current increases above the acceptable level, the heat caused by the resistance burns the fuse, thereby cutting off the circuit. If the MOV doesn't stop the power surge, the extra current will burn the fuse, saving the connected machine. This fuse only works once, as it is destroyed in the process.

Some surge protectors have a line-conditioning system for filtering out "line noise," smaller fluctuations in electrical current. Basic surge protectors with line-conditioning use a fairly simple system. On its way to the power strip outlet, the hot wire passes through a toroidal choke coil. The choke is a just ring of magnetic material, wrapped with wire — a basic electromagnet. The ups and downs of the passing current in the hot wire charge the electromagnet, causing it to emit electromagnetic forces that smooth out the small increases and decreases in current. This "conditioned" current is more stable, and so easier on your electronic devices.

Power Surges

Lightning storm
You might think lightning storms are one of the leading causes of power surges, but they're actually one of the least common. Kevin J Salisbury/Getty Images

Power surges occur when something boosts the electrical charge at some point in the power lines. This causes an increase in the electrical potential energy, which can increase the current flowing to your wall outlet. Several different things can cause this to happen.

The most familiar source is probably lightning, though it's actually one of the least common causes. When lightning strikes near a power line, whether it's underground, in a building or running along poles, the electrical energy can boost electrical pressure by millions of volts. This causes an extremely large power surge that will overpower almost any surge protector. In a lightning storm, you should never rely on your surge protector to save your electronics. The best protection is to unplug them.


A more common cause of power surges is the operation of high-power electrical devices, such as elevators, air conditioners and refrigerators. These high-powered pieces of equipment require a lot of energy to turn on and off components like compressors and motors. This can create sudden, brief demands for power, which can upset the steady voltage flow in the electrical system.

While these surges are nowhere near the intensity of a lightning surge, they can be severe enough to damage components, immediately or gradually, and they occur regularly in most buildings' electrical systems.

Other sources of power surges include faulty wiring, problems with the utility company's equipment, and downed power lines. The system of transformers and lines that brings electricity from a power generator to the outlets in our homes or offices is extraordinarily complex. There are dozens of possible points of failure, and many potential errors that can cause an uneven power flow. In today's system of electricity distribution, power surges are an unavoidable occurrence.

When to Use a Surge Protector

surge protector
You'll want to use a surge protector when you plug in electronics like computers, entertainment center equipment or high-end gaming consoles. Perry Mastrovito/Getty Images

In the last section, we saw that power surges are regular occurrences that are unavoidable in our current electrical system. This raises an interesting question: If we have to expect power surges, why didn't we need surge protectors in our homes 50 years ago?

The answer is that a lot of the components in sophisticated modern electronic devices (such as computers, game consoles and high-def TVs) are much smaller and more delicate than components in older machines, and are therefore more sensitive to current increases. Microprocessors, which are an integral part of all computers as well as many home appliances, are particularly sensitive to surges. They only function properly when they receive stable current at the right voltage.


So whether you should get a surge protector depends on what sort of device you're hooking up to the power supply.

  • You should use a surge protector with your computer. It is filled with voltage-sensitive components that a power surge could damage very easily. At the least, this damage will shorten the life of your computer, and it could wipe out all your data or destroy your system.
  • It's a good idea to use surge protectors for other high-end electronic equipment, such as entertainment center components. A surge protector will generally extend the life of these devices, and there's always a chance that a big power surge will cause severe damage.
  • It's best not to plug devices that naturally draw high amounts of amperage into a surge protector, as they can cause excessive wear on the protector during use. These include devices with heating elements or large electric motors, such as coffee makers, space heaters, fridges or vacuum cleaners. These appliances should be plugged directly into a wall outlet.
  • Do not chain multiple power strips together, as this can cause incorrect voltage output, overheating and even fire.

One problem with surge protectors is that the MOVs can burn out with one good surge. Therefore it's good to get a protector with an indicator light that tells you whether or not it's functioning properly.

Even if you connect surge protectors to all your outlets, your equipment might be exposed to damaging surges from other sources. Any lines carrying signals into your home can also carry a power surge, due to lightning or a number of other factors. If you have a coaxial cable line hooked up to expensive equipment, consider a cable surge protector. Surges on these lines can do just as much damage as surges over power lines.

Surge Protection Levels

All surge protectors are not created equal. In fact, there is a tremendous range in both performance and price of protection systems.

  • At one end, you have your basic $5 surge protector power strip, which will offer very little protection.
  • On the other end you have systems costing hundreds or even thousands of dollars, which will protect against pretty much everything short of lightning striking nearby.

Most systems have limitations of some sort; picking out a protector system that suits you is a matter of balancing the cost of the system with the cost of losing data or electronic equipment. As with insurance, you find the level of coverage you're comfortable with.


To protect your equipment from surges, you need individual surge protectors for each outlet. These power strips range a great deal in quality and capacity (as we'll see in the next section). There are three basic levels of power strip surge protectors:

  • Basic power strip: These are basic extension cord units with five or six outlets. Generally, these models provide only basic protection.
  • Better power strip: For $15 to $25 you can get a power strip surge protector with better ratings and extra features.
  • Surge station: These large surge protectors fit under your computer or on the floor. They offer superior voltage protection and advanced line conditioning. Most models also have a USB outlet, and may feature built-in circuit breakers. You can get one of these units for as little as $30, or you can spend upward of $100 for a more advanced model.
  • Uninterruptible Power Supply (UPS): Some units combine surge protection with a continuous UPS. The basic design of a continuous UPS is to convert AC power to DC power and store it on a battery. The UPS then converts the battery's DC power back to AC power and runs it to the AC outlets for your electronics. If the power goes out, your computer will continue to run, feeding off the stored battery power. This will give you a few minutes to save your work and shut down your computer. The conversion process also gets rid of most of the line noise coming from the AC outlet. These units tend to cost $150 or more.

Be aware that some cheap power strips will be labeled as a "relocatable power tap," rather than surge protectors. These devices are simply made to split one 120 volt outlet into multiple power sources, but won't provide any additional defense in the event of a power surge. These can look almost identical to surge protectors, so make sure the packaging clearly states the product's features before purchasing.

An ordinary UPS will give you a high level of protection, but you should still use a surge protector. A UPS will stop most surges from reaching your computer, but it will probably suffer severe damage itself. It's a good idea to use a basic surge protector, if just to save your UPS.

Once you've decided what level of surge protection you need, it's time to shop around for a good unit. In the next section, we'll find out what you should look for when considering different models.

Underwriters Laboratories Ratings

surge protector
Choose a surge protector that has a light that shows it's protecting your equipment, as well as one that is grounded. Tony Campbell/Shutterstock

Shopping for a surge protector is tricky because there are a lot of nearly worthless products on the market. Research into a particular model is the best way to ensure good results, but you can get a good idea of a product's performance level by looking for a few signs of quality.

First, look at the price. Generally, don't expect much from any surge protector that costs less than $10. These units typically use simple, inexpensive MOVs with limited capabilities, and won't protect your system from bigger surges or spikes.


Of course, high prices don't always promise quality. To find out what the unit is capable of, you need to check out its Underwriters Laboratories (UL) ratings. UL is an independent, not-for-profit company that tests electrical and electronic products for safety. If a protector doesn't have a UL listing, it's probably junk; there's a good chance it doesn't have any protection components at all. If it does use MOVs, they may be of inferior quality. Cheaper MOVs can easily overheat and set the surge protector on fire.

Many UL-listed products are also of inferior quality, of course, but you're at least guaranteed that they have some surge protection capabilities and meet a minimum safety standard. Be sure that the product is listed as a transient voltage surge suppressor. This means that it meets the criteria for UL 1449, UL's minimum performance standard for surge suppressors. There are a lot of power strips listed by UL that have no surge protection components at all. They are listed only for their performance as extension cords.

No surge protector is 100 percent effective, and even top-of-the-line equipment may have some serious problems. Electronics experts are somewhat divided over the best way to deal with power surges, and different manufacturers claim other technologies are inherently faulty.

Surge Protector Ratings

On a listed surge protector, you should find a couple of ratings. Look for:

  • Clamping voltage: This tells you what voltage will cause the MOVs to conduct electricity to the ground line. A lower clamping voltage indicates better protection. There are three levels of protection in the UL rating — 330 V, 400 V and 500 V. Generally, a clamping voltage more than 400 V is too high.
  • Energy absorption/dissipation: This rating, given in joules, tells you how much energy the surge protector can absorb before it fails. A higher number indicates greater protection. Look for a protector that is at least rated at 200 to 400 joules. For better protection, look for a rating of 600 joules or more.
  • Response time: Surge protectors don't kick in immediately; there is a very slight delay as they respond to the power surge. A longer response means your equipment will be exposed to the surge for a greater amount of time. Look for a surge protector that responds in less than one nanosecond.

Better surge protectors may come with some sort of guarantee of their performance. If you're shopping for more expensive units, look for a protector that comes with a guarantee on your equipment. If the unit fails to protect your equipment from a power surge, the company will replace it.