lightning modem protection
Telephone line surge protection
First version written by Tomi Engdahl at 1996, revised at June 1998
Why surge protection is needed ?
Telephone lines are long copper wires from central office to your house. Usually those wires are between few hundred meters to few kilometers. Those wires are connected in the central office to the telephone center/exchange. Normally there is only the 48V DC voltage from central office battery in the line, and when phone ring there is 70-130V AC voltage in the line. That's what in the line in normal operation.
But the situation in the thunderstorm is different. When the lightning hits the ground with it's millions of volts and thousands of ampreres strike every wire in the ground or hanging in the air will notice it. Near lightning strike gan generate a surge pulse of thousands of volts to telephone lines. If this can go to the electronics inside the telephone or modem, the electronics will destroy at the same moment.
What telecom operator has done to it ?
The surge can destroy electronics in both subscriber's and central office end, so you must think that telecom operator must have done something to this problem. Yes they have.
First they have installed surge suppression devices to their central office to make sure that small surges dos not damage it. They have usually also installed some sort of protection to subscriber's side of the line to the place where telephone cable enters the house or to the the telephone pole just next to the house. That type of protection devices are very common on rurals installations, where there are very long lines and they are usually hanging in the air from telecom poles so they are much more prone to lightning strike surges.
In bigger cities lightning protectors are not always used because different buildings are usually electrically so tightly connected to each other (though electric feed, plumbing etc.) and cables are in the ground so there is not possible to generate so big surge voltages because of potential differences between different points (central office and subscriber).
What types of voltage surges there are ?
There are two types of surges: differential and common mode. Differential surge is a voltage surge which is generated between the telephone wires. That is usually not great, because telephone wires are twisted pairs and is easy to stop by palacing suitable protection component between telphone line wires.
Other type of surge is common mode surge, in which the potential of the telephone wires raises thousands of volts from your ground potential. Usually the telephone wires are at potential quite near to the ground of your building. But if the lightning strike hits the central office (or some else place in telecom cabling near you), the potential of both of the telephone line wires can raise quite high (even thousands of volts). The common mode surges are usually bigger and more problematic than differential surges.
How telephone can handle surges ?
Telephones are devices sitting in the end of the telecommunication line electrically isolated from rest of the building. They are not very prone to common mode surges, unless they are special phones which are connected also to mains (for example those which have answering machine in them).
The thing which breaks ordinary telephone is differential surge, because it is the surge which can get to the electronics of the telephone. Modern telephones usually damage easier, because electronics will break down more easily than the transformers, carbon microphones and mechanical rotary dials used in older telephones. In cheap modern telephones there is usally not much done to protect them from surges, because good protection could easily cost more than the telephone itself. It is economically wise. If cheap telephone breaks down people go to shop and buy new one. If telephones are damaged often, then it migh be a good idea to install a good surge suppressor.
How about modems ?
Modems are more prone to lighting surges than telephones. There are two reasons for that: modems have more dedicate microelectronics in them than simple telephones and modems are connected to computers, which are connected to mains electricity. For those reasons modem electronics are prone to be damaged by both differential and common mode surges.
Differential surge easily can go through the modem line transfomer in the same way the normal modem signals go. To protect modem electronics, a good practice is to add protection to both sides of the line transformer: gas arrestor or VDR in the primary side of the transformer to take most of the surge and zener diodes to the secondary to take what goes through the primary protection and transformer. Quite typical circuit for modem line input protection is to put smal resistors (10 ohms or less) in resies with both lines coming to the modem. Then after those resistors there is the overvoltage protection device (gas arrestor and/or VDR, usually clamping at 130V). The resistors which are put in series with the telephone line work as sort of fuses for large surges and they are nonflammable type for fire safety.
The harder is the protection against common mode surges. The line transformer in the modem will keep the common mode voltage out of modem electronics, but it has limited isolation strength (few kilovolts). The telecommunication regulations demand that modem line transformer must withstand 1-2 kV of common mode voltage. This is enough for normal overvoltages, but large surges can easily have higher voltages. Overvoltage suppressors can't be generally used between phone line and modem case unless the modem is guaranteed to be properly grounded, because of user safety. So that's why they re not used in normal computer modems.
Why external surge suppressors are sometimes needed ?
The protection provided by telecom operator and the devices connected to telephone line are not always enough. The protection usually is designed so that it minimizes the costs. It is not worth to protect cheap telephones with expensive protectors, because strong lightning surge is quite rare and telephones are inexpensive to replace if such thing happens. Usually surge protection can be pretty sloppy is the electronics that are being protected are designed with surges in mind.
The situation migh be different, when there is something more expensive than ordinary telephone connected to line. For example expensive computer systems are usually worth to protect, because the damage caused by the lightning strike can cause very expensive damage. For example in PC case, lightning strike can not only destroy the modem (which is not usually very expensive), but also something else inside of the PC. That can become very expensive if valuable information is lost and the PC is very important at your business.
So sometimes extra protection is needed. The people which operate large computer systems know that sooner or later lightning strike will hit and break something. If there is enough protection, the damage is avoided or at least minimized.
Operation of the surge suppressors
To be able to get rid of surges the surge suppressors have to protect the equipments against two kinds of surge voltages:
- differential surge = surge voltage between line wires
- common mode surge = surge voltage between ground and line wires
When surge protectors are installed to a telephone/data communications room they are typically connected a common grounding bar which is connected to a good ground through a heavy grounding wire. The quality of the grounding (resistance and inductance) effect very much on how well the surge protector will protect against common mode surges. Even the best surge protector can't work well unless it is properly grounded. So it is recommended that you install the surge protector near the main grounding bar.
Some telephone line surge protectors sold in USA use the mains connector ground for grounding (those surge protectors which have mains surge and telehone line surge protectors in some case). The ground in the mains connector is not very good grounding point and using it as ground for surge protector can induce par of the surge also to the mains wiring.
The basic surge protectors use metal oxide varistor (MOV) to do the protection. This circuit below used one MOV between line wires for protection against differential surges and two MOVs from line wires to ground connection to protect against common mode surges. ----------o---------o--------- to equipment
The MOV between the line wire must be selected to have such voltage that it does not start to conduct at the normal telephone line voltages but stops the harmful higher voltages. Typica telephone line DC voltage is 48V and ring voltage is typically 90 vrms or 130V peak but can be upt to 130V RMS. Some modems use 130V RMS rated VDR which starts to conduct at about about 190V peak. Bell system Technical Reference #61100 mentions that a worst case telephone line voltage can be 105 VDC + 130 VAC= 289 V peak. If the MOV voltage is set too low the circuit will not pass "on-hook" requirments because it leaks too much current (for example FCC part 68 requirments demand 10 Mohm DC impedance in "on-hook" state).
| |
| MOV
TELCO | |
MOV o--- TO GROUND
LINE | |
| MOV
| |
----------o---------o-------- to equipment
The MOVs form line to ground connection to line wires have typically somewhat higher voltage rating to make sure that they do not start to conduct at normal ground potential differences seen in the situation where the surge protector is used. For example one AT&T unit has 130 V RMS MOV's in SERIES to ground, so they won't clip until about 380V peak and one PATTON says it clips at 310V in 500 Ns.
Because MOVs have limited surge handling capacity some surge protectors use resistors between the line and the surge protector to somewhat limit the surge current and dissipate the surge energy. Because the surges can have high energy the resistors must be able to handle high power surges safely. By having enough powerful non-flammable resistors (0.5-2W) usually is a safe choice. The fuses in the circuit are ment to cut the connection if something hazardous happens. If big surge happens it will quite propably burn the fuse. The fuse itself does not usually help much in fighting against surge but it will make sure that if for some reason the ground potential is no longer ground, no damaging currents will flow to telephone line (for example mains current accidentally entering to thin telephone cable wire can burn it and start fire). So the fuses melting the phone wire in catastrophic failure.
FUSE
The MOV arrangemen in this circuit is somewhat special. In this arrangement the rurge voltage in which the MOVs start to conduct ia approximately double the voltage ratings of the MOVs used in this circuit. In any surge case the surge energy will flow through MOVs (so the energy is divided between them). This arrangement is shown here because it is used also on some commercial surge protectors. The MOV arrangement in this circuii equivalent to the first circuit if the first MOVs have half the voltage rating and twice the capacitance of the second MOVs.
TELCO >----o/\o------/\/\/\----------o------------> OUT TO EQUIPMENT
|
LINE MOV
|---MOV--> TO GROUND
MOV
FUSE 20 OHMS 2W |
>----o\/o------/\/\/\----------o------------>
Always just VDRs no not provide enough protoection. DVRs are quite fast, but have limited surge handling capacity. If nore capacity is needed it is quite common to use higher power but slower gas tube arrestors in front of the VDR surge protection circuit. This arrangement (when derigned correctly) gives high surge handling capacity and quitre fast operation. If even faster opration is needed then is possible to add some fast special semiconductor surge protection devices after MOVs (zener diodes, avalanche diodes, surgectors, TISPs etc.).
Non-commercial designs
The following non-commercial surge suppressor circuits are collected from various sources (BBSs, FTP-sites etc). I am only including those schematics here to make a good collection of surge suppression circuits. I have not tried those circuits myself, so I can't say if they are effective or not.
Circuit one
This circuit is designed by Reijo Salminen, who posted it to MITS BBS at spring 1991. The circuit is designed for protecting modems and telephones connected to telephone line. The circuit is designed for telephone lines used in rural areas. The protector is connected between modem and incoming telephone line. The ground connector is connected to main electricity ground of the building through good grounding wire.
o---------------*----*----------RRRR-------*------
Line A I V ---> to modem
o---------------*----I---*------RRRR-------*------
B B
Ground--------------------*---*
A = 230V surge gas tube protector
B = 600V surge gas tube protector
RRRR = 10 ohm 1 W resistor
V = varistor 250V
The resistors in the circuit limit the surge energy passing through the circuit and they work as fuses in case of large surge. For safety, those resistors should be non flammable type.
Over voltage protectors B are rated so that in case of electricity grounding fault where ground pin becomes accidentally hot (220V AC), those surge protectors does not pass current to telephone line. If the surge protectors pass current to telephone line when electricity ground potential is raised against telephone line, there is apparent danger of fire.
In Finland modems are tested at 2000V voltage pulse and in USA with 1000V pulse, so the protection should be enough in both cases. It shold be mentioned that the circuit is not 100% proof, so the best protection is disconnect modem from telephone line connector when thunderstrom is coming.
Circuit two
This circuit was designed by Tim Jackson at 1990. It was presented in his article "TELEPHONE LINE SURGE ARRESTOR" found in telecommunication archives.
The trick is to install the unit in the line between the telephone jack and your modem (ie: not too far from the modem, like in another room) and connect the earth lead from the circuit to the earth pin on the SAME PLUG that feeds your PC. 10 ohm 5 Watt
Circuit drawn as best as possible with ASCII by Pat Verner.
>----------+-----/\/\/\/------------------+------------->
Phone Line A | |
<=>300V Gas Arrester ____|_____ To Modem
| | A |
| |TISP2290|
|--------+--------------------------|C |
Earth | | B |
| ----+-----
<=>300V Gas Arrester |
| |
| | To Modem
Phone Line B | |
>----------+-----/\/\/\/------------------+------------->
10 ohm 5 Watt
The phone line has a gas arrestor from each leg to earth. In other words, two gas arrestors. One from A to earth and one from B to earth.
The line then has a resistor in series with each leg (A and B) before being connected to the TISP2290 (the Texas Instruments chip mentioned earlier). This chip has three pins. The outer two (A and B in the diagram) are connected to the resistors while the centre one (C) is connected to earth. The metal tag of this component is internally connected to the earth pin (C), just for the record. The modem is fed from the outer two pins of the TISP2290.
The bulk of the energy involved in a surge is dissipated by trusted (and slow as treacle) gas arrestors. The TISP2290 absorbs the high speed spike that the gas arrestors miss and is itself protected by the two resistors which provide a little current limiting. The modem, being fed from the same point as the TISP2290 is protected by the whole circuit.
For those who have to know, the TISP2290 works in a manner similar to a zener array connected between the A and B wire and earth so as to limit the voltage between any of three points to about 200 Volts. As you know this is not entirely effective and so if the voltage rises to 290 Volts (hence TISP2*290*) then this crafty critter cuts in triacs to crowbar the offending points to earth until the surge has passed.
Commercial designs
The following circuit are the circuit used in two commercial units. The circuit diagram is drawn by looking inside those commercial units and drawing the circuit diagram. Those circuit diagrams are scanned from my memo I made when I examined those surge suppressors. Those circuit diagrams should be readable at most parts and the text should clear out the detais not easily readable from picture.
Furse ESP-TN
This circuit is a circuit diagram of commercial surge protector sold under name Furse ESP-TN for protecting normal telephone lines. The circuit is designed for protecting normal telephone lines and is packaged to metal box where there are connectors for telephone line and thick ground wire (the connector in the bottom of the circuit). The components were installed to circuit board where there is lots of copper thickened by lots of tin.
The circuit uses on 260 V doube gas arrestor as first protection. After that there is 2.2 ohm series resistors followed by surgesuppression network built from 180V TISP and three DVRs. After that there are two more serial resistors and network built from three 180V zener diodes. Wuite complicated circuit and no wonder why it costs quite much (about 100 US dollars).
Telematic Lighting Arrestor SAPN (Telematic Surge Barrier)
This is another telephone line surge arrestor sold under name Telmatic SAPN Line Barrier by Black Box. The protector has following specs:
- Clamping voltage: 200V+-10%
- Rise time: 15 ns
- Interface: 2-wire PSTN
- Connectors: Screw terminals
- Size: 2.5H x 2.1W x 14D cm
- Weight: 0.1 kg
- Protection mechanism: 5 kA gas discharge tube and high speed clamp diodes
The circuit is quite straighforward circuit made of gas arrestors, resistors and zener diodes. Firs protection for large surges are one gas arrestor connected between line leads and two other gas arrestors connected between line leads and ground connection. All gas arrestors are type JES 0394 2029-23-BY. After that there is 4.7 ohm resistors followed by surge suppression network built from zener diodes (type BWX50-180) which handle the surge which has passed those gas arrestors. There was a place and markings in the circuit board for capacitors C1 and C2, but those were not installed in the circuit.
Tomi Engdahl
- 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.
A lightning-induced surge event activates the SIDACtor protection device within nanoseconds. It effectively provides a short-circuit path that routes the offending signal away from the sensitive circuit.
SIDACtor solid state protection devices protect telecommunications equipment
such as modems, line cards, fax machines, and other CPE.
What is Transient Voltage?
Transient voltage is overvoltage in an electrical system, and can damage or destroy circuits. These voltage spikes can be caused by lightning strikes, electrostatic discharge (ESD), power outages, and short circuits. A transient voltage suppressor, or TVS, can be used to protect sensitive electronics. A TVS limits the voltage to a level that is safe for the circuit.
Metal oxide varistors (MOVs) are the primary voltage clamping component in many Transient Voltage Surge Suppressor, or TVSS products. A metal oxide varistor is an excellent transient voltage suppressor because transient energy is absorbed throughout the body and is dissipated evenly as heat, thus protecting the device.
Gas Discharge Tubes and TVSS
Another TVS component is the gas discharge tube (GDT). When a transient voltage surge occurs, the gas inside the GDT ionizes and impedance drops. The transient voltage is then diverted from the device. Gas discharge tubes return to their high impedance state when the transient voltage passes.
Silicon Avalanche Diodes and TVSS
Another TVS component is the silicon avalanche diode (SAD). When a transient voltage surge occurs that exceeds the devices standoff voltage the diode avalanches (conducts) and clamps the voltage to a low level. The transient voltage is then diverted from the device. When the transient has passed the SAD returns to it�s high impedance or stand off state.
Teccor Brand SIDACtor® Protection Thyristors
A SIDACtor device is a PNPN device that can be thought of as a TVS diode with a gate. Upon exceeding its peak off-state voltage (VDRM), a SIDACtor device will clamp a transient voltage to within the device�s switching voltage (VS) rating. Then, once the current flowing through the SIDACtor device exceeds its switching current, the device will crowbar and simulate a short-circuit condition. When the current flowing through the SIDACtor device is less than the device�s holding current (IH), the SIDACtor device will reset and return to its high off-state impedance.
Introduction to How Surge Protectors Work
 A surge protector power strip protects your computer from surges in power. See more surge protector pictures. |
When you put together a computer 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 useful device.
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 article, 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 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 to prevent that problem from destroying your computer.
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 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 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.
Surge Protection
 A simple MOV surge protector with line conditioning and a fuse |
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.
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 Arrestors
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 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.
 Inside a surge protector with line-conditioning chokes |
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 computer (or other electronic device).
Power Surges
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.
| 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 appliances. |
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 switch on and turn off components like compressors and motors. This 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.
When to Use a Surge Protector
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 computers, microwaves, 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.
 |
Even if you connect surge protectors to all of your outlets, your equipment might be exposed to damaging surges from other sources. Telephone and cable lines can also conduct high voltage -->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.
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.
 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.
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 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 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.
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
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 extension cords.
 |
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.
We’ll look at the different kinds of surge protector ratings in the next section.
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 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.
 A Belkin SurgeMaster II mid-range surge protector with connections for phone lines |
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.
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.
Earth's magnetic field:
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