When you put together a
system, one piece of standard equipment you'll probably buy is a surge
protector. Most designs serve one immediately obvious function -- they
let you plug multiple components into one power outlet. With all of the
different components that make up a computer system, this is definitely a
But the other function of a surge protector power strip -- protecting the
electronics in your computer from surges in power -- is far more important.
In this edition of
HowStuffWorks, we'll look at surge protectors, also called surge
suppressors, to find out what they do, when you need them, and how well they
work. We'll also find out what levels of protection are available and see
why you might not have all the protection you need, even if you do use a
quality surge protector.
The main job of a surge protector system is to protect
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 to prevent that problem from destroying your
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 end. 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.
As we'll see later on, various factors can cause a brief increase in
- 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 inflict some heavy 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.
Approximately the same thing happens when too much electrical pressure runs
through a wire -- the wire "bursts." Actually, it heats up like the
filament in a
light bulb and burns, but it's the same idea. Even if increased voltage
doesn't immediately break your machine, 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.
The Process of Protection
A standard surge protector passes the electrical current along from the
outlet to a number of 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
In the most common type of surge protector, a component called a metal
oxide varistor, or MOV, diverts the extra voltage. As you can see
in the diagram below, 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.
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.
A simple MOV surge protector with line conditioning
and a fuse
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.
Another common surge protection device is a gas discharge arrestor,
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. They do this by using an
inert gas as the
conductor between the two lines.
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 of these 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
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.
Inside a surge protector with line-conditioning
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
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 computer (or other electronic device).
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. A number
of different things can cause this to happen.
The most familiar source is probably
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 computer. The best protection is to unplug your
A more common cause of power surges is the operation of high-power
electrical devices, such as elevators,
air conditioners and
These high-powered pieces of equipment require a lot of energy to switch on
and turn off components like compressors and
switching creates sudden, brief demands for power, which 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
building's 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. In the next section, we'll see what
this could mean to you.
Protecting Your Equipment
In the last section, we saw that power surges are a regular occurrence,
unavoidable with our current system of providing electricity to homes and
offices. This raises an interesting question: If power surges are an
inherent part of our electrical system, 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
DVD players) 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 or not you should get a surge protector depends on what sort
of device you're hooking up to the power supply.
- There's no reason to hook up a light bulb to a surge protector because
the worst that is likely to happen due to a power surge is that your light
bulb will burn out.
- You should definitely 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 very easily wipe out all of your saved data or
destroy your system. Computers are very expensive items, and the data they
hold is often irreplaceable, so it's only good economic sense to invest in
a quality surge protector.
- 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 causes severe damage.
One problem with surge protectors is that the MOVs can burn out with one
good surge. This is why 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 of your outlets, your
equipment might be exposed to damaging surges from other sources.
cable lines can
also conduct high voltage -- for full protection, you should also guard
against surges from your telephone or cable lines. Any lines carrying
signals into your home can also carry a power surge, due to lightning or a
number of other factors. If your computer is connected to the phone lines
via a modem, you
should get a surge protector that has a phone-line input jack. 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.
Levels of Surge Protection
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 of thousands of
dollars, which will protect against pretty much everything short of
lightning striking nearby.
This inexpensive, quality protector features basic
MOV protection and line-conditioning systems.
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.
You can also install a "whole-house" surge
arrestor. You generally install these units near your electric
meter, where the power lines run to your building. This protects all the
circuits in your house or office from a certain range of voltage surges.
Units designed for whole-house protection are generally built for
outdoor installation. Better surge arrestors can handle surges up to
20,000 volts, while standard outlet surge protectors can't handle more
than 6,000 volts. Some high-end arrestors can actually monitor weather
conditions and will shut down the power supply to more sensitive
electronics when lightning is in the area.
A whole-house surge protector will suppress power surges stemming
from outside sources -- utility company problems, transformer switching,
etc. -- but won't do anything to suppress the high number of power
surges that originate inside your house, due to the operations of your
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
- Better power strip - For $15 to $25 you can get a power strip
surge protector with better ratings and extra features, such as a
protection indicator light and individual switches for each outlet.
- 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 an input for a phone
line, to protect your modem from power surges, and may feature built-in
circuit breakers. You can get one of these units for as little $30, or you
can spend upward of $100 for a more advanced model.
- Uninterruptable Power Supply (UPS) - Some units combine surge
protection with a continuous
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.
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.
Picking the Right Surge
Shopping for a surge protector is tricky business 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 of all, look at price. As a general rule, don't expect much
from any surge protector that costs less than $10. These units typically use
simple, inexpensive MOVs with fairly limited capacities, and won't protect
your system from bigger surges or spikes.
Of course, high price doesn't 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 electric and electronic products for
safety. If a protector doesn't have 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, setting the entire surge protector on fire. This is actually a
fairly common occurrence!
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 marginal 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
On a listed surge protector, you should find a couple of ratings. Look
- 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 time tells you that your computer (or other 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.
You should also look for a protector with an indicator light that
tells you if the protection components are functioning. All MOVs will burn
out after repeated power surges, but the protector will still function as a
power strip. Without an indicator light, you have no way of knowing if your
protector is still functioning properly.
Belkin SurgeMaster II mid-range surge protector with connections for
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 computer. If the unit fails to
protect your computer from a power surge, the company will actually replace
your computer. This isn't total insurance, of course -- you'll still lose
all the data on your hard drive, which could cost you plenty -- but it is a
good indication of the manufacturer's confidence in their product.
No surge protector is 100 percent effective, and even top of the line
equipment may have some serious problems. Electronics experts are actually
somewhat divided over the best way to deal with power surges, and different
manufacturers claim other technologies are inherently faulty. If you're
interested in learning more about these issues, and finding out all the ways
surge protection technology can fail, check out some of the sites listed in
the links section on the next page. Surprisingly, surge protectors are an
extremely controversial piece of technology, and they have sparked a great
deal of debate on the Web.